Hill Gardens of Maine - Powdery Mildew

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Powdery Mildew...Late Summer Scourge!
by Fred Davis, MG, Hill Gardens of Maine (To view other articles, click Archives)

Welcome through Fred's'Garden Gate! It’s that time of year again—the time when questions come rolling in about "…all that awful, white, powdery-looking stuff all over my phlox [or bee balm, veronica, lilacs, rudbeckias, sunflowers, hollyhocks, roses, peppers, cucumbers, peas and beans]!" It’s an annoying puzzle-problem made up of many intricately-interlocking pieces, so this time I’ll try to arrange a few of them into some sort of recognizable picture.

Powdery mildew (Erysiphe ssp.) is technically a rapidly-progressing fungus, becoming visible only when thin, nearly-microscopic-when-appearing-individually, threadlike mycelium form in large numbers on leaf surfaces. The actual powdery appearance is the mass-presence of reproductive spores growing on these mycelium. As these spores mature, they are launched into air streams where they are carried to new hosts.

Small white spots generally form on undersides of leaves, then rapidly spread to completely cover both the infected, and nearby, foliage. Infected leaves grow pale, turn brown and eventually fall to the ground.

Spores of powdery mildew are everywhere! It is only when atmospheric conditions are favorable, however, that, as they settle on host plants, growth and rapid proliferation occurs. Heat and humidity are chief factors. So also are conditions set in place by late afternoon sprinkling of susceptible plant foliage by uninformed—or misinformed—gardeners. The picture on the right shows mildew on Bee Balm.

Affected plants succumb very quickly unless preventive or curative measures are instituted immediately. A few of the primary preventives are: 1. SANITATION— maintaining a clean, neat garden and not allowing infected foliage to remain either on the plant or nearby soil surfaces—it is important to rogue-out infected plants and remove tainted garden refuse immediately; 2. VENTILATION—providing ample air-space between susceptible plants and surrounding species or varieties; included in this preventive is the placing of plants according to recommendations – planting a sun-loving variety in the shade, for example, is a formula for almost certain disaster; careful selection of mildew-resistant varieties is another key to avoiding problems; 3. CORRECT WATERING methods and timing—avoiding the wetting of foliage and never watering (sprinkling) susceptible plants after 3:00 p.m. (wetting foliage as the day ends is asking for trouble!).

Curative measures range from powerful and highly-toxic commercial fungicides, to less toxic ‘natural’ practices and materials. Commercially available fungicides, while effective, tend to indiscriminately destroy beneficial fungi as well. Additionally, most such materials are suspected of persistence in the environment – meaning the may remain toxic long after their intended ‘job’ is finished. Daconil, Funginex, Benlate and Captan are just a few of the many products available at your local farm ‘n garden store. (Note: I do not recommend the use of any of these powerful synthetic fungicides.)

A small number of far-less-toxic fungi-preventive products are also available (likely on a shelf near the chemicals) which will get the job done almost as fast…almost as well. In 1995, the Organic Gardening Book Club newsletter reported that using a 1% Neem oil, extracted from seeds of the Neem tree, "prevents rust and powdery mildew from infecting susceptible plants like lilacs and phlox, and it slows down blackspot on roses." I’ve also heard it said that Safer’s Insecticidal Soaps can prevent the spread of mildew. While unconfirmed in our gardens, any option which avoids the use of powerful chemicals deserves at least a trial in your garden.

Another ‘natural’ mildew control is thought to be a solution of common household baking soda (sodium bicarbonate) mixed at the rate of approximately one-half cupful in one gallon of water. Sprayed or sprinkled on susceptible plants either before or immediately after first symptoms appear, may help to prevent spread of the disease.

In either case – chemical or ‘natural’—all you can hope for once damage has become apparent is containment…not ‘cure’. None of these materials will remove disease-related damage, but may very well stop this dratted malady’s progress dead in its tracks.

After all is said and done, however, the real keys are SANITATION, VENTILATION, and adhering to cultural recommendations.

One final point: infected foliage from any plant—flower or vegetable—with spots or signs of mildew should be thoroughly cleaned up. Diseases can over-winter in fallen foliage and be ready to re-infect your plants shortly after the following spring. Also, unless you are a real ‘hot-rot’, biological composter, and routinely develop pile temperatures in the range of 145° to 150° (F) or more, it would be wise to keep infected trimmings and cleanings out of the compost bin. A better place for them would be your weekly rubbish pick-up bag. It’s highly unlikely the disease can be totally eliminated from the average home garden…but at least the odds can be skewed heavily to favor the gardener!

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