The warm, wet weather has caused sour rot to start to develop in some vineyards. Research in Ontario and New York has shown that applications of antimicrobials such as potassium metabisulphate (KMS at 5 kg/1,000 L) or peroxide (Oxidate) in combination with insecticides were the most effective way to manage the disease. Optimize drying conditions through canopy management and crop thinning to avoid clumped clusters. Minimizing fruit injury, especially by birds and insects (wasps and grape berry moth), is essential.
Manage insects as well
It is critical to manage not just the yeasts and acetic acid bacteria that cause sour rot, but also the Drosophila flies (spotted wing Drosophila as well as Drosophila melanogaster, the common “fruit fly) that can carry them.
Yellow jackets are also active now and can damage berries and allow sour rot to enter. Up-Cyde is the only product registered for yellow jacket control. It is registered for SWD in other crops so would have activity in grapes if applied for yellow jackets.
Except for Malathion, all of these products will provide good to excellent control of grape berry moth in preharvest sprays. Make note of restricted entry intervals (REI) with respect to hand labour and pre harvest intervals for hand vs machine harvest. Note that Up-Cyde cannot be used on table grapes.
Confirm with your winery or buyer regarding product acceptability and pre-harvest intervals.