Downy mildew resistance identification and SSR molecular marker screening of different grape germplasm resources

Downy mildew is a fungal disease that seriously threatens the normal growth and fruit quality of grapes; therefore, it is urgent to identify resistant germplasm resources. To identify the differences in the resistance rates of various Chinese wild grapevine plants to downy mildew, 120 grapevine germplasm resources were selected as experimental materials. Leaf disk in vitro testing was applied to evaluate the downy mildew resistance level of plants, and quantitative reverse transcription-polymerase chain reaction (RT-qPCR) was used to test the expression patterns of some key genes at different time courses after inoculation with Plasmopara viticola. Additionally, simple sequence repeat (SSR) markers were selected to identify resistance in different grapevine cultivars and to provide a basis for marker-assisted selection. The results showed that the disease index of 120 grapevine germplasm resources ranged from 0 to 78.70. Among the 120 grapevine varieties, 5 Vitis rotundifolia presented immunity to downy mildew; 3 wild grapevine strains, including Yunnan-Yuanmou 2, Yunnan-2, and Muzhaling-3, were highly resistant strains; 38 grapevine strains were resistant varieties, 71 grapevine strains were susceptible varieties, and 3 V. vinifera strains, namely Italian Riesling, Medoc Noir, and Gamay Teinturier were highly susceptible to downy mildew. After infection with P. viticola, the expression patterns of key genes in resistant and susceptible plants were different. Except for WRKY33, which showed a low expression level in all plants, all the genes presented an initial increasing trend followed by a decreasing trend over the inoculation period. Among the 30 pairs of SSR markers, 22 SSR markers, including CHr14V015, UDV-370 and GF09-46, could explain only 25.83-48.33% of the phenotypic variation, which means that they cannot be applied for downy mildew resistance identification. The RGA9 marker explained the highest phenotypic variation, at 80.83%, and was expected to be a standard marker that can be applied in disease-resistance production, thus providing the basis for both the rapid and mass identification of downy mildew resistance in grapevine germ-plasm resources and marker-assisted selection.