Abstract
1. The isolates of Phytophthora infestans, fifteen from potatoes and twenty from tomatoes, were tested for pathogenicity to the differential tomato varieties, Ponderosa, West Virginia accession-19 (W.V.-19), 36, 106 and 700. These isolates were classified into four races: 1) potato-type, weakly pathogenic or nonpathogenic to all tomato varieties; 2) tomato race O, pathogenic to Ponderosa and W.V.-19, but weakly pathogenic to W.V.-36, 106 and 700; 3) tomato race 1, pathogenic to Ponderosa, W.V.-19, 36, 106, but weakly pathogenic to W.V.-700; 4) tomato race X, pathogenic to all tomato varieties.
2. Thirty consecutive passages of the tomato race O through foliage of a susceptible variety did not increase its virulence. But ten passages through a resistant variety increased its virulence, showing virulence almost equal to tomato race 1.
3. According to the cross inoculation tests with the potato races and tomato races on the differential varieties for each, it was found that isolates belonging to a single potato race usually comprised several tomato races, and vice versa. There was no relationship in pathogenicity between the tomato and potato races.
4. With respect to the varietal susceptibility of tomatoes, all of eighteen commercial varieties tested were found to be highly susceptible not only to tomato race X but also to tomato race O.
