Texas root rot, more commonly called cotton root rot, is caused by Phymatotrichopsis omnivora, a soilborne fungus. It infects more than 2000 plants, including filberts and other nuts and plants--notably cotton, from which the common name is derived. It is found in soil throughout Mexico and the southwestern United States.
Texas root rot strikes in hot weather in the late spring, summer and early fall. When rotted roots prevent a filbert tree from getting enough water, it suddenly wilts and dies. Long periods of infection precede symptoms on mature trees.
A 10-power microscope will show light brown strands of fungus on the surface of the dying or dead roots; these are hyphae, branching webs of the fungus. Examination with a 100-power microscope will show hyphae in the form of the Christian cross with pointed ends. During periods of summer rain, the fungus sometimes produces a light tan to white mat of spores on top of the soil that looks like pancake batter. These mats are unimportant because they are not believed to germinate, and other fungi produce similar mats.
There is no known way to test soils for the presence of the fungus. The agricultural extension services in most states affected by Texas root rot can diagnose a sample of a rotting root. To collect a sample for analysis, remove a 6-inch or longer sample of a root from a tree that is dying or dead. The sample should be the size of a pencil or slightly larger. Put the sample in a plastic bag with the soil attached and keep cool until it is delivered to the testing service.
Texas root rot does not produce airborne spores. It spreads by dense collections of hyphal strands called sclerotia that form colonies on the roots of filbert trees and other hosts. Botanists at the University of Arizona report these strands have been found 12 feet underground. The fungus can live in the soil for years before it infects the roots. It strikes only dicots, plants that have leaves with webbed veins, not monocots, grasses and other plants that have parallel veins.
The spread of Texas root rot is unpredictable. There will be no pattern of the disease in a filbert orchard. The fungus might attack every third tree or maybe only one tree out of every seven or eight; it can jump across rows of trees.
Plant pathologists at Texas A&M University say there is no known control for Texas root rot. They advise growers to slowly add organic matter into the soil to lower the pH slightly. Texas A&M plant pathologists and those at the University of Arizona and Oklahoma State University all advise growers in areas commonly infected with Texas root rot to plant only trees that have shown resistance to the fungus.