Abstract
1. The isolates of Phytophthora infestans, twenty-seven from potatoes and forty-one from tomatoes, were obtained from different localities of eleven prefectures in Japan. These isolates were used for inoculation upon the seedlings of Irish Cobbler (Danshaku) potato and Ponderosa tomato. All isolates caused severe infection on potato seedings but on tomato caused different severities of infection. According to the relative strength of pathogenicity to tomato seedlings, these isolates were classified into three different groups. ‘Potato-type’ was lowp athogenic, ‘intermediate-type’ was moderately pathogenic and ‘tomato-type’ was highly pathogenic on tomato seedlings.
2. Of the potato isolates, seventeen isolates (60.0%) belonged to potato-type, five (18.5%) to intermediate type and four (14.8%) to tomato-type. Among the tomato isolates twenty-eight (68.3%) were tomato-type and eight (19.5%) were intermediate type.
3. Many potato isolates were collected from Shizuoka and Shimizu cities, in May and June from the year 1959 to 1961. The frequency of three types in these isolates were examined. All of fifty-one isolates collected in 1959 were potato type and thirty isolates in 1960 were identified as intermediate-type (4 isolates) and potato-type (26 isolates). Among one hundred and fifty-two isolates collected in 1961, five were tomato-type, six were intermediate-type and others were potato-type.
4. In serial passage experiments of the potato-type and intermediate-type through tomato foliage, it was found that the intermediate-type had acquired the virulence equivalent to the tomato-type after ten passages, but the potato-type had declined after four passages.
