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The Relationship Between Time of Cortical Senescence and Foliar Symptom Development of Phymatotrichum Root Rot of Cotton. C. M. Rush, Formerly Texas Agricultural Experiment Station, present address: Agricultural Research Service, U.S. Department of Agriculture, IAREC, P.O. Box 30, Prosser, WA 99350; S. D. Lyda(2), and T. J. Gerik(3). (2)Department of Plant Sciences, Texas A&M University, College Station 77843; (3)Texas Agricultural Experiment Station, Blackland Research Center, P.O. Box 748, Temple 76503. Phytopathology 74:1464-1466. Accepted for publication 11 July 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1464.

Borescopic observations revealed that Phymatotrichum omnivorum is capable of attacking roots of cotton seedlings 5 days after emergence. Roots in contact with the fungus became water-soaked and discolored. The fungus colonized the root and new strands originated from the colonized region. Disease symptoms were restricted to the root; no foliar symptoms appeared until the root cortex sloughed. Cortex sloughing occurred 18- 25 days, and plant death 27- 50 days after seedling emergence, regardless of the age at which cotton plants were exposed to sclerotial inoculum of P. omnivorum. The expression of Phymatotrichum root rot symptoms on cotton is intimately associated with cortical senescence.