How to Control Powdery Mildew of Grape

Q. I have white powdery mildew on my green, table grapes.  This has not been a problem in the past.  What now? Is this related to the cool, moist weather?

Grapes should have air movement around them to prevent powdery mildew and bunch rots. In the hot desert we have to be careful about giving them full sun throughout the day because of sunburn to the fruit.

A. Yes, powdery mildew on grape is seldom seen here because of our very dry and hot weather. This very cool spring, higher humidity and recent rains has made powdery mildew, as well as early blight on tomatoes a problem. Powdery mildew will disappear when it gets hot and dry.

Horticultural oils can help reduce powdery mildew problems

One thing that really helps control powdery mildew forming on grape bunches is improved air movement around grape bunches as well as sunlight during the morning hours. Sunlight on grape bunches in the late afternoon can cause sunburn on the berries so be careful about giving the fruit too much sun late in the day.

We can usually prevent powdery mildew by removing the leaves around grape bunches for better air movement and keeping bunches dry. Once powdery mildew has started, you may have to apply a fungicide to keep it at bay or eradicate it.

One of the best organic controls of powdery mildew on grape are the horticultural oils. Horticultural oils are mixed with water and sprayed on grape bunches in the early morning hours. There is a precaution in using oil sprays and not to apply it when temperatures are high. However, I have had no problem applying oils during the cool early morning hours.

Insecticidal soaps labeled for disease control have also given some control as well as dusting dry bunches with sulfur dust.