Studies on Physiological Specialization of Cucumber Downy Mildew
Fu Junfan;Fu Shuyun
Samples of the fungus collected from the principal cucumber growing regions in North China (including Beijing, Tianjing, Jinan, Zhengzhou, Xian, Harbin, Changchun, Shenyang, Dandong, Dalian, Jinzhou, Tieling and Chaoyang) have been examined for comparative studies on pathogenic reactions during 1982--1985. Seventy-five cucumber cultivars including resistant, moderate- resistant, moderate-susceptible and susceptible types, were tested for the resistance by "culture in vitro" method of artificM inoculation. The results showed that the differential resistance reaction to cucumber downy mildew was conspicuous, and the reaction was in agreement with the natural reaction in field (including North China and Hainan Island). The pathogenic reactions of all samples against one variety in various localities were similar, and the resistant varieties were found to be stable and widespread throughout China. Therefore, the results suggest that physiological specialization of the pathogen does not exist. The results also suggest that the lesion size and colour, sporulation, latent period and numbers of stomata on cucumber leaf may serve as indices for identification of resistance through arficial inoculation. Based on these results, it is believed that the "culture in vitro" method of artifical inoculation, in which detached leaves are inoculated with sporangial suspension and incubated in moist chamber, can be used for selecting and breeding cucumber cultivars with downy mildew resistance. A set of 20 cultivars from 100 varieties may be employed as differentials for resistance identification in China.