Answer: You might try thinning out the clusters so your plant has fewer grapes to try to ripen. The symptoms of browning grapes is probably due to a a fungal disease called Phomopsis. The first signs of infection on the grapes appears close to harvest time as a light-brown color. Then black, spore-producing structures of the fungus (pycnidia) will then break through the berry skin, causing the berry to shrivel. This disease overwinters on the grapevines so you'll need to control it early in the season with lime/sulfur applications. Multiple applications will be needed early in the spring. It's too late for fungicide applications this year so mark your calendar so you can plan to spray early next spring. The label will indicate at what growth stages you need to apply the product. Best wishes with your grapevines.
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