Cotton Root Rot of Sweet Potatoes
Phymatotrichum root rot, also called phymatotrichum cotton root rot, cotton root rot, Texas root rot or ozonium root rot, is a highly destructive fungal disease caused by the fungal pathogen Phymatotrichum omnivorous. This fungal disease affects more than 2,000 species of plants, with sweet potatoes being particularly susceptible. Monocots, or grass plants, are resistant to this disease. Sweet potato phymatotrichum root rot thrives in the chalky, clay soil of the Southwestern United States and Mexico, where summer soil temperatures consistently reach 82 F. (28 C.) and there is no killing winter freezes. In crop fields, symptoms may appear as patches of chlorotic sweet potato plants. Upon closer inspection, the plants’ foliage will have a yellow or bronze discoloration. Wilting will begin in the upper leaves but continue down the plant; however, leaves do not drop. Sudden death can occur very rapidly after symptoms appear. By this point, the underground tubers, or sweet potatoes, will be severely infected and rotted. Sweet potatoes will have dark sunken lesions, covered with wooly fungal strands of mycelium. If you dug up a plant, you would see the fuzzy, white to tan mold. This mycelium is what persists in soil and infects roots of susceptible plants like cotton, nut and shade trees, ornamental plants and other food crops.
Treating Sweet Potato Phymatotrichum Root Rot
Without freezing winter temperatures in the Southwest, sweet potato phymatotrichum root rot overwinters as fungal hyphae or sclerotia in the soil. The fungus is most common on calcareous soil where pH is high and summer temperatures soar. As temperatures rise with the arrival of summer, fungal spores form on the soil surface and spread this disease. Root rot of sweet potatoes can also spread from plant to plant beneath the soil, and its fungal strands have been found to spread as deep as 8 feet (2 m.). In crop fields, infected patches may reoccur year after year and spread up to 30 feet (9 m.) per year. The mycelium spreads from root to root and persists in the soil on even minute pieces of sweet potato root. Fungicides and soil fumigation are ineffective in treating phymatotrichum root rot on sweet potatoes. A 3- to 4-year crop rotation with resistant grass plants or green manure crops, such as sorghum, wheat or oats, is often implemented to inhibit the spread of this disease. Deep tillage can also disrupt spread of fuzzy fungal mycelium beneath the soil. Farmers also use early maturing varieties and apply nitrogen fertilizer in the form of ammonia to combat sweet potato cotton root rot. Soil amendments to improve the clay, chalky texture of sweet potato fields can help prevent this disease, as can lowering the pH.
