RESISTANCE OF TOMATO GENOTYPES TO 4 ISOLATES OF VERTICILLIUM-DAHLIAE RACE-2

Eight tomato (Lycopersicon esculentum Mill.) genotypes were evaluated based on shoot dry weight for resistance to four isolates of Verticill ium dahliae Kleb. race 2 in two greenhouse seedling experiments. The r ace 2 isolates, obtained from North Carolina, Brazil, and Spain, demon strated no differences in pathogenicity on the eight lines tested, thu s precluding the identification of a third V. dahliae race in this col lection. However, highly significant differences in virulence were obs erved among the isolates. The Brazilian isolate was the most virulent. No tomato genotype showed resistance comparable to that conferred by the single dominant Ve gene to V. dahliae race 1. While all tomato lin es were susceptible to all race 2 isolates tested, there were signific ant differences in susceptibility equal to differences in levels of re sistance. IRAT L3, Morden Lac, Okitsu Sozai, and 'UC82' significantly outperformed the lowest ranking line XXIV-a.Earlypak 7', Morden Mel, a nd Philippino 2 performance was statistically indistinguishable from t hat of either the highest- or lowest-ranked lines. Genetic diversity i n the host and pathogen and environmental conditions favoring the path ogen likely contributed to the genotype x isolate interactions observe d in Expt. 1. These results suggest using diverse isolates when screen ing for improved race 2 resistance.