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Aggressiveness of Random and Selected Isolates of Verticillium dahliae from Cotton and the Quantitative Relationship of Internal Inoculum to Defoliation. L. J. Ashworth, Jr., Plant pathologist, Plant Pathology Department, University of California, Berkeley 94720; Phytopathology 73:1292-1295. Accepted for publication 20 April 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1292.

Isolates of Verticillium dahliae from the Verticillium wilt-tolerant cotton cultivar Acala SJ-5 were considerably more aggressive than isolates from the less tolerant cultivars, 70-110 and Acala SJ-2, toward four differentially tolerant cultivars of cotton. Likewise, isolates from plants defoliated by Verticillium wilt were more aggressive than isolates from randomly selected, infected plants, whether isolates came from a highly susceptible cultivar (70-110) or a tolerant cultivar (Acala SJ-5). Isolates that uniformly caused defoliation did so independently relative to the Verticillium wilt tolerance of cotton cultivars 70-110, Acala SJ-2, Acala SJ-4, and Acala SJ-5 in the field. Also, isolates that caused uniform defoliation of cotton cultivars were as apt to be isolated from defoliated plants of the least tolerant and the most tolerant cultivars tested. Although the internal inoculum density (ID) of V. dahliae conidia of petiole tissue of nondehiscent leaves was greater for plants of culitvar Acala SJ-2 than for plants of Acala SJ-5, the ID of V. dahliae conidia in petioles of both cultivars were similar at dehiscence. Data reported here support the concept that the pathogenic aggressiveness of strains of soilborne V. dahliae occurs as a highly diverse continuum.

Additional keywords: pathogenic strains.