Powdery Mildew is native to eastern North America, but has since spread throughout the Americas and Europe. It is one of the most serious grape fungal diseases and can have great economic impact. Powdery Mildew can cause cluster loss, increased susceptibility to other fungus diseases and limit photosynthesis which can result in reduced brix and winter hardiness that can lead to vine death.
On leaves and berries, the fungus appears as a white/grey powder and on canes it appears as dark brown patches that remain visible on dormant cane. Later stages of the disease can result in cracking and shriveling of the berries.
There are two infection stages for Powdery Mildew, the ascospore stage in the spring that cause primary infection, and the conidial spore stage, which causes secondary infections. Secondary infections are the result of the primary ascospore infection. Three factors must be present to trigger the initial ascospore stage: vineyard temperatures exceeding 50 degrees, shoots have grown at least two inches and leaves are moist for more than 12 hours per day. Once initial infection occurs, ideal temperatures for growth of the fungus are between 70° and 85°F. Temperatures above 95° for 12 continuous hours or longer cause the fungus to stop growing. Leaf wetness is not required for continued growth. Late in the summer, the infected areas produce small black bodies called cleistothecia that mature and survive the winter in cracks on the bark. In spring, when the conditions are met, these cleistothecia burst and release ascospores into the wind that begin the cycle anew.
Control during the ascospore stage is critical to minimizing conidia outbreak later in the season. Spray management programs should be at their peak from prebloom (as early as 1 to 2 inches of shoot growth) through fruit set. When multiple rain events (four+) occur during this critical springtime period, powdery mildew can become a serious problem. The fungus overwinters on bark, so it's possible that cane pruning vs. spur pruning could reduce the risk of disease.