The influence of table grape rootstock and cultivar combinations on susceptibility to downy mildew

Downy mildew (Plasmopara viticola) is among the most important grape disease in the world. Disease control is mostly based on preventive management, including reduction of overwintering inoculum, green pruning, use of less susceptible cultivars, and fungicide applications. The objective of this study was to describe the incidence and severity of P. viticola and the resulting defoliation in three Vitis labrusca cultivars ('Concord,' 'Bord' and 'BRS Carmem') grafted onto three different rootstocks (Paulsen 1103, IAC 766 and VR 043-43) in a commercial vineyard in a subtropical area. To compare the responses of each cultivar and rootstock combination, the experiments were carried out on ridges with a semi-trellis system. Disease incidence and severity were assessed every 14 days from September to March in 2011-2012 and 2012-2013 on the last 10 leaves of specific shoots on six plants of each of the nine different combinations. During the growing season, weather conditions were favorable to the development of the disease. Symptoms started in the vegetative phase and continued during fruiting. There were significant differences between cultivars with 'BRS Carmem' being the most susceptible to downy mildew and 'Concord' the least susceptible, but rootstocks had no significant influence on downy mildew severity.