Powdery mildew is alive and thriving

Powdery mildew is a dusty white substance that often turns gray, it may even be mistaken for dust covering the leaves. Numerous fungi can be responsible for this disease. Powdery mildew begins as small white spots that can expand until the entire surface of the leaf is covered.

Symptoms usually begin late in the growing season on outdoor plants, as the weather begins to cool and becomes more humid. Shade, densely planted vegetation and closely growing plants promote an environment favorable to the fungi. Indoors, powdery mildew may occur any time of the year.

Open doors and windows allow spores to drift into the home and onto houseplants. The rate of spread and the development of the fungi is dependent on the relative humidity and temperature of the environment.

The fungus overwinters on infected plant debris in small black structures that resist winter conditions. In the spring, these structures shoot the spores into the air to drift on wind currents and to finally settle on leaves where the cycle begins anew. Indoors, powdery mildew can continue to thrive year-round.

Injuries to the plants by powdery mildew include stunting and distortion of fruit, leaves, stems and buds. The fungi may even cause the death of some plants, particularly begonia, even from a mild infection. Soon after infection, begonias will suffer leaf drop and poor growth as the underlying tissue dies. Lilacs however, are often infected late in the season and suffer little permanent harm and will survive year after year in spite of the disease.

If the disease continues to be a recurring problem, you may wish to switch to a more resistant variety or begin a spray schedule when the first signs of infection appear.

Cultural practices can be beneficial in controlling powdery mildew.

• Purchase disease-free plants from reputable nurseries, greenhouses and garden centers.

• Rake and destroy all dead leaves. Do not compost diseased material.

• Prune and destroy all infected growing tips and dead material.

• Resist putting susceptible plants in damp, shady locations.

• Do not work in or handle the plants when the leaves are wet.

• Avoid watering the foliage, use a soaker hose and water in the morning if possible.

The label is the law

There are several fungicides registered for use in Alaska, though they are used for different sites and plants.

Read the label carefully to ensure the fungicide will meet your needs. Some of the least toxic fungicides may contain the active ingredient; neem oil, sulfur or potassium bicarbonate.

For additional information, contact the integrated pest management (ipm) tech at your local Cooperative Extension Service.

Pam Compton is the IPM tech at the Mat-Su/Copper River District in Palmer.