This cluster was infected with downy mildew about two weeks before bloom. Washington growers would likely see this type of infection from rain early in the season
In the past, Dr. Gary Grove wasn’t concerned about downy mildew becoming a resident grape disease in the Pacific Northwest. The disease is not yet found in Washington State, but since working on climate change issues, the pathologist now worries the disease could gain a foothold.
Downy mildew, a serious grape disease found around the world, is problematic in regions with warm, moist summers, said Grove, Washington State University pathologist and director of the AgWeatherNet program. Vinifera grape varieties are susceptible to the disease, although susceptibility varies within juice grape or Labrusca varieties.
"The epidemiology is very explosive and could probably move faster than powdery mildew," he said. "Once the disease gets rolling and established in an area, the only way to deal with it is with a fungicide program."
But the problem with downy mildew, he notes, is that generally, the fungicides effective against powdery mildew are not effective against downy mildew. There are a few exceptions, but those products that work on both diseases, like the strobilurins, are very prone to developing resistance. Abound, Sovran, and Pristine control both, but are susceptible to resistance of powdery mildew. Widespread resistance to powdery mildew has been reported in New York and the eastern United States, according to Grove.
Some of the products were commercially available for only eight to ten years before they developed resistance, he said, stressing the importance for growers to follow fungicide resistance management.
Downy mildew is an easy disease to diagnose, he told grape growers attending the Washington State Grape Society annual meeting in November. Symptoms include oily spots on the leaves, with a white downy or mildewy coating on the underside of the leaves; the disease will also attack fruit. An easy diagnosis is the Magarey’s test of putting a suspect leaf in a wet paper bag and letting it sit overnight in temperatures of 70 to 75°F. Spots will appear in the morning if the disease is present.
Spores of downy mildew come in two types—the oospore, which allows the disease to survive cold winters, desiccation, and persists in the environment for up to ten years, and the sporangia, a spore that he describes as being able to swim. "Once we have this disease in an area, it’s going to stay," said Grove, referring to the hardiness of the oospores.
The disease cycle is difficult to break. Infected leaves fall to the ground in the fall, and the spores overwinter there. In the spring, when temperatures reach 50°F, combined with 0.4 inches of rain, spores are released and continue the disease
"We do have the weather conditions for this disease, though not to the same extent as other areas," Grove said. In reviewing past weather data, Grove found that in eight out of ten years, conditions were right for a primary infection, and in five out of ten years, conditions were ripe for a secondary infection. "In western Washington, it’s a no brainer that we have conducive conditions."
Grove believes that although Washington has the potential for downy mildew infection, it has not been detected because the fungus has yet to be brought into the state.
Downy mildew incidence has been reported sporadically in California. The hop version of downy mildew, which has similar epidemiology as the grape form, does quite well in eastern Washington, he noted.
"We do have the environmental conditions, we do have the host," he said. "I just don’t think we have the pathogen yet."
Clean plant material is a strong defense against the disease. He attributes the grape industry’s efforts to develop the Northwest Grape Foundation Service as part of an industry plant improvement program as the reason that the pathogen has not hitchhiked into the state.
"Two years ago, I would have said not to worry about it," Grove stated. "But since doing climate work and looking at future weather trends, now I’m worried about it.
"If it ever gets well established in California or Oregon, it could very easily move into Washington. And if it gets in, it would totally change our pest and disease management. Once it gets established, nothing will really manage or minimize it other than a really expensive fungicide program."