grape leaf problem

Asked May 18, 2020, 10:05 PM EDT

Something that I assume is a type of fungus is attacking the leaves on my grapes and destroying them. It creates bumps on the top surface of the leaf and a white scourge underneath which causes the leaf to wither and die. I have had seven types of table grapes for four decades without serious problems, but this new disease started in 2017 and is getting worse. So far the fruit has not been affected. I would like to know what this invader is and how to deal with it. Thanks for any helpful information you can provide. Wendell Cloepfil 7540 SW Florence Lane Portland, OR 97223

Washington County Oregon

1 Response

Thank you for the images. The "bubbling"of the leaves is not a disease. It's caused by very tiny grape erineum mites.

The mite feeding causes the upper leaf surface to be blistered, whereas the lower leaf surface of the blister appears hairy and white, resembling fungal growth. The white "fuzz" is actually enlarged leaf hairs caused by the mites' feeding. Later in the season, the white will change to pale brown then, finally, a dark brown.

These mites are far smaller than the spider mites you may be familiar with; they are microscopic.

Fortunately, erineum mites don't affect the vine's health nor fruit set and development when the infestation is moderate. The leaf in your image is very heavily infested which is probably why the such leaves yellow and drop.

It's seldom necessary to attempt to control erineum mites because they don't damage the vine's health or fruit production.

But if numerous leaves are dropping you may want to consider these products suggested for home use:
- Dormant-season oils and insecticides which are used for other pests in the fall; or
- Sulfur applications which have been used for powdery mildew.

It's challenging to eradicate grape erineum mites, so it's a good thing they don't negatively affect the vine nor fruit production. With one of the treatments mentioned above, you may be able to decrease the severity such that fewer leaves drop.