BELMONT, N.Y. — The dog-days of July and August challenge the gardener with a variety of new insects and diseases. Japanese beetles have started munching on beans, grapes, raspberries and other crops, and the Alternaria early-blight fungus is starting to show up on tomatoes. Plants from lilacs to squashes have begun to look like someone started sprinkling confectionary sugar all over the leaves. That white stuff signals the appearance of yet another group of fungal pathogens, the powdery mildews.
Weather conditions in the latter part of the summer are ideal for these fungi. Unlike most other microbes, powdery mildews don’t like standing water on leaf surfaces, which prevents their germination and growth, but they do prefer high humidity and warm temperature without much rainfall. Another factor is that antagonistic bacteria and fungi, which normally would keep the mildew growth in check, do need a film of moisture in which to grow, and this is often not available in the typically rain-starved months of the late summer.
Another unique feature of the powdery mildews is that these fungi are host-specific. Thus, the species that cause the leaf disease on roses is not the same as the one that infect green beans or squashes or maples or willows. Lilacs have their own powdery mildew pathogen, which is not the same genus and species as the one that is found on grasses. In fact, there are dozens of different kinds of powdery mildews, and they all need their own specific living host to survive, grow and reproduce.
The white powdery looking stuff on leaf surfaces is the body of the fungus, composed of millions of thread-like cells called hyphae. These send tiny penetrating structures into living host cells to gain access to the nutrients in the cell sap. Scientists call these little drills “haustoria,” from the Latin word that means “to take in.”
The fungus makes two kinds of spores. The asexual spores produced during the summer are used to re-infect the same plants, or to spread to nearby leaf surfaces by physical contact or via splashing rain. The other spores, those that are the products of sexual reproduction, overwinter in specialized pouched survival structures inside bowl-shaped protective covers.
Knowing the lifecycle of when and how the spores are produced is important in controlling the powdery mildew pathogens. Although a number of commercial or home-made fungicides are available, the best control methods may be non-chemical. These include resistant varieties of plants, removal of infected plants, pruning to improve air circulation, crop rotation, and sanitation. Not all of these methods are applicable for perennials, like lilacs or roses. Obviously, once established, shrubs are not going to be moved to be rotated for other plants. In such cases, the initial choice of a mildew-resistant cultivar may be of paramount importance, along with proper pruning to increase the air flow and cut down on stagnant, humid air pockets. But annual crops, like beans, squashes or cucumbers may be moved to another part of the garden to lessen the chances of infection next year.
For all plants susceptible to powdery mildew infection, sanitation is an important control method. Raking up infected leaves and pruned branches of ornamental perennials, or complete removal of annual plant debris, will reduce the amount of overwintering inoculum represented by the sexual spore packets. However, composting of infected plant debris is not recommended because the average compost heap does not get hot enough to kill the spores.
For my garden, the appearance of powdery mildew on squashes, beans and cucumber plants usually comes late enough in the season that I have already harvested nearly all the crop I can handle. Therefore, I don’t worry too much about these pathogens. On lilacs, some of the shaded foliage is pretty badly parasitized, but these leaves typically curl up and fall off without further damage to the overall health of the bush. I don’t grow roses, but many rose cultivars are significantly damaged by powdery mildew, and those plants need to be treated with fungicides. The fruit that I really would like to keep this pathogen away from is apple. When I bought the house I currently live in I made an attempt to rehabilitate some old, gnarly apple trees. Like many former orchard trees, my apples are ancient and have been neglected for a long time. Apple scab and powdery mildew are the two biggest fungal problems, but there are a number of insects that also munch on the fruit. I will probably start over with grafted, disease-resistant trees at some point, but I know that for commercial apple growers scab and mildew are two of the worst problems requiring chemical treatment.
Commercial apple producers watch weather events like hawks, and use a combination of integrated pest management tools, such as scouting for early symptoms of disease, correct timing of spraying of pesticides, judicious use of pruning, stringent sanitation, and proper timing of fertilizer applications. Many of those techniques can be used by home gardeners, as well, to reduce the damage done by the powdery mildew fungi.
–Steven Jakobi, Master Gardener volunteer
Cornell Cooperative Extension of Allegany County
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