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Selection of Pathogenic Strains of Verticillium dahliae and Their Influence on the Useful Life of Cotton Cultivars in the Field. L. J. Ashworth, Jr., Plant pathologist, Plant Pathology Department, University of California, Berkeley 94720; N. Galanopoulos(2), and S. Galanopoulou(3). (2)Plant pathologist, Hellenic Cotton Board, Thessaloniki, Greece; (3)Plant breeder, Cotton and Industrial Plants Institute, Thessaloniki, Sindos, Greece. Phytopathology 73:1637-1639. Accepted for publication 22 June 1983. Copyright 1983 American Phytopathological Society. DOI: 10.1094/Phyto-73-1637.

Three cotton cultivars were grown in the same rows at the University of California West Side Field Station for five successive years, 1976- 1980. The cultivars were 70-110, an Acala-type cotton used locally as a susceptible comparison in breeding trials; Acala SJ-2, a cultivar with moderate tolerance to Verticillium wilt; and Acala SJ-5, which is more tolerant of Verticillium wilt than either 70-110 or Acala SJ-2. In the beginning (1976) all cultivars were exposed to identical inoculum densities (ID), 2, 4, 15, and 21 microsclerotia per gram of soil. The rate of buildup of new inoculum at each original ID was greatest in 70-110 blocks, followed by Acala SJ-2 blocks, and it was least in Acala SJ-5 blocks. Within-cultivar differences in ID of Verticillium dahliae were not distinguishable after 2 yr, but differences occurred between blocks of 70-110 and Acala SJ-2 versus blocks of Acala SJ-5 each year. Sixty to 90% of plants of the most susceptible cultivar, 70-110, were defoliated each year by the time of harvest (a severe expression of Verticillium wilt), but the percentage of defoliated plants of the more tolerant cultivars increased in successive years. This observation suggested that differentially tolerant cultivars induced increases of soilborne inoculum of more aggressive strains of V. dahliae when planted year after year in the same infested soil. The data provide a quantitative basis for understanding the demise of cotton cultivars grown in fields infested with V. dahliae.

Additional keywords: cotton genotype effects on inoculum of V. dahliae.