Cotton root rot (Phymatotrichum omnivorum), also called Texas root rot, occurs on over 2,300 species of plants, according to New Mexico State University. One of the most destructive of all plant diseases, it causes the sudden wilt and death of the infected plant. Once soil is infected, the area can only sustain plants that have an immunity and tolerance to the disease.
A soil-born fungus, cotton root rot occurs through the southwestern United States and throughout Mexico. It prefers areas that are low in elevation but can survive in an elevation of up to 5,000 feet, according to the University of Arizona. The fungus thrives in arid desert conditions. It tolerates a wide range of soil types and has been found as deep as 12 feet beneath the soil's surface.
Cotton root rot does not reproduce through spores, as other fungi do. It spreads through the soil with hairlike strands which form a dense web. A plant can often live immune for years in soil infected with cotton root rot if the fungus is located deep beneath the soil's surface. Once the plant's roots reach into the infected fungal soil the fungus quickly colonizes the plant's root system.
The plant begins to show symptoms from late summer and into early fall. The plant often appears fine one day and the next it is dead with few or no symptoms prior to death. The gardener may notice a white- to tan-colored mat substance around the base of infected plants. Large trees or shrubs suffering from cotton root rot often exhibit yellowing leaves and wilt prior to death. The root system of the infected plant will appear rotted and brittle. Reddish lesions may be visible at the crown of the plant by the soil, according to North Carolina State University.
Fungicide applications only offer moderate control because the cotton root fungus can live so deeply inside the soil. Only the top layers of the soil will be treatable with the fungicide so once the plant's roots reach deeply into the soil they will become infected with the fungus. Treatment is virtually impossible and a gardener will need to focus on prevention.
Home owners should consider planting monocot plant species, such as palms, which are immune to cotton root rot, or other resistant plant species. Desert plants, such as the agave, also show an immunity to cotton root rot. Gardeners can consider growing plants in containers or in raised flower beds that have a barrier between the plant's root system and the infected soil. Bring in new soil that is free of cotton root rot when planting in raised flower beds.