Issue 7, July 15, 2019

Powdery Mildews on Ornamentals

Much of Illinois has settled into a hot and humid weather pattern conducive to powdery mildew infections. The six common genera of powdery mildew fungi in the Midwest all prefer warm, humid days. The spores germinate on foliage when the relative humidity is 23% to 99% but not in free moisture (rain). Powdery mildew is a common fungal disease of many annuals, perennials,  shrubs, and trees. Some species of powdery mildew fungi infect only a few closely related host plants, while others attack many genera of plants. This disease can spread quickly over a host plant; it doesn’t normally kill one.

Symptoms of powdery mildew are white or dusty gray patches on the leaves, shoots, buds, flowers or stems. This mildew is composed of threadlike mycelium and asexual spores of the fungus. Powdery mildew fungi overwinter on plant tissue and dormant buds. The spores are released in the spring damp weather and move to uninfected tissue in water or wind. In some cases the growth is superficial, and in other cases the leaves become distorted dwarfed and discolored.  The severity of the symptoms depends on the host species, age of the tissue infected, environmental conditions, and the fungus involved. New growth on plants is more sensitive than older leaves. 

Powdery mildew on common lilac

Managing powdery mildew infections can be done by several means.  It is important to provide conditions for adequate air flow. Pruning and thinning stands of plants, or branches will allow for better air movement. Watering in the morning or early part of the day to promote rapid drying will also help.  Resistant varieties are the first means of disease control but unfortunately are not always available. Fungicides are also available and should be utilized at the first sign of the disease. Once the disease has become widespread, it cannot be controlled in that year. Finally in the fall when cleaning up plant debris. Remove and destroy all infected plant parts. 

Maria Turner

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