Verticillium Wilt on Resistant Tomato Cultivars in California: Virulence of Isolates from Plants and Soil and Relationship of Inoculum Density to Disease Incidence

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Abstract
Verticillium wilt caused by race 2 of V. dahliae is common in California on tomato cultivars with the Ve gene for resistance to race 1. About 47% of 124 isolates of V. dahliae taken directly from tomato field soils were race 2; the remaining isolates were either race 1 (43%) or nonpathogenic on tomato (about 10%). In contrast, isolates from diseased tomato plants (race 1 resistant cultivars) from the same fields were predominantly race 2 (about 86% of 153 tested). Race 1 was more virulent than race 2 on cultivars lacking the Ve gene for resistance (susceptible). Average virulence of race 2 isolates was lower on cultivars with the Ve gene than on susceptible cultivars. Incidence of Verticillium wilt (DI) on race 1 resistant cultivars was essentially 100% in 46 fields where soil inoculum density (ID) of total microsclerotia (ms) (race 1 and 2 in undetermined proportions) was about 5.7 ms/g of soil. In 5 other fields, however, in which the numbers of race 2 ms were determined, a linear correlation was observed between numbers of race 2 ms (0.0-2.0 ms/g of soil) and DI (0 to 100%) when data were plotted arithmetically after conversion of DI percent to loge (1/1-DI) (slope = 2.0 and r = 0.877). The line for the same data plotted on a log10-loge-log10 scale had a slope of 1.0 (or 1.57 if assumed to be nonlinear and transformed to log10 before regression analysis) instead of 0.66, as predicted by Baker et al for abstract mathematical Model II. Thus, the models and equivalent interpretations for slopes of lines in arithmetic, log10-log10 and log10-loge-log10 plots apparently are of questionable validity.

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