How do plants get blight?
Bacterial blight of soybean
Bacterial blight of soybeans can enter leaves through wounds or natural openings such as stomata. After gaining entrance to the host leaves, "Pseudomonas syringae pv. glycinea" multiplies in the leaf intercellular fluid. The pathogen must then overcome the plants defenses. "Pseudomonas syringae pv. glycinea" accomplishes this by using the type three secretion system to inject a variety of pathogenicity effector proteins (Hrp proteins) into the plant cell cytoplasm. These proteins act by interfering with effector-triggered immunity and producing phytohormones/toxins that suppress plant defenses. The expression of these virulence factors depends on the environmental conditions at the time of infection (see "environment section). Furthermore, expression of virulence factors will only take place when a sufficiently large population of bacteria is present, which is determined through quorum sensing. When successful, the common symptoms of bacterial blight will be seen, with the main effect on the plant being a reduction in photosynthetic leaf area. Generally, the amount of photosynthetic area lost is not enough to impede plant growth. As the plant continues to grow it overcomes the loss of photosynthetic area and reduction in yield, if there is any, is negligible.
Phomopsis obscurans
The symptoms of leaf blight infections begin as one to several circular reddish-purple spots, 3/8” to 1/2” in diameter on a leaflet and enlarge to form V-shaped lesions with a dark brown inner zone, a light brown outer zone, and a purple, red, or yellow perimeter. Lesions follow major veins progressing inward, which is what gives rise to the characteristic V-shaped lesions that result after the pathogen has established a strong infection in the plant tissue. The whole leaflet may turn yellow and then brown due to infection and subsequent necrosis of the plant tissue. In severe cases where the pathogen has progressed substantially, the stolons, fruit, and petioles of the leaf become infected and may girdle and kill the stem. The disease is commonly found in young tissue of the strawberry plant, when defense mechanisms may not yet be fully established.
Rust (fungus)
Rust fungi are obligate plant pathogens that only infect living plants. Infections begin when a spore lands on the plant surface, germinates, and invades its host. Infection is limited to plant parts such as leaves, petioles, tender shoots, stem, fruits, etc. Plants with severe rust infection may appear stunted, chlorotic (yellowed), or may display signs of infection such as rust fruiting bodies. Rust fungi grow intracellularly, and make spore-producing fruiting bodies within or, more often, on the surfaces of affected plant parts. Some rust species form perennial systemic infections that may cause plant deformities such as growth retardation, witch's broom, stem canker, galls, or hypertrophy of affected plant parts.
History of the potato
A lack of genetic diversity from the low number of varieties left the crop vulnerable to disease. In the early 1800s, a strain of potato blight ("Phytophthora infestans") known as HERB-1 began to spread in the Americas, especially Central and North America destroying many crops. The blight spread to Europe in the 1840s where, because of an extreme lack of genetic diversity, the potato crops were even more susceptible. In Northern Europe there were major crop losses lasting throughout the rest of the 19th century. Ireland in particular, because of the extreme dependence of the poor, especially western Ireland, on this single staple crop, was devastated by the blight's arrival in 1845.
Phyllody
In many cultivated plants, phyllody is caused by infections of plant pathogens and/or infestations of ectoparasites. Aside from exhibiting phyllody, they may also exhibit other symptoms like virescence, witch's brooms, chlorosis, and stunted growth. Examples of these biotic factors include:In addition to causing phyllody itself, insects and other ectoparasites also serve as disease vectors that can spread phyllody to other nearby plants. The most common of these insect vectors are leafhoppers, an example of which is "Hishimonus phycitis", which transmits the phytoplasma-caused little leaf phyllody in eggplants. The broken-backed bug ("Taylorilygus apicalis") is another insect vector of a phytoplasma-caused phyllody in species of "Parthenium". Other ectoparasite vectors include eriophyid mites, like the rose leaf curl mite ("Phyllocoptes fructiplilus") which is known to be the primary vector of the rose rosette disease; and the chrysanthemum rust mite ("Paraphytoptus chrysanthemi") which transmits phytoplasma-caused phyllody in species of chrysanthemums.
Chestnut blight
Plant pathologists, Drs. William Powell and Charles Maynard, working at the State University of New York College of Environmental Science and Forestry, have developed American chestnuts which have full blight resistance. Full resistance was attained by introducing a wheat gene coding for the enzyme oxalate oxidase into the American chestnut genome. This enzyme breaks down the oxalic acid secreted by the fungus into carbon dioxide and hydrogen peroxide. Early studies on hypovirulence showed that less virulent strains of the chestnut blight produced less oxalic acid when attacking the cambium. The transgenic trees have blight resistance either equal to or surpassing that of Chinese chestnuts. In 2013, SUNY ESF had over 100 individual events being tested, with more than 400 slated to be in the field or in the lab for various assay tests in the next several years and more than 1,000 trees growing in several field sites in 2014. Government approval will be required before returning any of these blight resistant trees to the wild. The New York Botanical Garden has planted several of the transgenic trees for public display.
Phomopsis blight of juniper
"P. juniperovora", in wet years, is capable of killing off all seedlings. In the late 1940s a major disease outbreak happened, wiping out many immature juniper trees used for windbreaks in the U.S. In addition to killing off young plants, the value of older plants decreases due to infection. This is due to the decrease in aesthetic appeal, and the infection can increase the likelihood of the plant not surviving transplanting or taking longer to establish. An assessment of the economic losses from this disease has not been conducted.
Cochliobolus victoriae
Victoria type oats contain the "Pc-2" gene which confers resistance against crown rust caused by the fungus "Puccinia coronate". However, the oats that contained this gene are susceptible to Victoria blight. This is because the gene LOV1, which is part of the crown rot resistance, is responsible for susceptibility to Victoria blight. "Cochliobolus victoriae" produces the toxin victorin. Victorin only effects plants that contain the LOV1 gene, causing these plants to develop Victoria blight.
Southern corn leaf blight
Lesions caused by Race T are tan with yellow-green or chlorotic halos. Later on, lesions have red to dark brown borders and can spread to all other above-ground parts of the plant including the stem, sheath, and ear. The shape of lesions are elliptical or spindle and can be larger than lesions caused by Race O, at 6 to 12 millimeters wide by 6 to 27 millimeters long. Race T infection causes seedlings to wilt, and they die within three or four weeks.
Fungus
Many fungi are parasites on plants, animals (including humans), and other fungi. Serious pathogens of many cultivated plants causing extensive damage and losses to agriculture and forestry include the rice blast fungus "Magnaporthe oryzae", tree pathogens such as "Ophiostoma ulmi" and "Ophiostoma novo-ulmi" causing Dutch elm disease and "Cryphonectria parasitica" responsible for chestnut blight, and plant pathogens in the genera "Fusarium", "Ustilago", "Alternaria", and "Cochliobolus". Some carnivorous fungi, like "Paecilomyces lilacinus", are predators of nematodes, which they capture using an array of specialized structures such as constricting rings or adhesive nets. Many fungi that are plant pathogens, such as "Magnaporthe oryzae", can switch from being biotrophic (parasitic on living plants) to being necrotrophic (feeding on the dead tissues of plants they have killed).
Virus nanotechnology
Viruses cause several destructive plant diseases and are accountable for massive losses in crop production and quality in all parts of the world. Infected plants may show a range of symptoms depending on the disease but often there is severe leaf curling, stunting (abnormalities in the whole plant) and leaf yellowing (either of the whole leaf or in a pattern of stripes or blotches). Most plant viruses are therefore transmitted by a vector organism (insects, nematodes, plasmodiophorids and mites) that feeds on the plant or (in some diseases) are introduced through wounds made, for example during agriculture practices (e.g. pruning). Many plant viruses, for example, tobacco mosaic virus, have been used as model systems to provide a basic understanding of how viruses express genes and replicate. Others permitted the elucidation of the processes underlying RNA silencing, now recognised as a core epigenetic mechanism underpinning numerous areas of biology.Manifold plant virus platform technologies are being developed and studied for many applications including:
Xanthomonas oryzae pv. oryzae
Bacterial leaf blight is a prevalent and destructive disease which affects millions of hectares throughout Asia. In Japan alone, annual losses are estimated to be between 22,000 and 110,000 tons. In the Philippines, susceptible varieties lose up to 22.5% of the total harvest during wet seasons and up to 7.2% in the dry season. In resistant crops, these numbers are, respectively, 9.5% and 1.8%.
Chestnut blight
Removing blighted trees to control the disease was first attempted when the blight was discovered, but this proved to be an ineffective solution. Scientists then set out to introduce a hyperparasitic hypovirus into the chestnut blight fungus. The trees infected with virus-treated fungus responded immediately and began to heal over their cankers. However, the virus was so efficient at attenuating fungal growth that it prevented spreading of the virus from an infected fungus growing on one tree to that growing on another tree. Only the virus-treated trees recovered. Scientific opinion regarding the future of the stand varies.
Phomopsis obscurans
Temperature, inoculum density, moisture period, and host tissue maturity all have an effect on the disease development of the inoculated strawberry plant. Temperature takes a vital role in the infection and development of the blight disease caused by the pathogen, "Phomopsis obscurans". The most ideal temperature that leads to the greatest amount of disease growth, conidial germination, and germ tube elongation is between 26 and 32 °C (78.8 and 89.6 °F). Within this temperature range, inoculated plants experienced the greatest symptom development and the most severe infection at 30 °C (86 °F). These temperatures are commonly found in the midsummer environment of North Carolina, where leaf blight of strawberries caused by "Phomopsis obscurans" is especially prevalent.
Velvet blight
It is known to affect mainly tea plants ("Thea" genus).
The most studied of these species is "S. bogoriense", most notably due to the work of Ernst Albert Gäumann. "S. bogoriense" is named after the Herbarium Bogoriense (Bogor, West Java, Indonesia) which is the place where it was first identified on the bark of an unspecified tree and named by E. Nyman on June 3, 1898. This species was also listed in Otto Warburg's Monsunia in 1900.
Bacterial blight of cotton
Bacterial blight of cotton is a disease affecting the cotton plant resulting from infection by "Xanthomonas axonopodis pathovar malvacearum" (Xcm) a Gram negative, motile rod-shaped, non spore-forming bacterium with a single polar flagellum.
Phomopsis blight of juniper
"Phomopsis" blight of juniper is most aggressive during cool, moist weather often associated with spring. These conditions promote pycnidia to release their spores, which possess an optimal germination temperature range between 24 and 28 degrees Celsius. Continuously wet foliage is needed for infection to occur. Areas with heavy shade and poor drainage allow for moist, slow drying areas for the disease to thrive in. Exposure to 100 percent humidity for as little as 7 hours can propagate seedlings to be fully infected.
Fungicide use in the United States
Pod and stem blight attack soybean plants during very wet seasons. It is caused by a number of species of the fungal genera Diaporthe and Phomopsis. They cause small black fruiting bodies to form on plant stems and pods. These fungi cause moldy, cracked, and shriveled seeds which in return produce low-quality oil and meal. Foliar fungicides applied between mid-flowing and late pod provide the best fungicidal control.
Mountain Lake Wilderness Cluster
In the early 20th century the forest was dominated by chestnut trees to the extent that the forest was characterized as "oak-chestnut", but by 1930 most of these trees had been infected by chestnut blight. The root system of blight killed trees still produces sprouts but they do not get much larger than an inch in diameter before succumbing to the blight. A large number of hemlock trees are threatened by the woolly adelgid that arrived in 2000. Efforts are underway to introduce beetle predators known to feed on the woolly adelgid.
Chestnut blight
If cankers continue to form and expand, the fungus can girdle the stem, severing the flow of nutrients and water to the vital vegetative tissues. The absence of nutrient dispersal will result in tree death, however, the root system will survive. As a result, American chestnuts exist mainly as shrubs sprouting from the old, surviving roots. However, these sprouts usually succumb to infection by "C. parasitica" before reaching sexual maturity.
Outpost 2: Divided Destiny
Eden's terraforming process uses a bioagent that breaks apart oxygen bonds, carrying the potential to also destroy 'boptronic' (bio-optronic) computer systems and humans by breaking down their organic molecules. At first Eden performs a number of tests with early versions of the bioagent in testwells aligned with a fault line leading almost all the way up to Plymouth's colony. "The Blight", the uncontrolled expansion of the bioagent, is unleashed upon New Terra when one of their advanced labs explodes. Eden has to evacuate to run from the, at first, mostly unknown cause of the disaster. The water produced by the breaking of oxygen bonds lubricates dormant fault lines, and the air released begins to thicken and storm. These result in more frequent occurrences of earthquakes, lightning storms, and other natural disasters. In Plymouth, a dormant volcano erupts nearby, also forcing them to leave the area. To top off the disasters, the planet does not have enough atmosphere to burn off asteroids as it passes through an asteroid belt. Both factions soon come to the conclusion that New Terra is about to become uninhabitable, and the progressing Blight will soon engulf the entire planet. They decide to evacuate New Terra, but as they discover there are not enough resources left on the planet for two spaceships, confrontation is inevitable.
Phytophthora ramorum
In Europe, "Ramorum" blight was first observed on rhododendron and viburnum in the early 1990s, where it was initially found mainly on container-grown plants in nurseries. The principal symptoms were leaf and twig blight. By 2007, it had spread throughout nurseries and retail centers in 16 European countries, and had been detected in gardens, parks, and woodlands in at least eight countries. It has not caused significant harm to European oak species.
Bur oak blight
When affected by the disease, the oak develops vein necrosis confined to the leaf midvein and major lateral veins, eventually causing leaf death. Symptoms typically start in the lower crown of the tree and eventually spread to the entire tree. If this disease persists for many years, there can be lasting effects. One example is the possible loss of food reserves. This can leave the tree susceptible to greater insect predation, secondary infections from other pathogens, and death. A distinguishing characteristic of "T. iowensis" is the formation of black crustose asexual fruiting bodies at the base of the petioles that are retained until the following year. These distinctive fruiting bodies are pycnothyria (conidiomata) with radiating setae-like hyphae that form along the necrotic veins.
Burkholderia glumae
Symptoms of bacterial panicle blight include seedling blighting and sheath rot in addition to panicle blighting, which accounts for most of the damage from this disease. Affected panicles have blighted florets, which initially show white or light gray on the basal third with a dark-brown margin and eventually become straw-colored. The florets then turn dark with growth of fungi or bacteria on the surface. Extensive occurrence of upright panicles because of the failure of grain-filling and seed development is a typical phenomenon observed in a severely infested field. Prolonged high temperature during the growing season is an important environmental condition that promotes the development of bacterial panicle blight. 
Severe epidemics of this disease, which caused up to 40 percent yield losses in some fields, occurred during the 1995, 1996, 1998, 2000 and, most recently, 2010 growing seasons, when record-high night temperatures were experienced. Suspected global warming could make bacterial panicle blight a greater threat to rice production in the future. Indeed, occurrence of bacterial panicle blight is increasing not only in the southern United States but in other rice growing countries of Central and South America and Asia.
The disease cycle of bacterial panicle blight is not fully understood in spite of the economic importance of the disease.
The bacterial pathogens are considered to be seed-borne, but they also survive in the soil. After germination of the seed, the bacteria appear to survive on the leaves and sheath and spread upward as the plant grows. Their infection to rice panicles occurs at flowering, if the bacterial population reaches to a threshold level and environmental condition is favorable. They may also be disseminated from severely diseased panicles to neighboring healthy plants, according to the observed spatial distribution patterns of the disease in infested rice fields. However, it is not clear if long-distance dissemination can occur via insect transmission.
Unfortunately, there are few effective control measures for this disease so far.
Less nitrogen fertilization reduces disease severity but has not been successful. High temperatures at the vulnerable period can be avoided by early planting, but this cultural practice would become useless if hot weather comes early in the growing season. No pesticides are currently recommended or allowed for controlling this disease in the United States. Copper compounds are weakly effective but sometimes toxic to the plant. Oxolinic acid is useful for seed treatment, and its foliar spray is effective on infected plants, but this chemical is not commercially available in the United States.
The research goal on bacterial panicle blight is to develop effective disease control methods based on better understanding of the bacterial virulence mechanism and the rice defense system.
To achieve this goal, LSU AgCenter scientists are conducting several different areas of research. First, scientists are making efforts to develop new rice varieties and lines resistant to bacterial panicle blight through conventional breeding and line development processes. More than 15,000 lines are evaluated annually to select promising lines showing high levels of disease resistance to bacterial panicle blight and other good agronomic traits. The partial-resistant rice varieties and lines, such as Jupiter and LM-1, were developed from this program. Second, scientists are conducting genetic and molecular biological studies on the rice disease resistance to bacterial panicle blight. Genetic mapping to identify the rice genes associated with the disease resistance is under way. In addition, the induction of a rice defense system by pretreatment of various chemical materials, which leads to enhanced disease resistance, is being studied. Finally, scientists are studying molecular genetics and genomics of the major pathogen, B. glumae, to understand the mechanism underlying its bacterial pathogenesis in rice.
Genetic elements governing the production of virulence factors are being identified and characterized.
Exobasidium vexans
Detection of Tea Blister Blight is done via a visual inspection of the crop plants for the characteristic blisters. There are many methods of control throughout Asia, with varying levels of success. Most frequently fungicides such as carbendazim, hexaconazole, propiconazole, and tridemorph are used. A 0.2% mixture of nickel sulfate or copper oxide can be applied to foliage to control spread as well. Application of diluted polyoxin, an antibiotic, has also had some success at control and inhibition of the disease. Due to the short life cycle of "E. vexans" it is critical to apply chemicals weekly when conditions are favorable for disease. The basidiospores of this pathogen are very sensitive, direct sunlight in excess of 4 hours will readily destroy them, therefore other agricultural methods (such as planting rows greater distances apart to allow sunlight to penetrate to young target leaves) can be helpful in controlling disease spread.
Blueberry shock virus
Blueberry shock virus is pollen-borne and likely infects during pollination. Bees and other insects that come into contact with the infected pollen can transmit the virus as well. The virus can spread quickly once established in the field. Once infected, the plant suffers from flower and leaf blight and dieback. Symptoms begin to appear just prior to bloom and can continue to develop during bloom. Eventually, after one to two years the shoots grow back and the infected plant may regain fruit production again. When the plants fully recover, they once again produce a full crop. However, their pollen will continue to be a source of inoculum and spread the virus to other blueberry plants, making it difficult to control. Management of the disease involves preventing introduction of the virus to non-infected plants. Suckers and plant material should be tested for the virus before introduction into a nursery or field. If plants do become infected with the disease either the few plants infected can be removed and burned or the whole field may need to be. Herbicides are also sprayed to ensure that the root is killed, leaving no infected suckers in the ground. Research has shown that yields are not significantly affected in recovered bushes.
Boxwood blight
Boxwood blight (also known as box blight or boxwood leaf drop) is a widespread fungal disease affecting boxwoods (box plants), caused by "Cylindrocladium buxicola" (also called "C. pseudonaviculatum").
Norman Borlaug
In the last months of his undergraduate education, Borlaug attended a Sigma Xi lecture by Elvin Charles Stakman, a professor and soon-to-be head of the plant pathology group at the University of Minnesota. The event was a pivot for Borlaug's future. Stakman, in his speech entitled "These Shifty Little Enemies that Destroy our Food Crops", discussed the manifestation of the plant disease rust, a parasitic fungus that feeds on phytonutrients in wheat, oats, and barley crops. He had discovered that special plant breeding methods produced plants resistant to rust. His research greatly interested Borlaug, and when Borlaug's job at the Forest Service was eliminated because of budget cuts, he asked Stakman if he should go into forest pathology. Stakman advised him to focus on plant pathology instead. He subsequently enrolled at the University to study plant pathology under Stakman. Borlaug earned a master of science degree in 1940, and a Ph.D. in plant pathology and genetics in 1942.
Inducible plant defenses against herbivory
Induced defences require plant sensing the nature of injury, such as wounding from herbivore attack as opposed to wounding from mechanical damage. Plants therefore use a variety of cues, including the sense of touch, and salivary enzymes of the attacking herbivore. For example, in a study to test whether plants can distinguish mechanical damage from insect herbivory attack, Korth and Dixon (1997) discovered that the accumulation of induce defence transcription products occurred more rapidly in potato ("Solanum tuberosum L.") leaves chewed on by caterpillars than in leaves damaged mechanically. Distinct signal transduction pathway are activated in response either to insect damage or mechanical damage in plants. While chemicals released in wounding responses are the same in both cases, the pathway in which they accumulate are separate. Not all herbivore attack begins with feeding, but with insects laying eggs on the plant. The adults of butterflies and moths (order Lepidoptera), for example, do not feed on plants directly, but lay eggs on plants which are suitable food for their larva. In such cases, plants have been demonstrated to induce defences upon contact from the ovipositing of insects.
Morphological symptoms of plant diseases
Wilting is due to loss of turgor in plant tissue resulting in the dropping of plant parts. They are common symptom in diseases where the pathogen or the toxic metabolites it produces affects the vascular tissue of the host plant. Interference in water transport brought about by the infection of these vascular pathogens leads to wilting. Unlike wilting due to low soil moisture, wilting due to the activity of these pathogens cannot be overcome by watering the plants. Infected plants eventually die.
Bolting (horticulture)
Bolting is induced by plant hormones of the gibberellin family, and can occur as a result of several factors, including changes in day length, the prevalence of high temperatures at particular stages in a plant's growth cycle, and the existence of stresses such as insufficient water or minerals. These factors may interact in a complex way. Day length may affect the propensity to bolt in that some plants are "long day plants", some are "short day plants" and some are "day neutral" (see Photoperiodism), so for example when a long day plant, such as spinach, experiences increasingly long days that reach a particular length, it will be inclined to bolt. Low or high temperatures can affect the propensity of some plants to bolt if they are experienced for sufficient periods at particular points in the life cycle of the plant; once these conditions have been met, plants that require such a trigger will subsequently bolt regardless of subsequent temperatures. Plants under stress may respond by bolting so that they can produce seeds before they die.
Glossary of phytopathology
blight
Fungicide use in the United States
Uncontrolled bacterial blight has been shown to cause yield losses up to 20%. Symptoms include small, dry, and brittle yellowish-brown spots on the plant and stalks covered in bacterial ooze. The primary treatment of bacterial blight is applications of copper before the crop is fully mature.
American chestnut
American Chestnut Cooperators Foundation (ACCF) is not using Oriental genes for blight resistance, but intercrossing among American chestnuts selected for native resistance to the blight, a breeding strategy described by the ACCF as "All-American intercrosses." John Rush Elkins, a research chemist and professor emeritus of chemistry at Concord University, and Gary Griffin, professor of plant pathology at Virginia Tech, think there may be several different characteristics which favor blight resistance. Both Elkins and Griffin have written extensively about the American chestnut. They believe that by making intercrosses among resistant American chestnuts from many locations, they will continue to improve upon the levels of blight resistance to make an American chestnut that can compete in the forest. Griffin, who has been involved with American chestnut restoration for many years, developed a scale for assessing levels of blight resistance, which made it possible to make selections scientifically. He inoculated five-year-old chestnuts with a standard lethal strain of the blight fungus and measured growth of the cankers. Chestnuts with no resistance to blight make rapid-growing, sunken cankers that are deep and kill tissue right to the wood. Resistant chestnuts make slow-growing, swollen cankers that are superficial: live tissue can be recovered under these cankers. The level of blight resistance is judged by periodic measurement of cankers. Grafts from large survivors of the blight epidemic were evaluated following inoculations, and controlled crosses among resistant American chestnut trees were made beginning in 1980. The first "All-American intercrosses" were planted in Virginia Tech's Martin American Chestnut Planting in Giles County, Virginia, and in Beckley, West Virginia. They were inoculated in 1990 and evaluated in 1991 and 1992. Nine of the trees showed resistance equal to their parents, and four of these had resistance comparable to hybrids in the same test. Many ACCF chestnuts have expressed blight resistance equal to or greater than an original blight survivor but so far, only a handful have demonstrated superior, durable blight control. Time will tell if the progeny of these best chestnuts exhibit durable blight resistance in different stress environments.
Ovulinia azaleae
First reported in the 1930s on the Belgian-Indica hybrids, petal blight can attack most azaleas under favorable weather conditions. Infected flowers first exhibit small spots, about 1 mm in diameter, which appear water-soaked. The spots enlarge rapidly and become very slimy, causing entire petals to become slimy and limp, usually within 2–3 days after initial infection. Infected areas of flowers soon become tan or light brown, and eventually entire flowers turn prematurely brown. Infected flowers dry and generally cling to the plant longer than uninfected flowers do. One to 20 (commonly two to five) small, black sclerotia (a fungal resting stage) form on an infected flower 6–8 weeks later. They appear first as small white areas on the dried petals and slowly become dark as they mature.
Human interactions with microbes
Microorganisms including bacteria, fungi, and viruses are important as plant pathogens, causing disease to crop plants. Fungi cause serious crop diseases such as maize leaf rust, wheat stem rust, and powdery mildew. Bacteria cause plant diseases including leaf spot and crown galls. Viruses cause plant diseases such as leaf mosaic. The oomycete "Phytophthora infestans" causes potato blight, contributing to the Great Irish Famine of the 1840s.
Alternaria dauci
"Alternaria dauci" is one of two leading pathogens affecting carrots around the world. Most often found in temperate climates, the disease has been found in North America, the Netherlands, the Middle East, and even parts of Southern Asia and India. Carrot leaf blight is especially damaging in that its leaf lesions not only reduce photosynthetic area, but also weaken the leaves and petioles structurally. This makes mechanical harvesting of the carrot crop less efficient, and yields are even worse when blighted leaves have been exposed to heavy frosts.
Gummy stem blight
Gummy Stem Blight affects many cucurbits including watermelon, cantaloupe, cucumber, pumpkin, and some squash. Some symptoms are common of all Gummy Stem Blight infections while other symptoms can vary depending on the specific host the pathogen has infected. Hosts can become infected at any time in their life. When the pathogen is present in a young seedling, the cotyledons will sprout appearing dark and drenched. When older plants become infected, their leaves may appear water soaked and begin to develop dark tan lesions. The leaves begin to turn brown at the margins and necrosis progresses towards the base of the leaf.
Passiflora edulis
The fungus called fusarium wilt commonly occurs in adult plants and is caused by "Fusarium oxysporum". The pathogen has ability to survive for long periods, penetrating roots, invading the xylem and preventing the transport of water and nutrients to other organs of the plant. Once infected, this disease causes leaves to yellow and browning of the vascular system until it wilts and dies. It occurs in any type of soil infecting all plants. Management of crops include planting clean seedlings, uprooting and burning infected plants, and using sterilized tools.
Septobasidium pilosum
Septobasidium pilosum is a plant pathogen, one of a number of fungi in the genus "Septobasidium" responsible for the disease of tea plants known commonly as "velvet blight".
Halo blight
The seed is a source of inoculum and pathogen survives in seeds from the previous year. The pathogen of halo blight can overwinter in previously infected bean debris, contaminated seeds, weed hosts or volunteer beans. Halo blight can be dispersed by contact between wet leaves, rainfall, irrigation or people and animals moving through infested fields. The Pathogen can enter in either plant injuries or the natural openings in plants such as stomata and hydathodes during periods of high humidity or when the foliage is wet. Then the pathogen survives from defense mechanisms in intracellular spaces and obtain nutrients from the host. After 6–10 days of infection, bacteria oozes from lesions which causes secondary infections.
Fusarium ear blight
"Fusarium" fungi can overwinter as saprotrophs in the soil or on crop debris that can serve as inoculum for the following crop. The fungus can also spread via infected seed. The presence of "Fusarium" fungi on crop debris or seed can cause "Fusarium" seedling blight and foot and root rot. Later, infection of the heads can occur with spores spreading by rain splash from infected crop residues. Another major infection route is airborne inoculum as spores can travel long distances with the wind. The cereal crop is most susceptible at flowering and the probability of infection rises with high moisture and humidity at flowering.
Ascochyta diseases of pea
Ascochyta blights occur throughout the world and can be of significant economic importance. Three fungi contribute to the ascochyta blight disease complex of pea ("Pisum sativum"). "Ascochyta pinodes" (sexual stage: "Mycosphaerella pinodes") causes "Mycosphaerella" blight. "Ascochyta pinodella" (synonym: "Phoma medicaginis" var. "pinodella") causes "Ascochyta" foot rot, and "Ascochyta pisi" causes Ascochyta blight and pod spot. Of the three fungi, "Ascochyta pinodes" is of the most importance. These diseases are conducive under wet and humid conditions and can cause a yield loss of up to fifty percent if left uncontrolled. The best method to control ascochyta blights of pea is to reduce the amount of primary inoculum through sanitation, crop-rotation, and altering the sowing date. Other methods—chemical control, biological control, and development of resistant varieties—may also be used to effectively control ascochyta diseases.
Phomopsis blight of juniper
"Phomopsis" blight of juniper commonly infects the eastern red cedar, savin ("Juniperus sabina"), creeping juniper, and rocky mountain juniper plants ("Juniperus scopulorum"), but also has the ability to infect Chinese juniper and common juniper to varying degrees.
Fungicide use in the United States
Bean rust is a disease of the green bean that causes rust-colored pustules on the leaves. These pustules eventually rupture and eject thousands of spores into the air to be carried to other plants. Each individual spore can produce a fresh pustule on a plant within a week; therefore untreated rust can rapidly reach epidemic levels. When leaves become infected with rust they rapidly die causing defoliation. Research in the 1930s showed that sulfur was an effective treatment for rust and since then yield losses in treated fields have been reduced by 60%.
Barley
This plant is known or likely to be susceptible to barley mild mosaic bymovirus, as well as bacterial blight. It can be susceptible to many diseases, but plant breeders have been working hard to incorporate resistance. The devastation caused by any one disease will depend upon the susceptibility of the variety being grown and the environmental conditions during disease development. Serious diseases of barley include powdery mildew caused by "Blumeria graminis" f.sp. "hordei", leaf scald caused by "Rhynchosporium secalis", barley rust caused by "Puccinia hordei", crown rust caused by "Puccinia coronata", and various diseases caused by "Cochliobolus sativus". Barley is also susceptible to head blight.
Bacterial soft rot
There are many ways in which a plant can become infected by a bacterial soft rot. They can be host to the bacteria either by being infected as seed, or from direct inoculation into wounds or natural openings (stomata or lenticels) in mature plants, which is most common. But, when a plant is infected and the conditions are favorable, the bacteria immediately begin feeding on liquids released from injured cells and start replicating. As they replicate they release more and more pectolytic enzymes that degrade and break down cell walls. And, because of the high turgor pressure within the cells, this maceration effectively causes the cells to explode and die providing more food for the bacteria.
Dickeya dadantii
In the natural plant environment, "D. dadantii" causes plant maladies such as necrosis, blight and “soft rot,” which is a progressive tissue maceration. "D. dadantii" contains many pectinases that are able to macerate and break down the plant cell wall material. This exposed part of the plant releases nutrients that can facilitate bacterial growth. Commonly infected plants include potato tubers, bulbs of vegetables, and ornamental crops.
Castanea mollissima
Chinese chestnuts have evolved over a long period of time in coexistence with the bark fungal disease chestnut blight ("Cryphonectria parasitica", formerly "Endothia parasitica"), and have evolved a very successful resistance to the blight, probably more so than any other species of chestnut, so that, although it is not immune, it typically sustains no more than minor damage when infected. It's important to realize, though, that Chinese chestnut trees vary considerably in blight resistance. Some individuals are quite susceptible while others are essentially immune to the disease. Japanese chestnut is also comparatively resistant to blight, with European chestnut somewhat less so. In the 1890s, Chinese and Japanese chestnuts were imported to the United States with the intention of utilizing them as orchard trees due to their small, compact size compared to the towering American chestnut. The results unfortunately were disastrous as the imported Asian species introduced blight to which "C. dentata" lacked any resistance. The disease was first noticed on a tree in the Brooklyn Zoo in 1902 and quickly spread all out of control, ravaging American chestnuts. Within 30 years, there were very few left in their native range. An active program has been pursued in North America to cross-breed the Chinese and American chestnuts to try to maximize various desirable traits of the American chestnut, such as larger stature, larger leaf size, larger nut size, and greater nut sweetness, while also isolating and carrying the blight resistance from the Chinese chestnut.
Fungicide use in the United States
Bacterial blight is endemic to green bean crops grown east of the Rocky Mountains. During ideal conditions (usually humid weather) the disease can inflict losses as high as 60%. Symptoms include lesions resembling burn marks that gradually grow larger. In some states incidence of the disease can be as high as 100%. Trials have shown that properly timed copper sprays can reduce incidence by 90%.
Carrot
There are several diseases that can reduce the yield and market value of carrots. The most devastating carrot disease is "Alternaria" leaf blight, which has been known to eradicate entire crops. A bacterial leaf blight caused by "Xanthomonas campestris" can also be destructive in warm, humid areas. Root knot nematodes ("Meloidogyne" species) can cause stubby or forked roots, or galls. Cavity spot, caused by the oomycetes "Pythium violae" and "Pythium sulcatum", results in irregularly shaped, depressed lesions on the taproots.
Bacterial blight of soybean
Bacterial Blight of Soybean is caused by the bacterial agent "Pseudomonas syringae pv. glycinea". This bacterium also causes disease in snap bean (Phaseolus vulgaris) and lima bean (Phaseolus lunatus), however it is more commonly seen in soybean. "Pseudomonas syringae pv. glycinea" attacks all of the above-ground parts of soybean, but symptoms are typically seen on the mid-upper canopy of leaves and pods. After infection, small, water-soaked spots surrounded by a chlorotic halo appear on the leaves. The brown or black centers of these spots indicate that the tissue is dying. Typically these spots will enlarge and merge to form large, dead patches on the leaves. The leaves appear ragged if the dead tissue falls out. Lesions on pods are initially small and water-soaked but eventually enlarge, turn brown to black, and merge to encompass the whole pod. Infection can also occur on the stems, petioles and seeds.
Chestnut
Soon after that, though, the American chestnuts were nearly wiped out by chestnut blight. The discovery of the blight fungus on some Asian chestnut trees planted on Long Island, New York, was made public in 1904. Within 40 years, the nearly four billion-strong American chestnut population in North America was devastated; only a few clumps of trees remained in Michigan, Wisconsin, California and the Pacific Northwest. Due to disease, American chestnut wood almost disappeared from the market for decades, although quantities can still be obtained as reclaimed lumber. Today, they only survive as single trees separated from any others (very rare), and as living stumps, or "stools", with only a few growing enough shoots to produce seeds shortly before dying. This is just enough to preserve the genetic material used to engineer an American chestnut tree with the minimal necessary genetic input from any of the disease-immune Asiatic species. Efforts started in the 1930s are still ongoing to repopulate the country with these trees, in Massachusetts and many places elsewhere in the United States. In the 1970s, geneticist Charles Burnham began back-breeding Asian chestnut into American chestnut populations to confer blight resistance with the minimum difference in genes. In the 1950s, the Dunstan chestnut was developed in Greensboro, N.C., and constitutes the majority of blight-free chestnuts produced in the United States annually.
Fire blight
In the early 1800s, "E. amylovara" was the first bacterium that could be used in experiments to demonstrate that it did indeed cause disease in plants. It is accepted that this destructive crop bacterium had initially originated in North America. Today, "E. amylovara" can currently be found in all the provinces of Canada, as well as in some parts of the United States of America; states include Alabama, California, Colorado, Connecticut, Georgia, Illinois, Maine, Maryland, Massachusetts, Michigan, New York, North Carolina, Ohio, Oregon, Pennsylvania, Texas, Utah, Virginia, Washington, West Virginia and Wisconsin. Other American countries of its occurrence include but are not limited to Mexico and Bermuda. On the African continent, "E. amylovora" has been confirmed in Egypt.
Verticillium wilt
Verticillium wilt occurs in a broad range of hosts but has similar devastating effects on many of these plants. In general, it reduces the quality and quantity of a crop by causing discoloration in tissues, stunting, and premature defoliation and death. Stock from infested nurseries may be restricted. Once a plant is infected, there is no way to cure it. Verticillium wilt is especially a concern in temperate areas and areas that are irrigated. "Verticllium spp." can naturally occur in forest soils and when these soils are cultivated, the pathogen will infect the crop.
Pseudomonas cannabina
In crucifers, "P. cannabis pv. alisalensis" causes a blight on the leaves (appearing similarly to those caused by "P. syringae" as well as the common fungal disease Brown Leaf Spot) The infection can be diagnosed by the following:
Overwatering and stress increase plant susceptibility.
Beech blight aphid
The aphids themselves are a light bluish color with bodies covered with long, white, waxy filaments giving them a woolly appearance. They first become apparent in July and as populations continue to grow they become increasingly noticeable. Very high numbers can be seen on individual branches, sometimes extending onto leaves. Infested trees may appear to have their branches and twigs covered with snow. This aphid has a defensive behaviour in that it raises the posterior end of its body and sways from side to side when disturbed. Many aphids performing this action at the same time has led to this species being referred to as the "Boogie-Woogie Aphid".
Deposits of sooty mold caused by the fungus "Scorias spongiosa" build up below the colonies growing on the copious amounts of honeydew the insects exude. The aphids do not usually cause much damage to overall tree health, but dieback is occasionally seen on very heavily infested branches. If infestations are heavy, twigs may die, but damage to the tree is usually minor. The aphids can be blasted off with a jet of water or can be controlled with any insecticides labeled for aphids. A wetting agent should be included to help penetrate the waxy body covering of the insect. Horticultural oil sprays and insecticidal soap have also been used successfully. Several parasites are known to attack this aphid and it is thought that they will in time be effective in reducing the population numbers of aphids.
Fire blight
Fire blight, also written fireblight, is a contagious disease affecting apples, pears, and some other members of the family Rosaceae. It is a serious concern to apple and pear producers. Under optimal conditions, it can destroy an entire orchard in a single growing season.
Scirtothrips dorsalis
The characteristic feeding damage of "S. dorsalis" was recognized as "Murda disease" on chili long before the thrips was associated with and then determined to be the cause of the blight. Prolonged feeding by thrips curls tender leaves and buds, and will turn fruits and flowers from bronze to black in color, rendering the plant material unmarketable. Even slightly damaged or scarred vegetables or inflorescences are often viewed as unmarketable, and these damaged commodities will fetch a lower price, reducing a grower's return on their investment. Insects feeding in new growth limits and stunts overall plant growth, and may induce abortion of fruit. When thrips feed in high enough densities, or in sufficiently dry climates, this process results in the eventual desiccation and death of their host plant. Even low densities of thrips can contribute to the decline in fruit production and plant health, especially during times of drought.
American Chestnut Cooperators Foundation
Griffin developed a scale for assessing levels of blight resistance, which made it possible to make selections scientifically. He inoculated five-year-old chestnuts with a standard lethal strain of the blight fungus and measured growth of the cankers. Chestnuts with no resistance to blight make rapid-growing, sunken cankers that are deep and kill tissue right to the wood. Resistant chestnuts make slow-growing, swollen cankers that are superficial: live tissue can be recovered under these cankers. The level of blight resistance is judged by periodic measurement of cankers.
Gummy stem blight
Gummy Stem Blight is a cucurbit-rot disease caused by the fungal plant pathogen "Didymella bryoniae" (anamorph "Phoma cucurbitacearum").
Gummy Stem Blight can affect a host at any stage of growth in its development and affects all parts of the host including leaves, stems and fruits. Symptoms generally consist of circular dark tan lesions that blight the leaf, water soaked leaves, stem cankers, and gummy brown ooze that exudes from cankers, giving it the name Gummy Stem Blight. Gummy Stem Blight reduces yields of edible cucurbits by devastating the vines and leaves and rotting the fruits.
Plant disease resistance
The late blight Irish potato famine of the 1840s was caused by the oomycete Phytophthora infestans. The world’s first mass-cultivated banana cultivar Gros Michel was lost in the 1920s to Panama disease caused by the fungus Fusarium oxysporum. The current wheat stem rust, leaf rust and yellow stripe rust epidemics spreading from East Africa into the Indian subcontinent are caused by rust fungi Puccinia graminis and P. striiformis. Other epidemics include Chestnut blight, as well as recurrent severe plant diseases such as Rice blast, Soybean cyst nematode, Citrus canker.
Xanthomonas arboricola
"Xanthomonas arboricola" has an extraordinarily destructive potential when infecting crops. It is the most devastating blight on walnut, and can cause up to 100% yield loss if not properly managed . Its host range threatens stone fruits and other nuts. Epidemics have been reported in countries such as The United States, Iran, Turkey, and Italy. These outbreaks are not limited to just these countries, and without extensive epidemiological knowledge, it could spread with devastating effects. Through disease forecasting efforts, "X. arboricola" is known to have its highest virulence and growth rate at around 30 degrees Celsius. Temperature in addition to understanding regional humidity, could help prevent future epidemics in agriculture. Less moisture on the plants, and slightly cooler temperatures can hinder both the dispersal and growth of "X. arboricola". In Australia an estimated 3.1 million AUD were lost on average due to yield loses from "prunus" "spp." in years with pathogen prevalence. The pathogen’s capability to survive in woody tissue over winter, or after plant death proves a major challenge for plant pathologists to find an effective solution.
Quince
While quince is a hardy shrub, it may develop fungal diseases in hot weather, resulting in premature leaf fall. Quince leaf blight, caused by fungus "Diplocarpon mespili", presents a threat in wet summers, causing severe leaf spotting and early defoliation, also affecting fruit to a lesser extent. It may also affect other Rosaceae plants such as hawthorn and medlar, but is typically less damaging than on quince. Cedar-quince rust, caused by "Gymnosporangium clavipes", requires two hosts to complete the fungal lifecycle, one being a cedar (most commonly a juniper, "Juniperus virginiana") and the other a rosacea. Appearing as red excrescence on various parts of the plant, it may affect quinces grown in vicinity of junipers.
Morphological symptoms of plant diseases
Blights are characterized by the rapid death of entire leaves including the veins or parts of the leaves. Blights also could occur on flowers and stems.
Botryotinia
Plant diseases caused by "Botryotinia" species appear primarily as blossom blights and fruit rots but also as leaf spots and bulb rots in the field and in stored products. The fungi induce host cell death resulting in progressive decay of infected plant tissue, whence they take nutrients. Sexual reproduction takes place with ascospores produced in apothecia, conidia are the means of asexual reproduction. Sclerotia of plano-convexoid shape are typical. Some species also cause damping off, killing seeds or seedlings during or before germination.
Twig blight
A twig blight is a small, evil plant creature that looks like a woody shrub when rooted in soil. A twig blight has a thirst for blood, and when it moves about its leafless branches interlock to create a humanoid shape. A twig blight attacks with its claws that deliver a strength-draining poison.
Bilberry
Bilberry plants can suffer from bilberry blight, caused by "Phytophthora kernoviae". There have been severe outbreaks in Staffordshire, England.
Chestnut blight
American chinquapin is also highly susceptible to chestnut blight. The European chestnut and the West Asian species are also susceptible, but less so than the American species. Surviving chestnut trees are being bred for resistance to the blight, notably by The American Chestnut Foundation, which aims to reintroduce a blight-resistant American chestnut to its original forest range within the early decades of the 21st century. The resistant species—particularly Japanese chestnut and Chinese chestnut, as well as Seguin's chestnut and Henry's chestnut—have been used in these breeding programs in the US to create disease-resistant hybrids with the American chestnut. It's important to realize, though, that even Chinese chestnut trees vary considerably in blight resistance. Some individuals are quite susceptible while others are essentially immune to the disease. Many kinds of environmental stress may break down a tree's resistance to blight. Indeed, at higher elevations in areas exposed to severe climate, normally resistant, Oriental chestnuts have been killed by blight. The fungus will also infect other tree species such as oaks, red maples, staghorn sumacs, and shagbark hickories. Once infected, these trees will also exhibit orange bark with cankers. However, they will not exhibit shoot die back and death of the main tree. Instead the pathogen will be able to persist in trees, causing the fungus to be able to attack new growth in American chestnut trees.
Monilinia laxa
"Monilinia laxa" is an ascomycete fungus that is responsible for the brown rot blossom blight disease that infects many different types of stone fruit trees, such as apricots, cherries and peaches. It can also occasionally affect some pome fruits; for example, apples and pears. The pathogen overwinters on infected plant parts, particularly on infected twigs, branches, old flower parts or mummified fruits. In spring, the pathogen produces asexual conidia on the aforementioned infected plant debris. In addition, apothecia, which are small, open cup, mushroom-like sexual propagative structures of "M. laxa" that produce ascospores, also develop on the fallen fruits on the ground. Both asexual (conidia) and sexual (ascospores) spore types are spread during the spring via wind and rain in which they infect blossoms and young shoots. Floral tissue is the most susceptible to both spores’ infection when the trees are in full bloom. The infected floral tissues are responsible for the production of the secondary inoculums that further continues the disease cycle during the spring season. If the environmental conditions are very conducive (i.e. warm and wet environments), infection can also occur in non-flowering shoots or leaves. Infection is sometimes not visible until after the fruit begins to ripen and the pit hardens. These ripe fruits are at a high risk of being infected and passing the disease onto other plants during harvest.
Tomato
Various forms of mildew and blight are common tomato afflictions, which is why tomato cultivars are often marked with a combination of letters that refer to specific disease resistance. The most common letters are: LB – "late blight", V – "verticillium" wilt, F – "fusarium" wilt strain I, FF – "fusarium" wilt strain I and II, N – "nematodes", T – "tobacco mosaic virus", and A – "alternaria".
Leo Laliman
This blight was brought on by a species of aphid that originated in North America and was carried across the Atlantic Ocean sometime in the late 1850s/early 1860s. However, how the phyolloxera had survived the journey remained a point of much debate; Europeans had experimented with American vines for centuries without any pestilential problems. Eventually, it was decided that, following the invention of steamboats, the grape phylloxera were able to survive the shortened journey.
The first instance of the blight was recorded sometime in the early 1860s, and France suffered the blight for a 15-year period, without any solution. Eventually, Jules-Emile Planchon and two colleagues made an important discovery; the discovery that sourced the aphid phylloxera to the blight itself — up until then, the source of the damage was unknown. In 1870, American entomologist Charles Valentine Riley confirmed Planchon's theory. However, this discovery caused controversy; some met it with optimism, saying that now that the cause had been found, it would just be a matter of elimination. Others disagreed completely with the theory, saying that the grape phylloxera were merely a symptom, an effect, of the blight, rather than the source.
Physiological plant disorder
Frost and cold are major causes of crop damage to tender plants, although hardy plants can also suffer if new growth is exposed to a hard frost following a period of warm weather. Symptoms will often appear overnight, affecting many types of plants. Leaves and stems may turn black, and buds and flowers may be discoloured, and frosted blooms may not produce fruit. Many annual plants, or plants grown in frost free areas, can suffer from damage when the air temperature drops below 40 degrees Fahrenheit (4 degrees Celsius). Tropical plants may begin to experience cold damage when the temperature is 42 to 48 °F (5 to 9 °C), symptoms include wilting of the top of the stems and/or leaves, and blackening or softening of the plant tissue.
Blumeriella
Since 2014, the Royal Horticultural Society in the UK has received reports from gardeners and horticulturalists of damage to plants of the shrub "Kerria japonica". Symptoms include multiple red spots on leaves and lesions on the stems, resulting in defoliation and eventual death. It has been determined that this infection is caused by the fungus "Blumeriella kerriae", which is specific to the "Kerria". This highly contagious disease, known in English as kerria twig and leaf blight, was known in the U.S. but had not previously been observed on British plants. It is regarded as a serious threat to the cultivated kerria plants, which are popular garden shrubs.
American chestnut
Backcrossing as a treatment for blight was first proposed by Dr. Charles R. Burnham of the University of Minnesota in the 1970s. Dr. Burnham, a Professor Emeritus in agronomy and plant genetics who was considered one of the pioneers of maize genetics, realized that experiments conducted by the USDA to cross-breed American chestnuts with European and Asian chestnuts erroneously assumed that a large number of genes were responsible for blight resistance, while it is currently believed the number of responsible genes is low. The USDA abandoned their cross-breeding program and destroyed local plantings around 1960 after failing to produce a blight-resistant hybrid. Burnham's recognition of the USDA's error led to him joining with others to create The American Chestnut Foundation in 1983, with the sole purpose of breeding a blight-resistant American chestnut. The American Chestnut Foundation is backcrossing blight-resistant Chinese chestnut into American chestnut trees, to recover the American growth characteristics and genetic makeup, and then finally intercrossing the advanced backcross generations to eliminate genes for susceptibility to blight. The first backcrossed American chestnut tree, called "Clapper", survived blight for 25 years, and grafts of the tree have been used by The American Chestnut Foundation since 1983. The Pennsylvania chapter of The American Chestnut Foundation, which seeks to restore the American chestnut to the forests of the Mid-Atlantic states, has planted over 22,000 trees.
Buckeye rot of tomato
Buckeye rot of tomato affects tomato plants, although there are many other plants that are affected, including pepper, potato, tobacco, pineapple, and various other hosts. Upon infection, brown and greyish-green water soaked spots appear on the fruit. These spots develop into lesions that resemble the markings of a bull's eye with alternating dark and light brown concentric rings. These are very smooth lesions, barely raised, unlike the rough lesions found in late blight "(Phytophthora infestans)", which is a similar pathogen. Finally, a white fungal-like growth may appear within the lesion. While fruit rot is the most common symptom, damping off and stem cankers are possible. The disease does not affect the foliage of the plant, which further helps separate it from late blight .
Phytophthora infestans
Blight can be controlled by limiting the source of inoculum. Only good-quality seed potatoes and tomatoes obtained from certified suppliers should be planted. Often discarded potatoes from the previous season and self-sown tubers can act as sources of inoculum.
Alternaria dauci
Symptoms of "A. dauci" appear first as greenish-brown, then water-soaked, and finally necrotic lesions 8–10 days following an infection event. These lesions will appear on carrot leaflets and petioles, and have a characteristic chlorotic, yellow halo. The lesions can be irregularly shaped, and will often appear on older leaves first. Older leaves are the most susceptible to infection; when approximately 40% of the leaf surface area has become infected by "Alternaria dauci", the leaf will completely yellow, collapse, and die. It is during extended conditions of warm, moist weather that lesions can coalesce and cause entire tops of carrot plants to die off, a phenomenon that is sometimes mistaken for frost damage. The symptoms of this disease are also commonly confused with Cercospora Leaf Blight of carrots as well as bacterial blight, and microscopic analysis is frequently needed to accurately diagnose the pathogen. "A. dauci" produces characteristically dark to olive-brown hyphae and elongated conidiophores, with conidia typically borne singly. Petiole infection can also occur without any lesion development on leaflets, and "A. dauci" can additionally result in damping-off of seedlings, seed stalk blight, and inflorescence infections. These symptoms can significantly reduce yield due to lost photosynthetic activity, prevention of mechanical harvest, and infection of commercial carrot seeds.
Velvet blight
Velvet Blight is a disease that affects the stems, branches, leaves, fruits or trunks of plants and trees. This disease is primarily caused by three fungal species from the genus "Septobasidium": "S. bogoriense", "S. pilosum" and "S. theae".
Bacterial blight of soybean
"Pseudomonas syringae pv. glycinea" overwinters in crop residue and in seeds. Infection usually begins when the infected plant material is carried by a rainstorm with high winds to healthy soybean plants. Bacteria can enter through natural openings (stomata) or through wounds. In order to enter through natural openings, however, water must be present on the leaf surface. Bacterial Blight may appear after infected seedlings are planted and begin to emerge. In addition to being spread via wind and rain, transmission may occur when leaves of infected soybean rub against leaves of healthy soybean. Symptoms begin to appear 5–7 days after infection.
Papaya
The fungus phytophthora blight causes damping-off, root rot, stem rot, stem girdling, and fruit rot. Damping-off happens in young plants by wilting and death. The spots on established plants start out as white, water-soaked lesions at the fruit and branch scars. These spots enlarge and eventually cause death. The most dangerous feature of the disease is the infection of the fruit which may be toxic to consumers. The roots can also be severely and rapidly infected, causing the plant to brown and wilt away, collapsing within days.
Sphaeropsis blight
Sphaeropsis blight, also known as Diplodia tip blight, is a wide spread disease caused by an opportunistic fungal pathogen, "Diplodia sapinea".  It is found in “both hemispheres between the latitudes 30° and 50° north and south". The diseases symptoms include: damping off and collar rot of seedlings, stem canker, rood disease, and, most commonly, shoot blight.  These symptoms have caused significant economic loss to nurseries and pine plantations.  In a nursery in the north-central United States, losses of 35% have been reported.  Shoot blight and eventual die back can cause a reduction of marketable volume in timber by 63%.  Infection of terminal shoots can result in dead-top which significantly limits the usable length of the tree trunk. The presence of the  pathogen in concert with severe weather conditions can lead to extreme loss. Following a severe hailstorm in South Africa, nearly 5,000 acres of pine plantation were infected with Diplodia tip blight.  It was necessary to prematurely harvest large swaths of the plantations resulting in a loss of 45%.  Areas that were not harvested prematurely still suffered an average timber loss of 11%.
Fire blight
The pathogen spreads through the tree from the point of infection via the plant's vascular system, eventually reaching the roots and/or graft junction of the plant. Once the plant's roots are affected, the death of the plant often results. Over pruning and over fertilization (especially with nitrogen) can lead to watersprout and other midsummer growth that leave the tree more susceptible.
Spruce
The closely related species "Sirococcus conigenus" and "S. piceicola" cause shoot blight and seedling mortality of conifers in North America, Europe, and North Africa. Twig blight damage to seedlings of white and red spruces in a nursery near Asheville, North Carolina, was reported by Graves (1914). Hosts include white, black, Engelmann, Norway, and red spruces, although they are not the plants most commonly damaged. "Sirococcus" blight of spruces in nurseries show up randomly in seedlings to which the fungus was transmitted in infested seed. First-year seedlings are often killed, and larger plants may become too deformed for planting. Outbreaks involving < 30% of spruce seedlings in seedbeds have been traced to seed lots in which only 0.1% to 3% of seeds were infested. Seed infestation has in turn been traced to the colonization of spruce cones by "S. conigenus" in forests of the western interior. Infection develops readily if conidia are deposited on succulent plant parts that remain wet for at least 24 hours at 10 °C to 25 °C. Longer periods of wetness favour increasingly severe disease. Twig tips killed during growth the previous year shows a characteristic crook.
Phomopsis blight of juniper
Phomopsis blight of juniper is a foliar disease discovered in 1917 caused by the fungal pathogen "Phomopsis juniperovora". The fungus infects new growth of juniper trees or shrubs, i.e. the seedlings or young shoots of mature trees. Infection begins with the germination of asexual conidia, borne from pycnidia, on susceptible tissue, the mycelia gradually move inwards down the branch, and into the main stem. Management strategies mainly include removing and destroying diseased tissue and limiting the presence of moisture on plants. Junipers become resistant to infection as they mature and the young yellow shoots turn dark green. Preventative strategies include planting only resistant varieties and spraying new growth with fungicide until plants have matured.
Fire blight
The causal pathogen is Erwinia amylovora, a Gram-negative bacterium in the family Enterobacteriaceae. Pears are the most susceptible, but apples, loquat, crabapples, quinces, hawthorn, cotoneaster, "Pyracantha", raspberry and some other rosaceous plants are also vulnerable. The disease is believed to be indigenous to North America, from where it spread to most of the rest of the world.
American chestnut
Once an important hardwood timber tree, the American chestnut suffered a substantial population collapse due to the chestnut blight, a disease caused by an Asian bark fungus ("Cryphonectria parasitica", formerly "Endothia parasitica"). This disease was accidentally introduced into North America on imported Asiatic chestnut trees. Chestnut blight was first noticed on American chestnut trees in what was then the New York Zoological Park, now known as the Bronx Zoo, in the borough of The Bronx, New York City, in 1904, by chief forester Hermann Merkel. Merkel estimated that by 1906 blight had infected 98 percent of the chestnut trees in the borough. While Chinese chestnut evolved with the blight and developed a strong resistance, the American chestnut had little resistance. The airborne bark fungus spread a year and in a few decades girdled and killed up to three billion American chestnut trees. Salvage logging during the early years of the blight may have unwittingly destroyed trees which had high levels of resistance to this disease and thus aggravated the calamity. New shoots often sprout from the roots when the main stem dies, so the species has not yet become extinct. However, the stump sprouts rarely reach more than in height before blight infection returns.
Phytophthora infestans
Potato and tomato varieties vary in their susceptibility to blight. Most early varieties are very vulnerable; they should be planted early so that the crop matures before blight starts (usually in July in the Northern Hemisphere). Many old crop varieties, such as King Edward potato are also very susceptible but are grown because they are wanted commercially. Maincrop varieties which are very slow to develop blight include Cara, Stirling, Teena, Torridon, Remarka, and Romano. Some so-called resistant varieties can resist some strains of blight and not others, so their performance may vary depending on which are around. These crops have had polygenic resistance bred into them, and are known as "field resistant". New varieties such as Sarpo Mira and Sarpo Axona show great resistance to blight even in areas of heavy infestation. Defender is an American cultivar whose parentage includes Ranger Russet and Polish potatoes resistant to late blight. It is a long white-skinned cultivar with both foliar and tuber resistance to late blight. Defender was released in 2004.
Chestnut blight
The root collar and root system of the chestnut tree have some resistance to blight infection due to soil organisms adversely reacting to the fungus; consequently, a large number of small American chestnut trees still exist as shoots growing from existing root bases. However, these regrown shoots seldom reach the sexually reproductive stage before being killed by the fungus. They only survive as living stumps, or "stools," with only a few growing enough shoots to produce seeds. This is just enough to preserve the genetic material necessary to engineer an American chestnut tree using genes from any of the disease-immune Asiatic species to confer resistance to the disease.
Taro leaf blight
Symptoms on leaves initially occur where water droplets accumulate and eventually form small, brown spots surrounded by halos on the upper surface of leaves. These spots expand very quickly and form large brown lesions. The entire leaf can be destroyed within a few days of the initial appearance of symptoms under wet conditions. The undersides of leaves have spots that look water-soaked or gray, and as they expand, blight forms and the leaf is destroyed within a few days. Symptoms occur in a day/night pattern where water soaked areas expand during the night and then dry out during the day. As a result, additional water marks form leading to increasingly larger lesions. As the lesions expand, sporangia develop most actively at the margin of the lesion and progress to attack healthy tissue.
Blight
Blights are often named after their causative agent, for example "Colletotrichum" blight is named after the fungi "Colletotrichum capsici", and "Phytophthora" blight is named after the water mold "Phytophthora parasitica".
Wheat diseases
Wheat is subject to more diseases than other grains, and, in some seasons, especially in wet ones, heavier losses are sustained from those diseases than are in other cereal crops. Wheat may suffer from the attack of insects at the root; from blight, which primarily affects the leaf or straw, and ultimately deprives the grain of sufficient nourishment; from mildew on the ear; and from gum of different shades, which lodges on the chaff or cups in which the grain is deposited.
Botrytis squamosa
Botrytis squamosa (teleomorph: "Botryotinia squamosa") is a fungus that causes leaf blight on onion (often termed ‘blast’) that is distinctly characterized by the two stages – leaf spotting followed by blighting. The pathogen is an ascomycete that belongs to the family "Sclerotiniaceae" in the order "Helotiales". The lesions start out as whitish streaks and take on a yellow tinge as they mature. They cause yield losses up to 30%. This fungus is endemic to the USA and has also been reported in Europe, Asia, and Australia. Typical management of this disease includes chemical fungicides with significant efforts being made to establish a means of biological control.
Plant disease epidemiology
Disease epidemics in plants can cause huge losses in yield of crops as well threatening to wipe out an entire species such as was the case with Dutch Elm Disease and could occur with Sudden Oak Death. An epidemic of potato late blight, caused by "Phytophthora infestans", led to the Great Irish Famine and the loss of many lives.
Fungicide use in the United States
Leaf blight of almonds was first discovered in 1950, and by 1983 it had spread throughout the Sacramento Valley. Almond leaf blight is characterized by the death of leaves throughout the summer. The fungus interferes with water conduction in the leaf. In fall and winter, dormant buds are killed by an extension of the lesion and in the spring, flowers are killed. Repeated attacks of leaf blight, though rarely killing more than 20% of the leaves, ultimately reduce tree vigor. Experiments have shown that the use of ziram and captan can reduce the incidence of leaf blight by 75–80% in treated trees.
Chestnut blight
The chestnut blight is a fungal infection affecting the American chestnut tree that had a devastating economic and social impact on communities in the eastern United States. It later spread to other parts of the world including Italy. The fungus is spread by wind-borne ascospores and, over a shorter distance, conidia distributed by rain-splash action. In the first half of the 20th century it killed an estimated 4 billion trees. Infection is local in range, so some isolated American chestnuts survive where there is no other tree within 10 km. There are at least two viral pathogens that weaken the fungus through a mechanism termed "hypovirulence" that helps trees survive.
Notobubon galbanum
The surface of the plant is covered in a mix of chemicals including psoralen, xanthotoxin and bergapten that causes a phototoxic reaction resulting in blistering two or three days after exposure. The moment of exposure is innocuous, without any untoward sensations to the unwary. Exposure of the affected skin to ultraviolet light, such as contained in sunlight, triggers the effects of the plant's toxins, leading to severe itching and blistering. The welts and resulting blisters can be as small as the size of a coin to covering as much exposed skin as came into contact with the plant. Where more than 5% of the body is afflicted, the scarring can be a serious matter. Washing the affected area immediately after exposure may help but preventing any further exposure to ultraviolet light such as the sun or many artificial light sources will reduce and /or eliminate blistering.
Monilinia fructicola
Brown rot causes blossom blight, twig blight; twig canker and fruit rot. Brown rot is caused by a fungus that produces spores, and can be a major problem during particularly wet seasons. Prolonged wet weather during bloom may result in extensive blossom infection. The length of wet periods required for blossom infection depends upon the temperature. Humid wet conditions are when the fruit trees are most at risk from infection. Young green fruit can be infected just before autumn, but the infection often remains inactive until near maturity of the fruit. Brown rot can spread after harvest. Mature fruit can decay in only 2 days under warm conditions.
Chestnut blight
The pathogenic fungus Cryphonectria parasitica (formerly "Endothia parasitica") is a member of the Ascomycota (sac fungi) taxon. It is the main cause of chestnut blight, a devastating disease of the American chestnut tree that in the early 1900s caused a rapid, widespread die-off of the once plentiful tree from its historic range, in the Eastern United States.
Stem rust
The stem rust fungus attacks the parts of the plant that are above ground. Spores that land on green wheat plants form a pustule that invades the outer layers of the stalk. Infected plants produce fewer tillers and set fewer seed, and in cases of severe infection the plant may die. Infection can reduce what is an apparently healthy crop about three weeks before harvest into a black tangle of broken stems and shriveled grains by harvest.
Phloem
Because phloem tubes are located outside the xylem in most plants, a tree or other plant can be killed by stripping away the bark in a ring on the trunk or stem. With the phloem destroyed, nutrients cannot reach the roots, and the tree/plant will die. Trees located in areas with animals such as beavers are vulnerable since beavers chew off the bark at a fairly precise height. This process is known as girdling, and can be used for agricultural purposes. For example, enormous fruits and vegetables seen at fairs and carnivals are produced via girdling. A farmer would place a girdle at the base of a large branch, and remove all but one fruit/vegetable from that branch. Thus, all the sugars manufactured by leaves on that branch have no sinks to go to but the one fruit/vegetable, which thus expands to many times its normal size.