The comparison of the overwintering ability of metalaxyl-resistance and metalaxyl-sensitive isolates of Phytophthora infestans in seed potato

The ability of metalaxyl resistance, metalaxyl sensitive and mixed isolates in P. infestans to overwinter in infected potato seed tuber was studied with immersing inoculation and culturing potato piece method.The survival rate of pathogen in the seed potato inoculated with mixed isolates was the highest(100%).The survival rates of pathogen in the seed potato inoculated with metalaxyl resistant isolates were 80 0%.The survival rate of pathogen, however, in the seed potato inoculated with metalaxyl sensitive isolates was the lowest(46 7%).Pathogen was isolated and assayed metalaxyl resistance by an in vitro radical growth assay on fungicide amended agar media 39 cultures were isolated from seed potato inoculated by mixed isolates.Among 39 cultures, 38 were metalaxyl resistant ones and the frequencies were 97 4%,only 1 was metalaxyl sensitive and the frequency was 2 6%. It showed that the abilities of overwintering and competition of metalaxyl resistant isolates in the seed potato were stronger than that of metalaxyl sensitive ones.