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Garden Q&A: These strategies may help in battle against powdery mildew

Saturday, October 07, 2000

By Sandy Feather, Penn State Cooperative Extension Agent

Q. I have a lilac in my yard that gets sun most of the day. It did not bloom this spring. I noticed that it has a powdery white substance on the leaves. I had planted zucchini and cucumber in the same general area and have noticed that they also have a white powdery substance on the leaves, only it is in the form of a circle. Would you have any idea what the fungus(?) is, how I could get rid of it and what may be causing it to form? I would appreciate any help you could offer.


Send questions to Sandy Feather by email at or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208. Due to volume of mail, personal replies cannot be provided.


A. The powdery white substance you are seeing is a fungal disease called powdery mildew. Powdery mildew affects a wide range of plants, including apple and crabapple, azalea, black-eyed Susan, calendula, cucurbits, flowering dogwood, common lilac, monarda, phlox, rose, sycamore, verbena and zinnia.

Certain varieties of Kentucky bluegrass are susceptible as well, particularly when they are grown in shade. There are different strains of the fungus that causes powdery mildew. The powdery mildew on your lilac did not spread to your cucumbers and zucchini. Another strain caused the disease on those related plants. Although we usually think of powdery mildew as a dry weather problem (water on the foliage actually inhibits the germination of the fungal spores), the high humidity we have had all summer has created a very favorable environment for its development. I have seen powdery mildew on plants that I do not usually think of as susceptible to it.

Although it is not considered life-threatening to most plants, powdery mildew does cause leaves to drop prematurely and makes them unsightly. It can kill cucumber and zucchini plants outright. Even if it does not kill them, powdery mildew reduces the productivity of susceptible vegetables dramatically. Your best defense against powdery mildew is to grow resistant varieties when they are available. Although they can get the disease when conditions are extremely favorable, that does not happen often. When choosing vegetable varieties, it is wise to select those with multiple disease resistance. Seed catalogs are an excellent source for this information.

Cultural controls include spacing the plants carefully, pruning and thinning and controlling weeds to allow good air circulation. Plants grown in full sun are not affected as severely as those grown in shade. Avoid overhead irrigation late in the day or at night because it elevates the humidity level around susceptible plants. It is best to water early in the morning.

Removing infected leaves where practical helps reduce the amount of fungal spores present to continue the infection. Fungicide applications can be made preventatively to keep powdery mildew from becoming established on susceptible plants. Chlorothalonil (Daconil 2787) is labeled to control powdery mildew on vegetables.

Common lilac (Syringa vulgaris) is very susceptible to powdery mildew. Control is not necessary unless the plant's appearance bothers you. In that case, use an anti-transparent such as Wilt-Pruf. Begin applications before the disease appears in June and repeat at 30-day intervals. Reapply if the disease appears before the 30-day interval is over. Triforine (Funginex) is also labeled to control powdery mildew on lilacs. Other species of lilacs such as Meyer lilac (Syringa meyeri), littleleaf lilac (Syringa microphylla) and Manchurian lilac (Syringa patula) are resistant.

It is unlikely that powdery mildew prevented your lilac from blooming. We did have a late frost or two that may have killed those buds before they opened. Or perhaps you pruned your lilac at the wrong time of year. Lilacs bloom from buds produced the previous season. This is often referred to as blooming on old wood. Plants that bloom on old wood should be pruned as soon as they finish flowering to avoid removing mature flower buds. Finally, if you applied a fertilizer too high in nitrogen, it may have caused your lilac to put on vegetative growth at the expense of flowering.

Q. It seems to me that many professional products for lawn care are more effective than the ones available for home use. Since those products are not all restricted use, why aren't they available to home gardeners?

A. Professional-use products are usually stronger than those available to home gardeners. This makes the concentrated form more dangerous to handle when mixing. You must have the appropriate safety equipment -- goggles, chemical-resistant aprons, gloves, clothing and boots -- to work with these products.

A big reason that many of these products are not available for home use is that the equipment home gardeners use to apply pesticides is not as precise as that used by lawn-care professionals. Many products, especially some of the newer ones, have very low use rates. The equipment home gardeners use is not exact enough to apply products at very low rates. Overapplication of these products can damage your lawn; expose you, your family and neighbors to dangerous levels of products that are safe when used as directed; and pollute the environment.

Also, it is not cost-effective for most people to purchase commercial-grade equipment just to use on their own lawn. Lawn-care professionals must calibrate their equipment to avoid applying more pesticide than is required to control the insect, weed or disease being treated. Anyone who applies pesticides for pay must have a business license and liability insurance, as well as a commercial applicator's license. There are enforceable sanctions for those applicators who do not calibrate their equipment properly or who make improper pesticide applications. It would be impossible for the Pennsylvania Department of Agriculture to police pesticide applications by home gardeners since they cannot track them through pesticide license registrations.

The products available for home use are effective when used as directed. If you are having trouble controlling difficult weeds such as ground ivy or clover, try using a liquid broadleaf herbicide rather than a granular weed-and-feed. These weeds usually require two or three applications to get them under control, and you should not apply that much fertilizer. The liquid formulation will give you more flexibility. Follow label directions as to how often it is safe to apply the product you are using.

If you are having trouble controlling lawn insects such as white grubs, be sure your lawn does not have a thick layer of thatch that prevents the insecticide from getting down to the roots where the grubs are feeding. If it does, take steps to eliminate the thatch so the insecticide can get to the problem (see my column from Sept. 16 for more information on thatch).

Also, be sure you are following good cultural practices to encourage a healthy lawn. Maintain a mowing height of 2 to 3 inches. This encourages a deep root system and shades out germinating weed seeds. Water deeply and infrequently when we are not receiving sufficient rain to encourage a deep root system and provide enough water to keep your lawn growing well.

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