Powdery mildew: How to keep this fungus at bay

Home free? Not yet. Just when you think the plants have coped with summer heat, tent caterpillars and aphids, here comes the gray fuzz of powdery mildew.

Most gardeners see it in August: the gray, felty-looking spores, disfiguring many kinds of garden plants, including dahlias, azaleas and roses. At home, I've found it on my favorite nandina and, paradoxically, the sages in the bright, sunny herb garden.

Fungi in general thrive in our local climate conditions — humid, mild, with the absence of deep freezes. Indeed, people doing plant diagnoses in the Pacific Northwest joke that if a plant has a disease, it's probably a fungus amungus. (You don't want to know that in Western Washington we probably inhale 50 fungal spores with each breath, do you?)

Powdery mildew is our most common fungi, and new research reveals how much more of it we have than previously thought. Dr. Dean Glawe, a pathologist with Washington State University Extension, has devoted several years to studying this fungus in the Pacific Northwest. He took up the study of powdery mildew because these organisms can be severe and affect both farmers and gardeners. The hops crops (as in beer) in Eastern Washington, for example, were attacked in the early 1990s by a powdery mildew previously unknown, adding to growers' challenges.

Among Glawe's findings: Many more plants get powdery mildew than we ever imagined. Every gardener knows about squash. "If anyone ever finds a zucchini here in August that does NOT have powdery mildew, we should write a paper about it," Glawe says with a laugh. But at least 140 different plants not originally reported have been found to be hosts here, including Mahonia aquifolium, nandina and akebia.

Also surprising: Experts used to believe that we had 21 powdery mildews in this area. Glawe has raised the estimate to 150 — possibly more. Glawe suggests some approaches for gardeners:

Choose resistant plants. This is easier said than done. Many "nursery trials" have been conducted in parts of the United States and Europe on resistance in roses, phlox and bee balm. But it's hard to apply the findings from one region to another, because powdery mildews vary with location. If a grower in Tennessee hybridizezs a dogwood with resistance, for example, that resistance might not persist here. What's needed, Glawe says, are more scientific trials done locally.

That said, the local rose societies do conduct garden tests in varied conditions and publish lists of their favorite disease-resistant roses. Among those listed: orange 'Just Joey,' climbing red 'Altissimo,' as well as shrub roses 'Golden Wings' and 'Sally Holmes.' For more information, contact the Seattle Rose Society.

Choose settings carefully. Environment affects the severity of powdery mildews. Shade, crowded conditions, over-watering, fences and walls that reduce air circulation can all intensify the problem, because they raise the humidity levels at the leaf surface.

Glawe points out that gardeners can observe what scientists may have missed or failed to record, and often can take care of plants successfully by ground-level choices. In my experience, a Rosa rugosa 'Hansa' that should have been resistant became severely affected in the shade, but recovered when moved to sun.

Don't rely on washing leaves. Gardener scuttlebutt often suggests washing powdery mildew-infected leaves. A study of roses in Oregon, however, indicates this might not be enough. Researchers found that the leaves required washing 2-3 times a week, both sides, yet even then the problem was only reduced, not eliminated. Glawe says washing can also exacerbate the humidity problem without full sun and quick drying, and it can accelerate reinfection or other fungal problems.

Get rid of affected parts. Glawe recommends pruning mildewed shoots or leaves and burying the cuttings. "If you catch a developing epidemic early enough, and are ruthless enough, you might succeed in eradicating the fungus," he says. (Often we don't see it early enough to do this, and if I pursued it now, I'd have a naked nandina.) Picking up and disposing of fallen leaves makes sense, too.

Use chemical controls — carefully. Most registered fungicides cover a wide range of powdery mildews. One that works on apples will typically work on roses as well. (The key is to use a chemical control registered for edibles if you're dealing with a food plant.) In general, the least-toxic controls are potassium bicarbonate-based products, sulfur and some horticultural oils.

Finally, the best advice for dealing with powdery mildew may be to relax. Often the problem's more disfiguring than plant-threatening, and no action is needed.

Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension. She shares gardening tips every Wednesday. Her e-mail is marysophia@earthlink.net.