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Atypical Disease Symptoms Associated with Phymatotrichum Root Rot of Cotton. C. M. Rush, Texas Agricultural Experiment Station, Blackland Research Center, P.O. Box 748, Temple 76403. T. J. Gerik, Texas Agricultural Experiment Station, Blackland Research Center, P.O. Box 748, Temple 76403; and C. M. Kenerley, Department of Plant Pathology, Texas A&M University, College Station 77843. Plant Dis. 69:534-537. Accepted for publication 14 January 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-534.

Cotton plants showing atypical symptoms of Phymatotrichum root rot were observed during the 1983 growing season. Aboveground symptoms consisted of gradual wilting followed by leaf chlorosis and defoliation. Mature bolls and young leaves remained attached to the stem and the plant remained alive. In contrast, foliar wilting normally occurs very rapidly and the plant dies, but the leaves remain attached. Externally, roots displayed discrete discolored sunken lesions 1020 cm below the soil surface as opposed to typical lesion formation at the soil surface. Basipetal to this lesion, typical Phymatotrichum root rot symptoms had developed, the periderm and phloem were destroyed, and strands of Phymatotrichum omnivorum were visible. However, acropetal to the lesion, tissues appeared healthy with no evidence of the fungus. Internally and acropetal to the lesion, the root xylem showed extreme discoloration that usually extended to the transition zone between root and stem. Attempts to isolate the fungus from this region failed, and microscopic examination confirmed that the fungus had not advanced acropetal to the external lesion. The atypical symptoms were observed only during periods of low soil moisture availability. When soil moisture was replenished, strands of the fungus resumed growth on the root surface up to the soil line and plants previously showing atypical symptoms died.