By Amanda Sears
Powdery mildew is one of the most easily recognizable diseases of landscape and garden plants. While most fungal pathogens are favored by wet weather, powdery mildew pathogens favor high humidity but not wet conditions. Shady areas, areas with poor air circulation, and low areas that trap damp air are ideal environments for disease development.
A wide range of plants are susceptible to powdery mildew: annuals, perennials, shrubs, ornamental and fruit trees, small fruit, and vegetables. However, powdery mildews are host specific, which means they can only survive on one host plant. So if your lilac has powdery mildew, that strain of the pathogen will not affect any other type of plant. Likewise the powdery mildew on your zinnias will not infect your cucumbers.
Typical symptoms of powdery mildew include dusty fungal growth on leaves and on young succulent plant tissue. This disease is usually observed on the upper side of the leaf, although it can also affect the bottoms of leaves as well. Powdery mildew may occur as isolated blotches or cover entire leaves, stems, buds, or flowers. Early infections can lead to leaf stunting, curling, or other deformation. Infected fruit may become disfigured or fall prematurely. In the fall, small fruiting structures that resemble pepper flakes may be visible to the naked eye. These structures contain overwintering spores that serve as inoculum for the following year.
Management of powdery mildew begins with prevention. Plants should be properly spaced and thinned for improved air circulation and more rapid leaf drying. Resistant cultivars are available for crabapple, dogwood, phlox, zinnia, cucurbits, and several other plants and are an excellent means of disease prevention. Also, avoid late applications of nitrogen fertilizers, as this will promote the production of succulent tissue that is more susceptible to infection. As the disease begins to show, remove any infected plant parts, such as leaves. Do not compost material that has shown signs of powdery mildew because often the temperature of the compost is not hot enough to kill this pathogen. Fungicides usually are not warranted when cultural practices are implemented.
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