Powdery mildew is a common fungal disease of numerous ornamental plants including dogwood, rose, phlox and many bedding plants.
Unlike other foliar fungal diseases, which are normally favored by wet leaf surfaces, powdery mildew is favored by dry conditions.
In fact, powdery mildew is inhibited by moisture on leaves. Powdery mildew infection is greatest under high humidity and warm day and cool night temperatures.
It generally does not cause serious damage to its host, but it can interfere with the hosts’ normal growth and reproduction and if severe enough can cause significant plant decline.
The powdery mildew fungus grows over the surface of tender leaf and stem tissues where it produces columns of spores that give the leaf a white to gray appearance.
The fungus then sends structures into the cells of the leaf to act as anchors and absorb water and nutrients from the plant. As a result of infection, the plant slowly declines. Other symptoms of infection may include dwarfing, distortion, chlorosis, premature senescence and browning of leaves, and blemished or aborted fruits and flowers.
Throughout the growing season new infections may develop on plants that continue to produce new, succulent shoots.
Severe pruning and cutting back of plants to promote new growth actually favors powdery mildew infection. Spores are dispersed by air currents in greatest number during the middle of the day.
Because powdery mildew can cause premature defoliation of numerous plants, it causes alarm for gardeners. This is currently occurring on dogwoods throughout the state.
Often, infected plants will produce new growth. This new growth may or may not become infected depending upon environmental conditions.
Powdery mildew is most common on plants growing in shady areas. Since the disease is favored by high humidity, increasing air circulation and light penetration can reduce infection. Overgrown landscapes can be selectively pruned to accomplish this.
In greenhouses and homes, increasing ventilation and light intensity can reduce disease.
Chemical control is seldom recommended in the home landscape. When control is necessary, chlorothalonil (Daconil) is generally used.
Fungicides act as a preventative. They prevent infections from occurring, they do not get rid of existing infections. The white to gray powdery growth on the leaf will remain after spraying.
In other news: The Dawson County Produce Market is still open each Wednesday and Saturday. The summer of 2010 has been a very good year.
For most gardeners, the market has been excellent.
Clark Beusse is the Dawson County extension agent. For more information, call (706) 265-2442.