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Verticillium Wilt Disease of Cotton: Influence of Inoculum Density in the Field. L. J. Ashworth, Jr., Plant pathologist, University of California, Berkeley 94720; O. C. Huisman(2), D. M. Harper(3), L. K. Stromberg(4), and D. M. Bassett(5). (2)(3)Assistant professor, and research associate, University of California, Berkeley 94720; (4)Farm advisor, University of California, Cooperative Extension, Fresno 93702; (5)Agronomist, University of California, Shafter 93263. Phytopathology 69:483-489. Accepted for publication 22 November 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-483.

These are the first quantitative data on effects of inoculum of Verticillium dahliae on infection, disease severity, and yield of a tolerant host under field conditions. Infection rates were greater at high than at low inoculum densities. Degree of infection at harvest varied between genotypes; the most tolerant cultivars (Acala SJ-4 and Acala SJ-5) were more prone to infection than the less tolerant cultivars (70-110 and Acala SJ-2). However, the former showed fewer foliage symptoms and less defoliation than the latter and their yields were unaffected at the maximum inoculum density tested, 21 microsclerotia (MS) per gram of soil. The most sensitive cultivar was quite tolerant of Verticillium wilt disease, based upon comparative yields, although visually it appeared to be severely affected at even the lowest inoculum density, 1.7 MS/g soil. The data suggest that tolerant cultivars withstand defoliation at relatively large inoculum densities. An alternate but untested hypothesis, however, is that the less tolerant genotypes are susceptible to a greater proportion of the native soil population of V. dahliae than the most tolerant cultivars.

Additional keywords: Verticillium albo-atrum, soilborne pathogens, fungal disease, epidemiology, Gossypium hirsutum, disease resistance.