Grape Bunch Diseases are Now Here

Q. I am a big fan of your (Las Vegas Review Journal newspaper) column, but I haven’t seen any answers about what’s happening to my grapes. Please advise. This the stuff isn’t on all my grapes. I have four plants and maybe 10-15% have this white stuff on certain bunches.

This is what are called collectively, "bunch diseases". But are usually either Botrytis or powdery mildew.

A. I thought this might happen mostly because of our wet spring. Bayer tells us, when treating for grape bunch diseases, that about half of the improvement is due to better air circulation and the other half using a copper-based fungicide. They are right! Collectively, both botrytis and powdery mildew are called “bunch diseases”. Warning. Grapes must be present on the label because of testing and recommendations concerning the rate of application.

Sometimes, months after we see the disease during wet weather, the bunch disease shows up as dried berries. Too late to do much about it now.

            This looks like one of the grape diseases, probably grape botrytis or downy mildew. Try removing bunches and leaves so that you have one bunch every foot. If it is tight with leaves, remove some of the leaves as well. Don’t remove too many leaves so that you get direct sunlight on the grapes or limbs. You want to improve the air circulation around the bunches and the berries but without putting the bunches (and limbs) in direct strong sunlight for any length of time.

In severe cases, the grape bunch may just dry up!

            You can apply a spray mixture of a copper-based fungicide according to the label. Copper sulfate, Liqui-Cop, and Bordeaux sprays come to mind, but any copper-based fungicide should work. You may have to repeat the application. Read the label. Remember fungicides help stop plant diseases but do not “cure” the plant of a particular disease. You are spraying to prevent the spread of grape botrytis and powdery mildew. There is some evidence that Neem oil has given some protection, but copper-based fungicides are better.

One of the copper-based fungicides. Any fungicide that contains copper in it is considered a "copper-based" fungicide. https://www.gardeningknowhow.com/garden-how-to/info/what-is-copper-fungicide.htm

            You may be too late with this application, but it is worth a try. Do it very soon and follow label directions for controlling these “bunch diseases” on grapes.