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Enlisting Mites to Fight the Number One Grape Disease

Entomologists and plant pathologists pit fungus-eating mites against grape powdery mildew

photo of mite
Scanning electron micrograph showing a tydeid mite taking cover in the hairs of a grape leaf. Actual size of this mite is that of a small pinhead.
Scan by A. Norton/ H. Hoch

Grape powdery mildew packs a punch. Not only is it the most destructive of all known grape diseases in the Northeast, adversely affecting vine health, grape quality, and yield, but it has an amazing capacity for resistance. Several new materials have been developed to combat this disease, but their effectiveness is often short-lived. This resistance problem, coupled with increasing concerns about environmental hazards that may accompany the use of fungicides, has led to the idea of biological control via fungus-eating mites.

Entomologists and plant pathologists from Geneva–Greg Loeb, David Gadoury, Andrew Norton, Bob Seem, and Wayne Wilcox–are focusing on fungus-eating "tydeid" mites. A certain species of these mites has been discovered busily protecting wild grapes from powdery mildew. The question is: can this species do the same for cultivated grapes under vineyard conditions?

The full answer to this question will take two to four more years to answer, but so far, so good. In the first season Loeb and the others successfully established over 700 rooted cuttings of both commercial and wild grapes in a new vineyard planting at the Experiment Station in Geneva. Vines were assigned to one of three treatments: 1) mites but no fungicides, 2) mites and a fungicide active against powdery mildew, or 3) neither mites nor fungicides.

According to Loeb, "The mites became established on all of the grapes we are working with, both cultivated and wild. We weren’t necessarily expecting this." Tydeid mites have shown a preference for grapes with pubescent (hairy) leaf veins, probably because the hairs protect them from predators and harsh weather. Several of the 15 grape types being tested have smooth, non-hairy leaves. While mite density per leaf ranged from 3 to 26, no major differences were detected as a function of grape species. This bodes well for their future usefulness in vineyards.

Mildew levels were assessed on a subset of vines at the end of the season. Vines that received mites and no fungicide had slightly lower levels of mildew than vines without mites or fungicide, but the difference was not statistically significant. (Vines receiving both mites and fungicide were not evaluated.) A more comprehensive evaluation of which cultivars are most suitable for these mites and of the extent to which the mites provide protection against powdery mildew will be carried out over the next several field seasons.