I don’t want to cause any unwarranted panic, but there has been a report of a substantial mite infestation in a vineyard block in North Georgia. Nothing a little miticide can’t handle, but I wanted to remind everyone was aware that mites can be a problem, especially as the weather continues to heat up.
First of all, if you are not familiar with mites, mites are tiny spider-like creatures that feed by penetrating the plant tissue with their mouthparts and sucking out the plant juices. High populations can result in “bronzing” of the leaves (see image below), which reduces the photosynthetic ability of the vine, which can impact vine health and grape development.
There are several species of mites that can attack grapes, but the two most common are the two-spotted spider mite and the European red mite (see images below).
Each of the mite species cause similar injury to grape leaves. Their feeding creates tiny white or yellow spots or “stippling” on leaves that eventually results in the “bronzed” appearance. These symptoms may be confused with drought stress, so monitoring may be necessary. Additionally, the presence of webbing is an easy way to distinguish whether spider mites are the culprit (European red mites do not create webbing).
Not all mites are bad. Predatory mites are common and feed on the pest mites. When observing mites on a leaf, the predatory mites are generally faster, have no spots, are shinier with few or no bristles, and are more pear-shaped than the pest mites (see images below).
To the naked eye, mites look like tiny, moving green, yellow, or red dots, but since most mites are are less than 0.016 inch long, they are much easier to see with a 10x hand lens. If you find discolored leaves and suspect spider mites, hold a white sheet of paper or paper plate under the leaves and shake the vine or leaves. If mites are present, you will find the tiny spider-like creatures moving around on the paper. Verify mite infestation with a hand lens or microscope.
Because miticides also kill the predatory mites, it is important to determine which mites are infesting your leaves. Chemical control should be considered only if European red mites exceed an average of 10 mites per leaf and/or for spider mites, 5 mites per leaf. Lower numbers of pest mites may be naturally controlled if predatory mites are present.
Table 1. Recommended miticides for mite management in grapes.
Note: Always check the chemical label for directions and rates.
Additional management recommendations can be found at smallfruits.org: http://www.smallfruits.org/assets/documents/ipm-guides/BunchGrapeSprayGuide.pdf