Garden Detective

Garden Detective: Leafhoppers stomp all over grapevines

What’s wrong with these grape leaves? The most likely culprits are leafhoppers.
What’s wrong with these grape leaves? The most likely culprits are leafhoppers.

Q: I have several varieties of wine grapes in my yard in Roseville and my vines appear infected with some kind of disease or fungus. I have tried neem oil, thinking it was from some type of pest, but that has not been working. Attached are photos of one of the vines (I believe chenin blanc). I have also planted cabernet sauvignon vines that are infected (but not as bad), and have new zinfandel vines that have not yet been infected.

Pat Zenzola, Roseville

According to UC master gardener Rachel Tooker, it appears from your photos that you have a particularly strong infestation of grape leafhoppers. Nymphs (juvenile) and adult leafhoppers remove the contents of leaf cells, leaving behind empty cells that appear as pale yellow spots or stippling. If populations are high, the entire leaf may be pale yellow or white.

Leaf drop can occur when densities are extremely high. In addition, honeydew excreted from the insects cause small varnish spots on fruit and can occasionally accumulate and cause sooty mold.

Because there are many natural enemies that feed on leafhoppers, it may not be necessary to apply chemical treatment. A first line of defense is always to review your cultural practices.

There are many steps you can take to discourage leafhopper populations and allow time for natural predators to get to work on your behalf. A last step would be to try chemical treatment.

The UC Integrated Pest Management Website has information available on its website at www.ipm.ucdavis.edu Here is a brief overview of steps that you can try:

▪ Healthy, properly maintained vines can tolerate some pest damage without developing major fruit quality or vine health issues. Select grape varieties that will grow and produce well in your area. Plant and support vines in a sunny area of your yard.

Before watering, check for moisture by digging down into the soil. Water deeply and use mulch to reduce soil moisture loss. Avoid overfertilization of the vines since that can favor development of more pests than just the leafhoppers. Learn to prune vines properly.

▪ Predators of grape leafhoppers include spiders, green lacewings, minute pirate bugs, lady beetles, predacious mites and small parasitic wasps. Avoid applying chemicals that will also reduce these beneficial populations.

▪ Insecticidal soap or narrow-range oil can be applied to infested foliage to reduce high populations of leafhopper nymphs but will not reduce virus transmission significantly; thorough coverage of leaf undersides is important. It is very difficult to control adult leafhoppers effectively and no control is recommended.

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