Studies on Downy Mildew of Grapes (Vitis vinifera L.) in Kashmir valley

Downy mildew of grapes (Plasmopara viticola) attained the status of a major disease of grape in Kashmir valley. The disease was prevalent in all the three districts of the valley surveyed during the year 2007 and 2008 with disease incidence and intensity ranging from 36.78 to 43.22 and 17.70 to 22.95 per cent, respectively. The highest disease incidence (53.28%) and intensity (26.75 %) was recorded in district Srinagar and least in district Ganderbal (24.77 and 12.94%, respectively). Among the locations surveyed, Shalimar of district Srinagar exhibited highest disease incidence (67.70%) and intensity (37.49%) and Repora of district Ganderbal exhibited least disease incidence (18.92%) and intensity (7.73%). The disease is characterised by small pale yellow, translucent spots measuring approximately 0.3 mm in size on adaxial leaf surface, which later on coalesced and formed irregular necrotic patches surrounded by chocolate coloured halo. On the corresponding abaxial surface, white cottony growth of the fungus appeared which later turned dirty grey to dark brown. The disease ultimately resulted in premature defoliation. Infected shoot tips exhibited Shepherd’s crook symptoms, dried up and dropped. Young infected berries turned brown, got covered with white cottony growth of the fungus and later withered. However, late infected berries remained stunted, developed a purplish grey colour, shrivelled and dropped. The disease appeared in the month of June and reached its maximum by the end of September. Maximum apparent infection rate of 0.166 and 0.179 units/day was observed during third week of July in the year 2007 and 2008, respectively. Weather factors viz. precipitation and relative humidity showed positive correlation with the disease development in terms of infection rate (units/day) with 65 per cent contribution alone by rainfall. The pathogen perpetuated through winter in the form of oospores on fallen diseased leaves in/on orchard floor and also in infected buds of pruned snag and intact shoots. The oospore production and their viability in soil decreased with increasing depth of placement of infected leaves. Maximum oospore production in leaves on orchard floor was observed during second fortnight of May to first fortnight of June and in buds of pruned snag during first to second fortnight of June. The infected leaves and buds of pruned snag lying on the orchard floor served as the most important overwintering site for P. viticola. Among the fungitoxicants tested, metalaxyl + mancozeb 72 WP (0.3%) followed by dimethomorph 50WP (0.02%), cymoxanil 50WP (0.2%) and mancozeb 75WP (0.3%) proved highly efficacious in controlling the disease when sprayed at five different phenological stages of grape vine. Disease tolerance of varying degrees was observed under natural conditions in the available grape germplasm. Of the eighteen grape cultivars screened for tolerance/resistance to P. viticola during 2007 and 2008, Himrod was the only tolerant cultivar whereas Sahibi, Khalili, Tas-e-Ganesh, Hussaine, Kishmish Rosavis, Perlette, Anab-e-Shahi and Thompson Seedless were highly susceptible