Crop loss in grapes due to downy mildew infection on clusters at pre- and post bloom stages under non-epiphytotic conditions

Abstract

Severe necrosis of young clusters of grape cvs. Thompson, Tas-A-Ganesh and Sonaka was observed in many vineyards in Maharashtra resulting in about 20 to 80% mortality. The symptoms were similar to those caused by downy mildew disease, but there was no sporulation on or near the necrotic tissues. There was negligible leaf infection in these vineyards, and there was no preceding rainy period suggesting some other cause of necrosis. Investigations revealed structures resembling immature oospores in some of the infected clusters. Molecular detection using Plasmopara viticola specific primers confirmed that the mortality was due to downy mildew infection. Though the estimated possibility of downy mildew infection, based on the downy mildew forecasting module of Metwin 2 software was low to nil, analysis of weather parameters indicated that dew formation had led to build up of 95 to 100% RH and leaf wetness of minimum 4 to 12 h from near midnight to early morning hours. These environmental conditions supported infection, but not incubation and sporulation of P. viticola. Nutrient analysis showed that vines with high cluster mortality had significantly lower potassium content in petioles (1.18-1.57%) as well as in blades (0.90-1.10%), which further predisposed the vines to infection.