Evaluation of 17 Table Grape Accessions for Foliar Resistance to Downy Mildew

Grapevine downy mildew (DM) could cause serious damage to vines and young clusters in a humid climate, and multiple fungicides are applied to control this disease. Since most grapes are produced for fresh fruit consumption in Japan, we evaluated foliar DM resistance of 17 table grape cultivars/selections that are important for Japanese table grape breeding by visual rating. By analysis of variance (ANOVA), we estimated the variance component of blocks, cultivars, years and the interaction between cultivars and years. ANOVA revealed that the effect of cultivars is the main source of the variance (79.6%). The effect of year was significant, whereas the estimated variance component was only 3.0% of total variance. The effect of blocks and the interaction were not significant. These results suggested that our evaluation of the DM resistance was reliable. Two Vitis vinifera cultivars 'Rizamat' and 'Neo Muscat' were highly susceptible, and were easily defoliated comparing to other accessions. Most of the other V. vinifera cultivars were susceptible, whereas 'Muscat Hamburg' showed some resistance to this disease. In contrast, some American hybrids such as 'Concord', 'Campbell Early' and 'Ontario' were highly resistant. However, one of the American hybrids, 'Steuben', was not as resistant as those resistant hybrids. Japanese tetraploid cultivars, 'Kyoho', 'Pione' and 'Darkridge', are rated as medium, while 'Ishiharawase', which is the tetraploid bud sport of 'Campbell Early', was highly resistant. These results suggest that there is a variation of DM resistance among table grape cultivars, and that the resistance of Japanese leading cultivars 'Kyoho' and 'Pione' are medium.