Cool evening, warm days cause powdery mildew

This is one in an occasional series of columns on Green Gardening practices.

Late summer in Western Washington brings many predictable sights in the garden - beautiful spider webs, leaves changing from green to khaki and the white deposits of powdery mildew on plant leaves. While watering container plants one recent morning, I noticed the "powder" had gathered so thickly on a potted feverfew (Chrysanthemum parthenium) that some green leaves had vanished.

Powdery mildew is caused by a very common fungus organism. The disease worsens in late summer and early fall because of cool nights and warm days. The symptoms of this fungal disease are uncomplicated: a gradual whitening of leaf and flower surfaces as the fungus infests the plants and produces spores. Small black specks may also appear late in the season.

What makes powdery mildew aggravating to gardeners is the extraordinary variety of plants it can affect including vegetables such as squash, pumpkins and beans; rose bushes notoriously so and deciduous azaleas, the ones that lose their leaves, often look completely gray by September. The native Oregon grape (Mahonia aquifolium) gets the disease, as do other native plants. Many annual flowers such as cosmos and petunias, and perennials such as delphinium and phlox seem to attract the problem. And you can count on begonias in hanging pots to contract powdery mildew.

The good news is that powdery mildew seldom kills a plant. It's disfiguring and troubling but not often fatal. Some basic care can help. The Green Gardening program stresses cultural care for plants before choosing pesticides.

-- Avoid using too much fertilizer or water, because the organism moves first to succulent new growth. You can often see this happening on roses. (However, in fall when we aren't fertilizing shrubs, you may see mildew on all leaves, mature as well as new. Many shrubs stop growing in fall but still get the problem.)

-- There's no direct apparent link between bad drainage and powdery mildew, but it's essential to correct any poor drainage to avoid root rots, which place even more stress on plants defoliating from powdery mildew.

-- Make sure plants have good air circulation. In late summer, many annuals have crowded leaf foliage. So do some remedial thinning and pruning.

-- Cleaning up fallen leaves may help reduce the source of infection. Raking and destroying fallen leaves that have been affected by diseases is a good gardening practice in fall, and should be done throughout the garden.

-- If it's practical, remove affected leaves.

-- Hose off diseased plants. Oddly enough, the organism can be checked somewhat by being washed off. It won't go completely away nor will water end the infection. It simply seems to reduce the problem.

-- Don't worry if the plant is an annual. Just don't compost annual plants pulled from the garden after blooming or fruiting.

-- On plants such as deciduous azaleas, which will be losing their leaves, fall treatment isn't necessary. But rake up the fallen leaves.

-- Use a combination of techniques, including tolerating some powdery mildew damage.

-- Fungicides, which are pesticides formulated to kill fungal organisms, can help with prevention and management of powdery mildew. The best time to apply them is when the disease is first noticed. Unlike many other fungal problems, powdery mildew will succumb to fungicides after the symptoms show up. But again, if the plant will be pulled out of the garden soon, or lose its leaves, save the fungicide treatment for next year's crop.

-- Several fungicides are registered for powdery mildew. Be sure to check the label to find the plant you're treating: the label must list "roses" or "azaleas" or "vegetables." It's especially important if you are treating vegetables that the product be labeled for edibles.

Fungicides containing sulfur (such as Safer Garden Fungicide) or triforine (Funginex) will help. Some gardeners are finding that triforine does not work as well as it has in the past due to the appearance of powdery mildew fungi resistant to the chemical.

A new fungicide, Remedy, which is a potassium bicarbonate (made by the Bonide Company), is registered for the problem. Remedy is chemically related to baking soda and has very low toxicity. It's a good choice for powdery mildew when it appears on your roses next year.

-- With all fungicides, thorough leaf coverage is essential. Be very careful. Read all label instructions and wear protective goggles and gloves. Funginex (triforine) can be corrosive to eyes if splashed into them.

For further garden questions, e-mail robsonm@wsu.edu. Or write to Mary Robson, Area Horticulture Agent, Washington State University Cooperative Extension, 500 7th Ave. SW, Suite 200, Renton, Washington, 98055-2983.

The Practical Gardener runs Sunday in Home/Real Estate. It is prepared by Mary Robson, area horticulture agent, Washington State University/King County Cooperative Extension.