Powdery and Downy Mildews

by R.K. Horst


Powdery Mildew

Family: Erysiphaceae (species of Erysiphe, Sphaerotheca,Phyllactinia, Microsphaera, Podosphaera and Uncinula)

Characteristics: Light, powdery spore growth covering shoots, leaf surfaces and sometimes flowers

Range: Throughout North America, but more prevalent in semiarid regions

Crops affected: Beans, cucurbits, lettuce, peas, roses, phlox, lilacs, grasses, grapes, small fruits and fruit trees, euonymus

Controls: Garden cleanup, resistant varieties, spacing plants for good air movement, pruning infected tissue, fungicide sprays

Powdery mildew is one of the oldest plant diseases known to man -- the ancient Greeks described it, and several books in the Old Testament speak of its effects. Probably every gardener, unless they live at the Arctic or in high mountain regions, has witnessed its symptoms: a delicate webbing or thick felt that covers leaf surfaces and stems, giving the appearance that they've been dusted with flour. Leaves may turn yellow, die and fall off, reducing yields and weakening -- or killing -- the plant if infection is severe. On grapes and fruit trees, the disease is especially serious, attacking new growth -- buds, shoots, flowers and just-forming fruit -- as well as leaves and stems. Subsequent growth is dwarfed and distorted, often covered with the mildew; young fruit develop scars and corky skin.

Most garden plants are susceptible to powdery mildews (only pine trees and their relatives, in fact, are totally immune). However, different species and genera of the fungus attack different plant species to varying degrees. Powdery mildew is especially serious on cucumber, squash, rose, phlox and lilac. However, grasses, beans, peas, lettuces, grapes, small fruit trees and euonymus are also quite susceptible. The most common genera found around and in gardens are Erysiphe, Sphaerotheca and Podosphaera.

All powdery mildew fungi infect the plant tissue by sending hollow tubes from a spore on the leaf surface into the plant to suck out nutrients. The growth then forms in a radius from the initial point of inoculation, and after about four days, spores start forming, produced in chains on upright stalks. The spores are what actually give the powdery effect, and when they become airborne they can spread the fungus rapidly. On perennial hosts such as roses, fruit trees and soft fruits, the mildew may survive from season to season in infected buds; special spores are also produced to enable the fungus to overwinter in lettuce, peas and certain other crops. Unlike most fungi, powdery mildew spores do not need water for germination. Some species require high humidity, but the moisture provided at the leaf surface when cold nights change to warm days or when plants are grown in crowded, low or shady locations without sufficient air circulation is sufficient to initiate an epidemic.

The most favorable conditions for powdery mildew are nighttime temperatures of 60F and relative humidity of 90 to 99%, with 80F temps and 40 to 79% relative humidity during the days -- conditions common in late spring and early fall. Since the spores are sensitive to extreme heat and direct sunlight, plants in partial shade are especially vulnerable.


Downy Mildew

Family: Peronosporaceae (species of Bremia, Peronospora, Plasmopera, and Pseudoperonspora)

Characteristics: Tops of leaves have yellow blotches, while the undersides develop a faint frost of gray, white, blue or violet fuzz

Range: Cool, moist climates such as in the eastern and northern U.S. and into Canada

Crops affected:Lettuce, onions, spinach, strawberries, cabbage family crops, peas, rhubarb, beets, lima beans, carrots, grapes, cucumbers, melons, roses, snapdragons and marigolds

Controls: Garden sanitation, resistant varieties, spacing plants for good air movement, pruning infected tissue, fungicide sprays

Downy mildew differs from its powdery cousin in a number of significant ways. Unlike powdery mildew, which appears on both sides of the leaf surface, downy mildew produces spores primarily on the undersides of leaves. The tops of leaves will have yellow blotches, while the undersides develop a faint frost of gray, white, blue or violet fuzz. Fruits of infected plants are often small and bitter, if they even make it to harvest.

As with the powdery mildews, different genera and species of the downy mildew fungi attack different plant species. However, all downy mildew fungi are in the family Peronosporaceae, with the most common genera found in and around the garden being Bremia, Peronospora, Plasmopara and Pseudoperonospora.

Downy mildews like it cool and moist, making them most troublesome in the eastern and northern areas of the U.S. Under humid, cool conditions, spores appear copiously on the lower surfaces of leaves, growing in tree-like formation on branched fruiting structures, unlike the powdery mildew spores, which are produced in chains. In the presence of water from a recent rain or heavy fog, the spores will germinate within four hours; sporulation on leaf surfaces may occur in three days under ideal conditions of 65F temperatures. Below 40F the spores won't germinate, and they're killed by exposure to 80F temperatures for 24 hours. Dry winds and warm, clear days inhibit spore production. Roses, for example, are unaffected by downy mildew when humidity is less than 85%. Unlike powdery mildew spores, which are spread by the wind, downy mildew is spread by splashing water.


Controls

Planting resistant varieties (designated as PM or DM) and following good cultural practices are the first line of defense against both mildews. Provide plenty of space and sunshine for good air movement. Remove all leaves and debris that might contain overwintering spores. For powdery mildew, avoid shaded areas and excess fertilizer (which produces succulent, susceptible growth). Since the spores cannot germinate in wet conditions, frequent water sprays or overhead sprinkling systems may help reduce the incidence of infection. Prune and train to promote good air circulation and remove any infected shoots, buds and branches that might harbor the disease from season to season. To avoid an outbreak of downy mildew, allow plants to dry between irrigations and keep leaves as dry as possible. Adjusting planting times to avoid the peak infection periods of early spring and late fall may also help.

Should cultural practices fail and the infestation is severe, you may need to spray. Sulfur, applied at two-week intervals beginning when buds start to open and until small fruits are present, has long been used to control powdery mildew on apples; however, timing is critical to control, and the fungicide may cause damage if applied when temperatures exceed 90F. Sulfur may also cause injury to foliage and fruit of certain types of muskmelons.

Other fungicides that can be used on crops include Safer Garden Fungicide-Miticide and Karathane. Piprone is also used for powdery mildew control, but is registered for use only on ornamentals. Manzate is another fungicide currently used on downy mildew. Unfortunately, once you start using these products and the mildews become established in your garden, frequent applications -- every seven to 10 days -- are often necessary. And that brings a host of environmental concerns.

However, environmentally friendly sprays may soon beavailable. My present work at Cornell University has been to investigate the effects of biocompatible chemicals such as bicarbonates (common baking soda) and horticultural oils on controlling mildews. So far we've found that sodium or potassium bicarbonate in combination with Safer Sunspray effectively controls powdery mildew on roses, cucurbits, herbs, zinnias and chrysanthemums. The most effective treatment has been a weekly spray of a 0.5% aqueous solution (three teaspoons per gallon of water) of either sodium or potassium bicarbonate (we used the familiar Arm and Hammer brand baking soda), combined with a 0.5% solution of Safer Sunspray horticultural oil (2.5 tablespoons per gallon of water). Registration is underway to approve bicarbonates for the control of mildews, but as yet they are not labeled for that use. However, they promise to have an exciting impact on control of these diseases in the future.


R. K. Horst has been an professor of plant pathology at Cornell University since 1968, focusing his work primarily on diseases of ornamental plants. He is author of Westcott's Plant Diseases Handbook, 4th and 5th editions (Van Nostrand Reinhold Publishing Co.) and the Compendium of Rose Diseases (published by the American Phytopathological Society, St. Paul, MN; 1983.


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