Michelle Grabowski, University of Minnesota Extension Educator
Two types of mildew can be seen on pumpkins, squash and cucumbers in Minnesota this August. Powdery mildew and downy mildew have both been reported. Despite the similarities in their names, these two diseases are caused by very different pathogens and have very different symptoms and control strategies.
Powdery mildew is caused by the fungus Podosphaera xanthii. As its name implies, this disease can be recognized by the powdery white fungal growth on leaves and stems. Infections often start as a few white powdery spots but can quickly grow to cover the entire leaf in a layer of powdery white fungal spores and mycelia.
Although the powdery mildew fungi do not commonly attack squash fruit, yield of infected plants can be reduced because infected plants have less energy to invest in fruit. Often severely infected plants produce fewer and smaller squash, cucumbers or pumpkins.
Spores of the powdery mildew fungus blow in on the wind and are impossible to keep out of the garden. The best way to prevent powdery mildew is by planting disease resistant varieties. Many powdery mildew resistant varieties of pumpkin, squash and cucumber are available. Home gardeners can also use a fungicide with sulfur as the active ingredient to protect susceptible plants. In order for fungicides to be effective, they must be applied when the first small spot of powdery mildew is observed and the fungicide must be sprayed to cover both the upper and lower leaf surfaces. Perhaps the simplest solution for the home gardener is to include a few extra plants in the garden, to make up for the yield lost to powdery mildew.
Downy mildew is caused by Pseudoperonospora cubensis, a fungus like organism, often called a water mold because it thrives in moist conditions. This disease can be recognized by the almost square yellow to brown spots that appear on leaves. Leaves infected with Downy Mildew often look like a patchwork quilt of yellows, greens and brown. On the underside of infected leaves, a purplish gray fuzzy mold can be seen. Under warm wet conditions, downy mildew can spread rapidly. Many leaf spots grow together, turning infected leaves brown so quickly; they almost appear to have been hit by frost.
The downy mildew pathogen cannot survive Minnesota's harsh winters. Each year new spores must move into the state on moist wind from areas to the south. This means that gardeners experiencing problems with downy mildew this year may not see it at all next year.
Although there are several varieties of cucumber that are resistant to downy mildew, gardeners may not be able to find squash or pumpkin varieties with good resistance. Once downy mildew has started an infection, it is very difficult to control. In a home garden, the best solution is to remove infected plants as soon as symptoms appear to reduce the spread of the disease to other cucurbits in the garden.