Cotton root rot (Phymatotrichum omnivorum) affects more then 2,000 plant species, according to Texas A&M University. The fungus thrives in clay loam soil conditions where pH is between 7.0 to 8.5. Cotton root rot requires high heat conditions to flourish, with a soil temperature of about 85 degrees F. The fungus is a widespread problem throughout the southeastern and southwestern United States and is most prevalent from June through September. Plants afflicted die suddenly, but trees and shrubs succumb more slowly. The spores of the disease often appear on the soil surface as a white cotton mass. Control can be difficult.
Avoid planting species of plants, trees and shrubs that are susceptible to cotton root rot. Most fruit and nut trees are at risk. Roses, oleander and pomegranate show extreme susceptibility to cotton root rot. Annuals tend to be safe to plant in an area with cotton root rot. Yucca, evergreens, eucalyptus and citrus appear to be immune to the fungus, even though they have shown signs of infection on their root systems.
Apply 3 to 4 inches of mulch around plants to try to keep the soil cool in the heat of summer. Cotton root rot requires soil to be around 85 degrees F to flourish. Use peat moss, bark chips, recycled mulch, leaf debris, pine needles or other mulch.
Add an abundance of organic matter to clay soils when planting. Peat moss or manure works well. Add it at a ratio of 50 percent organic humus with 50 percent soil. Organic matter is believed to help cut down on the cotton rot fungus spores in the soil.
Plant your plants in raised containers that hold purchased garden soil. Ensure that the containers offer proper drainage so excessive water does not build up in the plant's root system.
Space plants for adequate airflow and so that roots do not become entangled. Ensure that the soil dries out around the plants' root systems between waterings.