Root rot in Arizona plants is the result of a fungus found in many of the Southwest's low-organic soils. Root rot has an economic impact on cotton, alfalfa, sugar beet, grape and stone fruit crops. It also threatens many ornamental plants in home landscapes, note University of Arizona Cooperative Extension plant pathology specialist Mary Olson and agronomist Jeffery C. Silvertooth.
Phymatotrichopsis omnivora is the soil fungus responsible for root rot in Arizona. This fungus attacks plants with yellow, branching strands. The strands destroy a plants' bark and then enter its vascular, or sap-transporting, system. When the plant dies, the fungus produces dense, white cellular masses called sclerotia. They change to brown and finally black as they age, according to the Arizona Department of Agriculture. With the sclerotia's protection, the fungus can survive in soil up to 12 feet deep for a minimum of five years. Phymatotrichopsis also forms white or light brown mats of spores on top of the soil, but their spores appear to be sterile.
The root rot fungus thrives in the mild winters of Arizona's low deserts. It also survives, however, at elevations of up to 5000 feet when it can find suitable plant hosts. Phymatotrichopsis tolerates a wide range of soils, including central and Western Arizona's river flood plains and southern Arizona's grassland slopes.
Phymatotrichopsis affects more different plants than any other known plant pathogen. Its broadleaf plant hosts number more than 2,300. Even broadleafs that don't develop the disease--like mesquite trees--can pass it on to other plants, cautions the Arizona Department of Agriculture.
Leaves on some fungus-infected plants become slightly yellow or brown before they wilt. All leaves, however, wilt and die in hot summer weather while remaining on the plants. Trees that succumb to root rot often have red lesions encircling their crowns. Phymatotrichopsis also creates dead patches in the middle of crop fields. Destroyed root systems mean affected plants lift easily from the soil.
This fungus slowly infects areas by spreading through the soil one plant at a time. A more rapid spread occurs with the transplanting of plants from a fungus-infested location to a fungus-free one.
Symptoms of root rot fungus don't surface until a plant's root system is destroyed. Affected plants aren't treatable, cautions the Maricopa County Cooperative Extension. Reducing the spread of the disease in affected soils is a matter of different approaches. Mixing 20 tons of manure per acre of soil has helped in some areas. So has the summer rotation of grass crops immune to the fungus and crops of susceptible plants, advise Silvertooth and Olson. Homeowners can replace infected plants with ornamentals immune to the fungus. They include agaves, palms, aloes and spider lilies.