Diseases of grape (
Cotton (Texas) root rot
Texas or Cotton root rot of grape is caused by the soil borne fungus Phymatotrichopsis omnivora (Phymatotrichum omnivorum). At low elevations, the first symptom of infection is wilting of the leaves or a fast decline. Disease may develop quickly in summer months and plants die with leaves still on the plant in a few days. At high elevations, vines usually have a slow decline and may not die immediately, and may even survive for several years. P. omnivora infects roots of mature plants and causes rotting of the entire tap root. It grows as mycelial strands (photo 1) on the root surface and is readily identified by microscopic observation of its cross-shaped hyphae (photo 2) that are found emerging from strands.
All varieties of grapes are susceptible to Texas (Cotton) root rot, but Vitis vinifera varieties seem to be exceptionally susceptible (photo 3). There is no resistance to Cotton (Texas) root rot, and there are no prevention or control measures that are consistently effective in calcareous soils. Trials with reportedly resistant rootstocks have not held up in areas of severe disease. Do not plant grapes in areas where Texas root rot has occurred.
However, in non-calcareous soils, in which the pH can be lowered by additions of acidifying agents such as sulfur, sulfuric acid or ammonium thiosulfate, disease has been controlled. These chemicals should be added every one to two years, preferably by drip irrigation, during the winter when vines are dormant. The pH in the root zone must be maintained below 6.5 in order to prevent disease development.
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September 20 , 2002