Continuing Education and
Communication Services
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
8/6/01
Contacts:
Sara Helland, Department of Plant Pathology, (515) 294-0589,
sarathom@iastate.edu
Del Marks, Continuing Education and Communication Services,
(515) 294-9807, dmarks@iastate.edu
Yard and Garden Column for the Week Beginning Aug. 10
Musing about Mildew
By Sara Helland
Research Associate
Department of Plant Pathology
Iowa State University Extension
Have you noticed a white dusting on leaves in your lawn, vegetable garden or flowerbed recently? Despite our crazy Iowa weather, a snow in August is unlikely, so that white stuff must be something else -- perhaps downy mildew or powdery mildew. These two fungal diseases attack a wide variety of Iowa vegetables, fruits, trees, grasses, flowers and shrubs. Although they have similar names, they are very different fungi. They often have different symptoms and require different methods to control successfully.
The fungus that causes downy mildew thrives in cool temperatures with high humidity and is especially fond of young shoots and leaves with water on them. Downy mildew gets its name from the fluffy, gray to white mat that forms on the underside of leaves and on fruits when the disease is in full swing. Surely a vigilant gardener like you will notice your unhealthy plants before this occurs by scouting for the following downy mildew symptoms: brown patches on strawberry stems and leaves; yellow to brown spots on cucumber, squash and melon leaves; and brownish spots on bean, onion and snapdragon stalks and leaves. In turfgrass, the fungus weakens and yellows grass blades, but it seldom kills the plants. The best control for downy mildew is through the use of resistant varieties and irrigating with a soaker hose rather than a sprinkler.
If you take precautions and still find symptoms of downy mildew on your plants, you can sympathize with French grape growers of the past. The most notorious downy mildew epidemic occurred in France in 1885, after the United States gave the French a gift of cold-hardy, downy mildew-tolerant native American grape vines. Unfortunately, the majority of native French vines (and the wine industry) were devastated by the downy mildew fungus that was carried from these "gift" vines to the vulnerable French vines. All was not lost, however, because this epidemic resulted in the discovery of Bordeaux mixture, the first useful fungicide against downy mildew.
Powdery mildew is an entirely different creature than downy mildew. It is more common than downy mildew and prefers a different habitat. Unlike downy mildew, powdery mildew does not need rain to cause pain. Powdery mildew is especially worrisome in warm, dry weather, but also will survive on warm, humid days. It affects young and old leaves, stems and fruit in a great variety of plants. These insidious fungi will infect dogwoods, lilacs, phlox, vegetables and fruit trees, and will even invade your house to infect ivies and African violets. The good (or less bad) news is that although powdery mildew is widespread, different plants are generally attacked by different species of powdery mildew fungi. Therefore the fungus in your lawn can't climb up into your dogwood tree to cause problems, or vice versa.
Cucumbers, squash and melons can be clobbered by powdery mildew infection. The leaves will develop yellow spots and may begin to look like someone sprinkled flour over them. They will then wither and die, exposing the fruits to sunscald and reducing yield. Powdery mildew is especially happy on older leaves in shady areas of the squash patch, so you may want to look under the canopy when scouting for this sneaky disease. Tomatoes will exhibit symptoms similar to melons, and strawberries will exhibit a white powdery growth on the underside of the leaves. The best method for control of the fungus in cucurbits and strawberries is through resistant varieties. However, threatened tomatoes must be sprayed with a fungicide, since no resistant varieties exist. Make sure you use a fungicide labeled against powdery mildew, as many fungicides have no impact on these fungi.
Your favorite rose varieties are also especially susceptible to powdery mildew. Their leaves will curl up after growing red blisters with a gray, powdery coating. Apple and pear trees exhibit similar symptoms, with the added bonus of brown webbing on the fruit's skin. The especially sinister characteristic of the powdery mildew fungi that infect woody plants, like roses, lilacs, apples, etc., is that they will survive in the shoot tips over the winter. If you notice powdery mildew in buds or on stems, these branches should be pruned off and discarded -- far away from your tree or shrub! As for lawns, powdery mildew will grow in shady areas of the yard with poor air circulation and high humidity. Infected grass will have a white powdery dusting. Although the fungus will rarely kill the grass, it will weaken it enough to make it vulnerable to other stresses. To control this disease, plant shade-tolerant turfgrass varieties or remove over-hanging trees and shrubs, mow high and don't over-fertilize susceptible areas.
Next time you find a white dusting on your plants, you might pause to ponder its identity. It might be downy mildew, a cool, wet-weather-loving fungus that greatly enjoyed the past spring. Or, it might be warm, dry-weather-loving powdery mildew, which is quite happy with our current weather conditions. It also might be the snow mirage of an Iowa gardener tired of the hot, humid weather and longing for the restful winter months. If you are uncertain about which type of fungus is attacking your plant, an ISU extension plant pathologist will be happy to help. The Plant Disease Clinic at ISU can be reached at (515) 294-0580. They cannot, however, provide therapy for hot, tired gardeners.
ml: isugarden
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