Sweet Potato Cotton Root Rot: What Causes Texas Root Rot In Sweet Potatoes

Sweet Potato Cotton Root Rot: What Causes Texas Root Rot In Sweet Potatoes

By: Bonnie L. Grant, Certified Urban Agriculturist

Fungal diseases are extremely common and can affect a plant’s roots, stems, foliage, flowers and fruit. Sweet potato Texas root rot is a disease which goes under several scientific names, but one thing is clear – the disease is prevalent in the southwestern United States and can have economic significance. It is not isolated only to Texas, but the type of soil, pH and heat level make the area ideal to foster the fungus. A few cultural changes, along with continual monitoring can help you prevent Texas root rot in sweet potatoes.

About Sweet Potato Texas Root Rot

Suddenly wilting plants during late spring through early fall may signal the presence of sweet potato cotton root rot. The only obvious way to diagnose the disease is to dig up plants and look for the cottony growths on the roots. Other, less invasive, cues can also help you diagnose Texas root rot in sweet potatoes, the first step to halting it and preventing widespread infection and huge crop loss.

The fungus Phymatotrichum has the ability to survive in soil for several years. Once plants have cotton root rot, the foliage turns brownish bronze or yellows. Wilting occurs within a day, starting with the uppermost leaves. By the third day, all the leaves have discolored and wilted and most are dead, although they remain attached to the plant.

Plants that have been infected for 3 or 4 days will also die very suddenly, even if they were producing vigorous growth the day before. If you dug up a plant, you would notice a 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.

Controlling Cotton Root Rot of Sweet Potato

The fungus lives in soil, often even as deep as roots penetrate. In areas that have annual issues with sweet potato cotton root rot, the disease spreads in a circular pattern at a rate of 5 to 30 feet (1.5 to 9 m.). The fungus is most common on calcareous soil where pH is high and summer temperatures soar.

This is why the American Southwest is such a good host for the fungus. When soil temperatures reach 82 degrees Fahrenheit (28 C.), the fungus is in full growth and wilt symptoms begin to appear in plants. The mycelium spreads from root to root and then persists in the soil on even minute pieces of sweet potato root.

Good sanitation practices as well as field clean up are crucial to stomping out cotton root rot of sweet potato. Crop rotation to grain or grasses, which are resistant to the disease, can help squash the fungus. These types of plants need to be grown for at least 3 years before reintroducing a plant that is susceptible to the root disease.

Deep plowing has also been shown to reduce the march of the mycelium. There are not yet any resistant varieties nor is there a registered fungicide to combat the disease. The best practices are cultural and sanitary in nature, organic tried and true approaches that work with vigilance and commitment to stamping out the problem.

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