Texas Root Rot
Phymatotrichopsis omnivora (Duggar) Hennebert
[Syn. Phymatotrichum omnivorum (Shear) Duggar]
Solomon, J.D.; Leininger, T.D.; Wilson, A.D.; Anderson, R.L.; Thompson, L.C.; McCracken, F.I. 1993. Ash pests: A guide to major insects, diseases, air pollution injury and chemical injury. Gen. Tech. Rep. SO-96. New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station. 45 p.
Importance. - The Texas root rot pathogen has a wide range of broad-leaved hosts throughout certain areas of the Southwestern United States. Ashes planted in old fields or residential areas may be affected by this disease.
Identifying the Disease. - The disease is characterized by sudden wilting and death of infected seedlings. Older trees may exhibit reduced growth and vigor. Coalescing, necrotic lesions on roots may appear down to a soil depth of 30 cm. The inner bark and cambium turn brown or black and mushy, and the root collar may be girdled. As the roots die, leaves of affected trees turn yellow or bronze before they wilt. Dieback and thin crowns are common.
Identifying the Fungus. - Spore mats develop on the soil surface during warm, wet periods. Infected roots are covered by yellowish, fluffy mycelia that penetrate the bark. Mycelial strands, with distinct, cross-shaped hyphae, are the best diagnostic microscopic feature. Black sclerotia formed from compact hyphae on dying roots may be found in the soil.
Biology. - The fungus persists in the soil for many years as sclerotia, which germinate to produce infectious hyphae that enter roots through natural openings and wounds. The hyphae colonize the root and grow up to 9 cm per year along infected roots and in the soil. The fungus is favored by warm, calcareous soils with high clay content and a pH of 7.2 to 8.5.
Control. - Soil amendments to increase soil acidity in alkaline soils inhibit growth of the fungus. Ammonium sulfate or ammonium phosphate fertilizer should be applied at 4.5 kg per 9.3 m2. The soil should then be soaked to 30 to 60 cm. Planting trees in infected areas or on land previously planted in cotton should be avoided.
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