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PLANT DISEASES. Plant diseases significantly limit the production of commercial crops and landscape plants in Texas. Here as elsewhere biotic agents, including fungi, bacteria, nematodes, viruses, mycoplasmas, and parasitic plants, cause diseases that alter the appearance, function, and value of plants. Other plant disorders such as nutrient deficiency or toxicity, pollution injury, and moisture imbalance are caused by abiotic factors. Most Texas crops are threatened by ten to twenty major diseases. The likelihood of disease occurring in a given planting depends on the presence of the causal organism, susceptibility of the host, and favorable weather conditions. Control includes resistant cultivators, crop rotation, land preparation procedures, and chemical applications to seed, soil, or foliage. The most damaging diseases of the seven most economically important Texas field crops are Phymatotrichum root rot and Verticillium wilt or cotton head smut, maize dwarf mosaic and stalk rot on grain sorghum, maize dwarf mosaic and stalk rot on corn, sheath blight on rice, pod and stem blight and anthracnose of soybeans, leaf spot and southern blight on peanuts, and leaf rust and Septoria disease of wheat. Vegetables, fruit and nut crops, and commercially grown ornamentals often experience serious loss.
Cotton root rot, or Phymatotrichum root rot, is caused by the fungus Phymatotrichum omnivorum. This soilborne fungus affects some 2,000 species of plants and occurs commonly in the calcareous blackland soils that run from the Dallas area to the lower Rio Grande valley. Host plants affected by the fungus die rapidly during the summer months, when the organism is most active. Cotton is the most affected commercial field crop, but Phymatotrichum root rot also affects peaches, grapes, roses, and apples and limits production of crops such as alfalfa. Losses can be reduced by cultural control procedures.
Another common disease kills thousands of live oaks in Texas annually. The oak wilt fungus (Ceratocystis fagacearum) occurs throughout Central and South Texas, where the live oak is a dominant tree that decorates the landscape. The organism is moved short distances through root grafts, and its spores are carried long distances by wood-boring insects. Control is by cultural means, in this case by the removal of the fungus.
Farmers and homeowners receive assistance in disease control from a variety of public and private sources. The most commonly available assistance is from county extension agents, who are supported by professional scientists called extension subject-matter specialists. This service is provided by the Texas Agricultural Extension Service and funded by counties, the state of Texas, and the United States Department of Agriculture. New information is made available by agencies such as the Texas Agricultural Experiment Station and the United States Department of Agriculture.
BIBLIOGRAPHY:George Agrios, Plant Pathology (New York: Academic Press, 1969; 2d ed. 1978; 3d ed. 1988). C. Wendell Horne et al., Texas Plant Disease Handbook (College Station: Texas Agricultural Extension Service, 1983). Jerral D. Johnson and David N. Appel, Major Oak Diseases and Their Control (College Station: Texas Agricultural Extension Service, 1984).
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