Cotton root rot, also known as Texas root rot, is caused by the soil-borne fungus Phymatotrichopsis omnivora. It affects more than 2,000 species of dicotyledonous plants (plants whose seeds contain two embryonic leaves), among them fruit and nut trees, ash, some elms, sycamore and fig trees. The African sumac tree, a heat-tolerant native of Africa, is particularly susceptible to cotton root rot.
Confirm Your Suspicions
The treatment for cotton root rot is usually either labor-intensive, expensive or drastic, so make sure that you're really dealing with cotton root rot before proceeding. A slow and inexplicable loss of foliage density combined with yellowing leaves over several years suggests an infected tree, but the only way to be sure is to look at the roots. You might be able to see the mycelial strands of the fungus with your eyes. To be certain, send a root sample to a plant pathology lab for testing.
Since the fungus persists in the soil and may strike again at any time, it is extremely important to have your African sumac tested for cotton root rot even if there is no hope of saving it. If your tree suddenly dies sometime from late spring through early fall when soil temperatures surpass 82 degrees Fahrenheit and its foliage is wilting from the top down over the course of 72 hours, suspect cotton root rot and seek laboratory confirmation.
Improve the Soil
P. omnivorum thrives in calcareous clay soils with a pH in the range of 7.0 to 8.5. By slowly working organic matter into the soil, it is possible to improve the soil's consistency and lower its pH below that range. ForestsPests.org recommends applying ammonium sulfate or ammonium phosphate fertilizer at 4.5 kilograms per 9.3 square meters in order to increase acidity, but even after doing so it's recommended not to plant trees in previously infected areas.
Once you have determined the presence of P. omnivorum in your soil, your best course of action is simply not to plant African sumac or any other susceptible plant. The Texas A&M University Extension maintains a list of plants considered resistant to or tolerant of cotton root rot. You can also plant any monocot (plants whose seeds have only one seedling leaf), as these are immune.
Unfortunately, there is no way to tell whether your soil is harboring P. omnivorum ahead of time. The only way to be sure is to plant a susceptible plant, like the African sumac, and collect a root sample for testing should the plant develop symptoms of cotton root rot.