Prashant Swamy and Naidu Rayapati*
*Department of Plant Pathology, Washington State University, 24106 N. Bunn Rd, Prosser, WA 99350 (naidu.rayapati@wsu.edu)

Grapevine leafroll (GLD) and red blotch (GRBD) diseases are significant threats to the productivity of Washington vineyards. Studies were conducted in commercial vineyards to examine impacts of GLD and GRBD on fruit yield and quality in own-rooted, red-fruited winegrape cultivars. In each cultivar, 15 to 20 symptomatic grapevines and an equal number of unsymptomatic vines were selected for this study. Vines showing symptoms of GLD and GRBD and those without symptoms were tested by molecular diagnostic assays to ensure that symptomatic vines were positive for Grapevine leafroll-associated virus 3 (GLRaV-3) or Grapevine red blotch-associated virus (GRBaV) and healthy vines were negative for the two viruses. Data on fruit yield (number of clusters/vine and total weight/vine) were collected at commercial harvest in September/October of 2015, depending on the cultivar. At commercial harvest, ~30 berries/vine were collected randomly from different clusters of symptomatic and unsymptomatic grapevines and fruit quality was assessed. The data were analyzed by two-tailed Student’s t-test to determine significant differences. Symptoms of GLD and GRBD were consistently observed after veraison in all cultivars studied, even though GLRaV-3 and GRBaV can be detected throughout the season. No differences in leaf physiological parameters between healthy and asymptomatic leaves of virus-infected vines were observed preveraison. In contrast, appreciable differences in leaf physiological parameters between healthy and symptomatic leaves of vines infected with GRBD were observed postveraison. The results indicated that GLD and GRBD mainly affect sugar accumulation in berries and that in some cultivars, disease can significantly reduce fruit yield. However, the observed negative impacts were variable among winegrape cultivars and seasons and in different geographic locations. These results provided some insights on how two distinct viral diseases affect vine health and fruit quality in Washington vineyards.

Funding Support: WSU-Agricultural Research Center, WSDA-Speciality Crop Block Grant Program, Washington Wine Commission, and Washington State Grape and Wine Research Program