These spots turn necrotic on older leaves and may result in complete defoliation, the sporophores turn dark grey. The pathogen is Plasmopara viticola . It is biotrophic and grows intercellularly and sends haustoria into cells. Sporangiophores come out of stomata on under surface of leaf in humiod weather. These sporangiophores are the basis of identification.
American grapes are resistant and germplasm has been introgressed into euroipean varieties.
Even resistant varioeties require some chemical protection and Bourdeaux mixture is still one of the best controls. Other contact fungicides eg zineb, mancozeb, captain are effective. Applied at bloom and every 10 days. Disease prediction is used. Covered on Pages 283 - 286.