Description: Many fungi fall under the category of powdery mildew. Some plants can withstand being infected and not have serious injury. Begonias can die from a mild infection. Some rose varieties are affected more than others. Crepe Myrtles are often infected with powdery mildew because they are topped, leading to a crowded crown with minimal air circulation.
Powdery mildew looks like a dusty white or grey coating on leaf surfaces or other plant parts. It can usually be removed by rubbing it. Dusty plants have been confused as having powdery mildew. However, powdery mildew begins as powdery white spots. As the spots expand the entire surface is covered.
Powdery mildew is host specific. This means that different powdery mildew fungi infect different plants. The powdery mildew on your lilac will not spread to your roses.
Fungus favors periods of high relative humidity. Close spacing of plants and shade also promote fungus.
Symptoms: Powdery mildews cause stunting and distortion of leaves, buds, growing tips and fruit. Yellowing of leaves and death of tissue may cause premature leaf drop. Nutrients are removed from the plant by the fungus and this can cause a general decline in growth and vigor. The fungi causing powdery mildew are spread by spores produced in the white patches. The wind carries it to other parts of the plant or other plants entirely. During the winter the fungus will survive on infected plant parts and in debris such as fallen leaves. Small black dots appear within the white powdery patches. The next spring these dots are launched into the air and carried by air currents to new plants.
Action: There are several effective fungicides available that treat most fungi. Check labels to make sure the plant you are treating is listed. Other things to do to prevent infection include gathering up and destroying fallen leaves in autumn. Improve air circulation by thinning and pruning (not topping). Powdery mildew loves new growth, delay fertilization until it is under control. Don’t water from above. Do buy resistant varieties. Begin fungicide treatments when the first white patch is noticed. Most fungicides need repeat applications every 7-14 days, for continuous protection.
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