The most important disease of woody dicotyledonous plants in Arizona is Phymatotrichopsis root rot (Cotton or Texas root rot) caused by a unique and widely distributed soil-borne fungus, Phymatotrichopsis omnivora. The fungus is indigenous to the alkaline, low-organic matter soils of the southwestern United States and central and northern Mexico.
Cotton (Texas) root rot often causes a rapid wilt and death of the host in the late spring, summer and early fall when temperatures are high (Figures 1a - c). Dead and dying leaves remain attached to the plant. However, infected plants also may decline more slowly, especially at cooler temperatures and when plants are well cared for. The roots of dying or declining plants are rotted.
Most woody trees and shrubs show no symptoms during the first few years after planting into infested areas. This is in contrast with certain tap rooted crop plants such as cotton and alfalfa that become infected and die the first summer after planting. The fungus is deep seated in the soil, and it may simply be that roots of many susceptible plants do not grow into the deeper, infested areas for a number of years.
It should be emphasized that the fungus is only a pathogen of mature roots of dicotyledonous plants. The only effective method of control is to plant immune or highly resistant species in infested area