Cactus
Fungi, bacteria and viruses attack cacti, the first two particularly when plants are over-watered. Fusarium rot can gain entry through a wound and cause rotting accompanied by red-violet mold. ""Helminosporium" rot" is caused by "Bipolaris cactivora" ( "Helminosporium cactivorum"); "Phytophthora" species also cause similar rotting in cacti. Fungicides may be of limited value in combating these diseases. Several viruses have been found in cacti, including cactus virus X. These appear to cause only limited visible symptoms, such as chlorotic (pale green) spots and mosaic effects (streaks and patches of paler color). However, in an "Agave" species, cactus virus X has been shown to reduce growth, particularly when the roots are dry. There are no treatments for virus diseases.
St. Johns River
Wetlands in the upper and middle basin are fed by rainwater, trapped by the structure of the surrounding land. It is an oxygen- and nutrient-poor environment; what grows usually does so in peat which is created by centuries of decaying plant material. Water levels fluctuate with the subtropical wet and dry seasons. Rain in central and north Florida occurs seasonally during summer and winter, but farther south rain in winter is rare. All plants in these basins must tolerate water fluctuation, both flooding and drought. Sweetbay ("Magnolia virginiana"), cypress ("Taxodium"), and swamp tupelo ("Nyssa biflora") trees often find great success in this region on raised land called hammocks. Trees that live in water for long periods usually have buttressed trunks, tangled, braided roots, or protrusions like cypress knees to obtain oxygen when under water, but the majority of plant life is aquatic. Wetland staples include the American white waterlily ("Nymphaea odorata"), pitcher plants, and Virginia iris ("Iris virginica"). In the southernmost points of the river, "Cladium", or sawgrass, grows in vast swaths of wet prairie that at one time extended into the Everglades. These wetland flora are remarkably successful in filtering pollutants that otherwise find their way into the river.