In the war against a fungus devastating to grapes, Cornell University scientists may have learned mites' real might.
To do battle against powdery mildew, Cornell scientists have turned to the shady underworld of wild grapes. Underneath wild grape leaves exist tiny hairlike structures called acarodomatia. Tydeid mites, as well as other potentially beneficial mites, make themselves at home among them, says Gregory English-Loeb, a Cornell assistant professor of entomology at the university's New York State Agricultural Experiment Station in Geneva, N.Y. What English-Loeb and other Cornell scientists learned is that the mites feed on powdery mildew, which is considered a nemesis to vineyards.
Originally, English-Loeb wondered what ecological role these tydeid mites might play on the grape leaf. They did not appear to feed on the leaf tissue, and, interestingly, he did not find much powdery mildew fungus on plants where the mites were abundant.
Along with English-Loeb, a group of Cornell scientists at the research station began to study interactions between the tydeid mites, acarodomatia and powdery mildew. The scientists are: Robert C. Seem and Wayne F. Wilcox, professors of plant pathology; David M. Gadoury, senior research associate in plant pathology; and Andrew P. Norton, postdoctoral researcher in entomology.
In a series of experiments conducted in the greenhouse and outdoors, the researchers learned that the tydeid mites are capable of dramatically reducing powdery mildew. In one experiment, the mites reduced by 85 percent the powdery mildew's mycelia, the part of the fungus that penetrates and robs the leaf of nutrients. Cleistothecia, the stage of the fungus that overwinters, was reduced by an even greater amount.
Having shown that the mites play an important role in controlling powdery mildew on wild grapes, they wondered if the mites could do the same for popular varieties like Chardonnay, Riesling, Sauvignon Blanc and others. "We are trying to learn how much of an ecological role tydeid mites will have in the world of cultivated grapes," English-Loeb says.
That ecological role could easily be measured economically in New York state. New York is the second-leading state in production of grape juice, behind the state of Washington, and it's second only to California in wine production. The state's grape producers harvest 350 million pounds of grapes annually, worth about $40 million. With over 100 million bottles of wine made each year in the state, ringing up $500 million in sales and about $85 million in state and local revenues, scientists must pay attention to this harmful fungus.
During the 1998 growing season, the researchers successfully established the mites on over 700 rooted cuttings of both commercial and wild grapes in a new vineyard at the Geneva Experiment Station. Some of the vines received mites but no fungicides, some vines received both mites and fungicide active against powdery mildew, and the remaining vines received neither mites nor fungicide.
In this current growing season, mildew levels were assessed. Grapevines that received mites and no fungicide had significantly lower levels of mildew than vines without mites or fungicide. The vines that received both the mites and fungicide had the lowest level of disease, according to English-Loeb. "We will need several more field seasons to fully evaluate the potential of this mite as a biological control agent of powdery mildew, but our initial results are promising," he says.
Meanwhile, research is continuing on why the mites spend so much time in acarodomatia. The scientists initially hypothesized that the acarodomatia provide the mites a healthy climate in which to live and reproduce, but the researchers now believe the most likely explanation is that acarodomatia provide the mites protection from their predators.
"Our research shows that acarodomatia provide a defense mutualism between beneficial mites and grape plants. The grape plants are being protected from fungus and plant-feeding spider mites, and the mites are getting protection from their own predators," says English-Loeb. "These types of mutualisms may be quite common in nature, and we may be able to take advantage of them to better manage agriculturally important pests."
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