Effect of Plant Age and Acibenzolar S methyl on Development of Downy Mildew of Basil
Sweet basil (Ocimum basilicum) is an important annual culinary herb grown in the United States. Recently, basil production was drastically affected by downy mildew caused by Peronospora belbahrii, a recently discovered foliar disease of basil in Homestead, FL. The disease has spread to more than 30 states in the United States causing significant losses to basil growers. As a result of the recent emergence of the disease, limited management tools are available for control of downy mildew, and it is critical for growers to apply management measures at appropriate times. This study was designed to evaluate 2to 7 week old basil plants for their susceptibility to downy mildew. Another objective of the study was to evaluate the effect of a pre inoculation application of acibenzolar S methyl (ASM) for control of downy mildew. The results suggested that 2to 3 week old basil was more susceptible to downy mildew than 4to 7 week old plants. The area under the disease progress curve (AUDPC) was smaller for 5to 7 week old ASM treated basil plants than for 2to 4 week old ASM treated basil plants. This study indicated that 2to 3 week old basil plants need to be protected, and ASM should be applied before pathogen infection on 5to 7 week old plants to reduce downy mildew to a greater extent. Sweet basil (Ocimum basilicum L. Lamiales: Lamiaceae) is commercially the most important culinary herbal crop grown in the United States. Basil, a member of the mint family, is native to central Asia and northwest India and is grown for both fresh and dry consumption as well as used for manufacturing perfumes, food flavors, and aromatherapy products (Simon et al. 1990) The product quality of basil is ensured by appearance (color and absence of disease and insect damage) flavor, and moisture content for the fresh market and volatile oil content for the processing market. Downy mildew of basil, caused by the oomycete Peronospora belbahrii Thines (Peronosporales: Peronosporaceae) (Thines et al. 2009) has become the most devastating disease of basil in the United States. Basil downy mildew was first discovered in Uganda in 1933 (Hansford, 1933) Until its first outbreak in the United States in 2007 (Roberts et al. 2009) basil downy mildew had been reported from Europe (Garibaldi et al. 2004, 2005) the Middle East (Khateri et al. 2007) and Africa (McLeod et al. 2006) Since its first discovery in Florida, P. belbahrii has been spreading to other basil growing regions in the United States and making commercially produced basil unsuitable for the fresh market. Presently, downy mildew of basil has occurred in more than 30 U.S. states including all eastern states, the Midwest region, California, and even the Hawaiian islands. In 2013, some growers who had produced basil in Florida for many years ceased this line of operation because of disastrous crop losses from downy mildew (Vern Meyer, personal communication) P. belbahrii is believed to be seed borne (Garibaldi, 2004) and spread by wind borne sporangia and movement of seeds, infected plants, and cuttings. Initial symptoms of basil downy mildew on infected basil resemble a nutrient disorder such as nitrogen deficiency. Infected leaves of basil plants become chlorotic mainly between the veins, and like other downy mildew pathogens, P. belbahrii typically sporulates on the abaxial surface of infected leaves. In mass, sporulation is grayish to black and has a dirty appearance. In the case of severe infection, leaves may show necrotic lesions and finally defoliation of foliage. The most commonly grown sweet basil (O. basilicum) is substantially more susceptible to downy mildew than other cultivars such as red leaf, lemon, and blue spice basil (Wyenandt et al. 2010) It appears that disease severity of basil downy mildew is strongly associated with the age of the plant, environmental conditions, and cultivar susceptibility. Cool, wet, and humid weather, which are characteristics of the climate of Florida, favor infection by P. belbahrii, and blowing wind and water splashing help spread sporangia from infected to healthy plants. Moreover, there are very few fungicides and biologicals registered for the use on basil for control of downy mildew (Homa et al. 2014) The limited disease management tools, favorable weather conditions in south Florida, and lack of resistant cultivars make basil production very vulnerable to downy mildew. Downy mildew has the potential to destroy basil production as has already occurred in many regions of the United States where basil has been grown. The use of resistant cultivars is the most economical means to manage crop diseases. Because downy mildew resistant basil cultivars are not commercially available, it is important to determine if the susceptibility to downy mildew of available basil cultivars varies at the different stages of plant growth. Determining the susceptibility stage of the basil will help to understand the epidemiology of downy mildew of basil and to apply registered fungicides at an appropriate stage of the basil to prevent or slow down the advancement of this disease. However, little information is currently available on susceptibility of basil at different growth stages to downy mildew infection. The primary objective of this study was to evaluate 2to 7 week old basil plants for their susceptibility to downy mildew. In addition, we evaluated the effect of pre inoculation application of ASM for control of downy mildew on 2to 7 week old basil. Materials and Methods Plant materials. Seeds of basil cv. Genovese (Eden Brothers, Dohlonega, GA) were planted in 10 cm diameter plastic pots containing Fafard growing mix (Fafard Inc. Agawam, MA) Plants were supplemented with Miracle Gro 18 18 21 (Miracle Gro Lawn Products, Inc. Marysville, OH) as required. The seeds were covered with a 5 mm deep layer of Fafard super fine germinating mix (Fafard Inc. Basil was seeded each week for 6 consecutive weeks in a nursery greenhouse before ASM (Actigard 50 WG; Syngenta Crop Protection LLC, Greensboro, NC) treatment and pathogen inoculation. Plants were regularly watered four times a day each for 5 min using an automated overhead irrigation system. ASM treatment and pathogen inoculation. One week after the last seeding, ASM at 50 mg*L was sprayed uniformly on basil in nine pots (as nine replications) for each seeding and water was sprayed on plants in other nine pots in the same seeding as the non treated control. Each pot contained four basil plants. A week after ASM treatment, basil plants were inoculated with P. belbahrii by spraying the suspensions at 2 10 sporangia/mL onto both ASM treated and untreated basil plants until runoff. The inoculum was prepared by washing off freshly Received for publication 4 June 2014. Accepted for publication 8 Sept. 2014. This research was supported by the U.S. Department of Agriculture Specialty Crop Research Initiative program (project number 2011 51181 30646) Current address: Department de Ciencias Agrarias, Unimontes, Campus de Janauba, Caixa postal 91, Rua Reinaldo Viana, 2630, CEP 39440 000, Janauba MG, Brazil; Sponsored by CNPq. To whom reprint requests should be addressed; e mail szhang0007@ufl.edu. 1392 HORTSCIENCE VOL. 49(11) NOVEMBER 2014 produced sporangia from underside of basil leaves into distilled water, filtering through four layers of cheesecloth, and determining the sporangial concentration using a hemocytometer under a microscope. Inoculated plants were maintained in a humidity chamber with greater than 85% humidity and 23 3 C in an air conditioned greenhouse for at least 24 h before they were moved to benches. At the time of inoculation, the age of the basil plants ranged from 2 to 7 weeks after seeding. The plants were watered three times a day each for 5 min by overhead irrigation using 360 NN GR Nifty nozzles (Dramm Corporation, Manitowoc, WI) The temperature in the greenhouse was set to 23 C; however, temperature varies 5 C) each day depending on the ambient solar radiation. The experiment was conducted two times. Data collection and statistical analysis. Disease severity was assessed as the percentage of symptomatic leaf area (sign and symptoms) Disease severity of basil downy mildew was rated 9, 13, 16, and 18 d after inoculation (DAI) in Expt. 1 and 10, 12, 15, and 17 DAI in Expt. 2. Disease severity was rated as percentage of leaves showing downy mildew symptoms and signs of the pathogen. The AUDPC was calculated based on the four ratings of disease severity for each seeding using the formula: Pn 1 i Yi Yi 1 d Th 2 h i Ti 1 d Th Ti 1/2 where Yi disease severity at i observation and Ti days at i observation. To determine the effect of ASM on growth of the plant, the number of fully expanded leaves was counted from each of four plants per replication regardless of downy mildew infection in Expts. 1 and 2 on 13 and 17 DAI, respectively. Similarly, plant height was measured from four plants per replication in Expt. 2 on 17 DAI. Data were analyzed using GLM procedure in SAS 9.3 software (SAS Institute Inc. Cary, NC) Means of treatments were separated by Fisher's protected least significant difference test at P 0.05.