Ask Your PCA: What is best way to handle powdery mildew in grapes?

Currently, powdery mildew is the main focus for vineyards. Mildew is the No. 1 issue with winegrapes, especially on the coast. It is constantly being treated.

Spring mildew will be seen from March through mid- to late July. Left untreated, powdery mildew will cause scarring on the canes and leaves in vineyards. The mildew will cover the leaves and they won't photosynthesize, which will physically damage and deform the leaves.

Late in the season, if mildew is on the clusters, it will cause scarring, incomplete pollination and fruit set, and the berries can become infected.

The frequency of treatment for powdery mildew is weather dependent. With very hot or cool temperatures, the mildew won't grow as quickly, so spray applications can be extended. Temperatures between 70 and 80 degrees require additional spray applications.

The type of material applied is determined by the weather and the active ingredient in the product used. Certain chemistries only last seven days on the plant, while other chemistries are 14 to 21 days.

Some chemistries will translocate, which means they will move in the leaf as the plant is growing.

There are really good chemistries to treat powdery mildew. Sulfur is the main chemistry used, and it is very, very effective.

Sulfur hasn't had problems with resistance. It has multiple modes of action, whereas synthetic chemistries such as systemic fungicides usually work on just one specific site on the pathogen. Sulfur has multiple modes of action, so it is very good in resistance management.

Mildew and water don't mix. Free water kills mildew. On the other hand, if the temperatures are moderate and dry, problems with powdery mildew will increase.

It's still early to determine if there will be high powdery mildew pressure this year. If the weather stays between say 70 and 80 degrees during the day, problems with powdery mildew will increase.