Grape Bunch Disease Need Applications of Fungicides in the Spring

Q. I am a big fan of your RJ 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 from the reader and I would call it a "bunch rot" a bunch disease that must be controlled in the spring, according to the fungicide label. Most homeowners use a "copper based fungicide for control.

A. It’s a disease of the bunches; a “bunch disease”. 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 downy and powdery mildew are called “bunch diseases”. The fungicide must be applied right after a rain or during high humidity days just after it flowers. Grapes flower in mid March in Las Vegas.

One of the bunch diseases later in the summer (about May) from about three or four years prior. To late for the first application of most fungicides but give it a try.

            This looks like one of the grape diseases, probably grape 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 to improve air circulation. 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 the desert.

One of the grape bunch diseases from a few years ago in Las Vegas.

            You can apply a spray mixture of a copper-based fungicide according to the label. Liqui-Cop and Bordeaux sprays come to mind, but any copper-based fungicide (a fungicide that includes copper in the ingredients) should work. You may have to repeat the application. Read the label. 

Warning. Grapes must be on the label of a fungicide because of testing and recommendations concerning the rate of application.

One of the many copper-based fungicides. Check the label to make sure your crop and rates are mentioned.

            Remember fungicides help prevent plant diseases but do not “cure” the plant of a particular disease. You are spraying early to prevent the spread of a bunch disease. There is some evidence that Neem oil has given some protection, but copper-based fungicides are better.

Grape leaf removal around grape bunches to improve the air circulation.

            You may be too late with a spray of any kind, but it is worth a try. Do it very soon and follow label directions for controlling these “bunch diseases” on grapes. Next time apply your favorite spray earlier in the season, right after rain or during high humidity, to prevent the spread of “bunch diseases”.