Grape Damage May Be Due to Intense Sunlight Not Black Rot

Picture from reader most likely sunburn on the berries

Q. My husband and I attended your seminar earlier this year regarding growing grapes and making wine. It was very interesting and helpful. We have been growing the Thompson "tasteless" for about four years now and they are doing quite well, however, our second year wine grapes, Cabernet and Black Spanish, appear to have black rot according to the internet pictures. Can you tell me if this year’s crop can be saved?


A. We don’t have this disease west of the Mississippi to my knowledge. That is an East Coast grape disease problem. We have very few grape disease problems in our climate due to our very low humidity and geographical isolation. It is more likely to be sunburn. From the picture you sent me the discoloration is typical of sunburn and it seems to be facing the sides toward the sun.

Make sure when the berries are developing that they are in the shade of the canopy as much as possible during the heat of the day. They should have as much indirect light as possible to help in color development (which also helps in the color of the juice or wine).

Grape trellis with parallel catchwires on the top
to "catch" the new growth and provide shade
to the berries

When you trellis the grapes you want to make sure that the bunches are shaded from direct light as much as possible by the overhead canopy of the vines. We use a catchwire about 18 inches above the cordon wire to do this so that the fruit remains protected but still allows for plenty of indirect light. The fruit needs sunlight to develop good color and antioxidants.

They should also have plenty of air movement around bunches so that the few disease problems we have are minimized. We sometimes pull leaves off of the vine surrounding the bunches to increase indirect light and air movement.

Grapes berries usually turn color (called veraison) in July through September depending on the variety being grown. Most of our wine grapes are ready for harvest in late July and into most of August. Veraison will also turn the berry a different color as it spreads through the berries but from your picture this is not what I am seeing.

Our wine grapes entering into veraison
at 115F (46C) daytime temps

Not all the grapes will be turning color at the same time nor will bunches be ready for harvest all turn at the same time. In the extreme heat of July and August color development can be very spotty through the bunches with some much further along than others.

I have attached a picture of our wine grapes entering into veraison with some berries totally dark while others are still green and yet others turning color.