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Verticillium Wilt Disease of Tomato: Influence of Inoculum Density and Root Extension Upon Disease Severity. L. J. Ashworth, Jr., Plant pathologist, University of California, Berkeley 94720; O. C. Huisman(2), D. M. Harper(3), L. K. Stromberg(4). (2)(3)Assistant professor, and research associate, University of California, Berkeley 94720; (4)Farm advisor, University of California, Cooperative Extension, Fresno 93702. Phytopathology 69:490-492. Accepted for publication 22 November 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-490.

Rate of infection of tomato by Verticillium dahliae increased with increases of inoculum density in four field experiments in which microsclerotia (MS) in naturally infected tomato plants was the inoculum. However, essentially all plants were infected by the end of the growing season regardless of inoculum density (range = 0.127 MS/g soil). Only slight external symptoms appeared in plants grown at inoculum densities of 0.19 MS/g in a friable clay loam soil in which root growth was unrestricted and plant growth otherwise was favored. Maximum loss under these conditions was about 40%, depending upon the experiment. In a sandy loam soil with a dense layer 1560 cm below the surface however, the combination of wilt disease, restricted root growth, and poor water penetration was devastating to tomato. Maximum yield under these conditions was about 4.3 103 kg/ha compared with about 14 103 kg/ha under the more favorable growing conditions.

Additional keywords: V. albo-atrum, soilborne pathogens, fungal disease, epidemiology.