Plasmopara viticola infects and sporulates through stomata of susceptible grape leaves. Sporulation, chlorosis, and necrosis ratings were made in 1994 and 1995 on grape selections and cultivars and Vitis species grown in a fungicide-free vineyard. Cellulose-acetate impressions were made of the abaxial leaf surfaces and stomata were carefully counted within a circle 100 μm in diameter under a light microscope. Leaves were rated as either pubescent or glabrous. There were significant differences among genotypes for sporulation, chlorosis, and necrosis for 1994 and 1995, with highly significant correlations over both years. Stomatal densities were significantly different, but there were no correlations among levels of downy mildew and stomata! densities. Pubescent leaves had significantly higher sporulation, chlorosis, and necrosis ratings for downy mildew than glabrous leaves over both years.