Water Sprays, Sulfur Can Reduce Powdery Mildew

Q: My fibrous begonia leaves are all moldy. It's spreading.

A: Late summer in the Pacific Northwest produces lots of gardener frustration from mildew problems. We've recently received questions about similar symptoms on lilacs, pumpkin leaves and deciduous azaleas. These fungal problems infest a wide range of flowering plants. Infected plants typically have a grayish-white powdery coating over leaves and shoots. Leaves may be distorted, curled or twisted.

Powdery mildews develop under humid conditions when days are warm and nights are cool (such as late summer). The disease can be worse if the planting is crowded, and it will show up more on plants that have succulent new leaves from being over-watered and fertilized.

An odd fact about powdery mildew is that, while it likes damp conditions, it doesn't survive in a film of water. A hard, directed spray of water can often reduce the amount of mildew by washing the spores off, so you may want to try washing your plants with water after removing the worst affected leaves. This won't help return the plant to full health, but you'll improve the appearance of the plants.

Most fungal problems aren't controllable once you see the infection. Powdery mildew, however, because it is external to the plant, can be controlled by sulfur applications. The spores rest on the leaf surfaces and penetrate the leaf surface with rootlike organs called haustoria. If you use sulfur, follow the directions on the label and do not apply sulfur if the air temperature is above 90 degrees F. (If your disease is a downy mildew, it will not respond to these measures.)

If you choose to remove the affected plant, clean up all fallen leaves and destroy the plant parts rather than composting them. For many plants, especially where crop production is not being affected, powdery mildew is more a problem of the plant's appearance than its health.

But it can affect so many of the leaves that the plant's ability to take in nutrients and produce new leaves is reduced. For more information, call the Dial-Extension number, 296-DIAL and request tape #1614 "Powdery Mildew."

Q: I planted miniature snapdragons this summer. Leaves have brown spots on the back and some are withered. What can I do?

A: It's probable that your plants have snapdragon rust problems. A common fungus, often carried on cuttings or present in soil, snapdragon rust will cause symptoms like these with the entire plant ultimately appearing stunted and dry. At this point in the season, removing the plants makes sense. Clean up all fallen leaves and plant debris.

Don't compost the affected foliage, and do not replant snapdragons in the same spot next year. When you do plant snapdragons, make sure the plants have plenty of air circulation so the leaves don't stay damp - dampness provides good growing conditions for the fungus. Dusting with sulfur, a relatively low-toxicity fungicide, can help but will not stop a fully established infection.

This fungus can sometimes be averted by planting seeds of rust-resistant varieties rather than installing plants. Or, ask your nursery if their spring bedding plants are rust-resistant. One type, "Sweetheart Mix," is reported by the grower to have some rust resistance, but you'd have to experiment with it in your own growing conditions.

Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by Mary Robson, Master Gardener program director, Holly Kennell, Washington State University/King County Cooperative Extension agent, Susan Miller, integrated pest management specialist, and volunteer Master Gardeners.