Are you seeing downy or powdery mildew on grape clusters?
It’s easy to imagine that a cluster infected with powdery mildew would look powdery in appearance; however, downy mildew cluster infections can also look somewhat powdery, so how can growers distinguish the two from each other? The above photos exemplify both downy mildew (Plasmopara viticola) (1) and powdery mildew (Uncinula necator) (2) infections on grape clusters. At a glance, they look very similar, but the powdery mildew on fruit clusters has an appearance similar to an actual powder whereas downy mildew may look more like granulated sugar or wet powdered sugar. On a leaf surface, downy symptoms are yellow to light green spots or “oil spots” (upperside) and white powder (underside), while powdery symptoms tend to be white/grayish powdery coating (upperside).
In regards to management, both diseases are best managed with a preventative approach using appropriate fungicides timed throughout the growing season. In regards to downy mildew, one positive note is that clusters tend to become more resistant to the disease as they develop. During circumstances where cluster infections occur, removing infected clusters from the vine(s), out of the vineyard, and destroying them is a good way to reduce inoculum and mitigate issues down the road. Leaving mummified fruits on the vineyard floor can help encourage decomposition of overwintering spores, but will carry any inoculum that does not break down, leading to increased disease pressure. Refer to the Midwest Fruit Pest Management Guide for information related to chemical management for both downy and powdery mildew.
Additional reading:
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- For more information on powdery mildew, refer to the July 3rd, 2024 fruit update.
- For more information on downy mildew, refer to the June 18th, 2024 fruit update.
Russeting and cracking on grape berries
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Russeting in a physiological response to micro-cracks forming on the outer epidermal fruit layer, known as the cuticle. The russeting itself is composed of a number of cells for the purpose of sealing microcrack wounds to protect the fruit. Some apple fruit cultivars are reputedly known for and even appreciated for their russeting, but it is not usually desirable in grapes. Russeting on grapes often occurs with high-humidity conditions, with fast changes in grapevine water uptake, and high winds can be an additional factor leading to microcrack formation. Grapes with a high degree of russeting on the skin may impact wine quality by disrupting the contribution from compounds located in the grape skins, especially for red wine grapes, which are traditionally fermented with their skins on. However, for the most part, minor russeting on grapes is not a major concern in regards to fruit quality and most growers can expect to tolerate the condition. — By Madeline Wimmer, UMN Fruit Production Extension Educator with contributions from Soon Li Teh and Drew Horton from UMN
Article By
Madeline Wimmer
Fruit Production Extension Educator, University of Minnesota Extension
Madeline Wimmer is a fruit production extension educator with University of Minnesota Extension, based at the regional office in Rochester, where she works with growers across the state on apples, grapes, honeyberries and strawberries. She studied at the University of Wisconsin-Madison, working with cold-climate wine grapes during her graduate years, and has worked in vineyards in Wisconsin and Missouri. Her programming focuses on building the confidence of new growers producing fruit in Minnesota conditions.