ISU Extension News

Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915

9/22/03

Contacts:
Mark Gleason, Plant Pathology, (515) 294-0579, mgleason@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Yard and Garden Column for the Week Beginning Sept. 26, 2003

Falling into Mildew

By Mark Gleason
Extension Plant Pathologist
Iowa State University

As fall descends on Iowa landscapes, many garden plants take on a snowy appearance. It's too early for real snow, of course. This flocking comes courtesy of a fungal "snow" called powdery mildew.

Next to mushrooms, powdery mildews are the most familiar fungi in Iowa landscapes. Powdery mildews produce patches of grayish-white, granular-looking material on leaves of all sorts of plants, from zinnia and squash to oak and dogwood. The patches appear on the top or bottom surfaces of leaves, and sometimes coat the entire leaf as well as stems and flowers.

Despite their near-decorative appearance, powdery mildews are plant parasites. They don't kill their hosts, unlike more brutal fungi. Instead, they merely sip away at a plant's stored sugars. As powdery mildews creep across plant surfaces, they punch into the goodies stored underneath and steal only what they need.

Powdery mildew-infested plants don't die, but they may start feeling queasy as their sugars are stolen. Leaves that are attacked often turn yellow. When severe powdery mildew overwhelms a cucumber or melon patch, the fruit ripen prematurely and don't taste very good.

Powdery mildews have a powerful motive for keeping their hosts alive: if the host dies, they die too. Like rusts and a few other fungi, the powdery mildews are obligate parasites, programmed to keep their victims alive even as they drain away their precious sugars.

A bonus for management of obligate parasites is that they can't survive Iowa winters. When the first killing frosts destroy the foliage of garden plants, most powdery mildew fungi die as well. So there's no need to clean up dead garden foliage in order to stop powdery mildew next year. Infections in 2004 are likely to come from spores blown into Iowa from southern states, courtesy of summertime weather systems.

Despite their shameless sugar-sucking, powdery mildews frequently inflict only negligible damage on host plants. Lilacs, for example, seem to shrug off even a heavy case of PMS (powdery mildew stress).

Many plants suffer little from powdery mildews because the fungi arrive so late in the season. A powdery mildew attack launched in late August or early September doesn't really dent a perennial plant, since most of its sugar-making and sugar-storing is already done by then. Even annuals don't suffer much from late-season attacks.

Mid-season epidemics can be another matter. When a pumpkin patch shows powdery mildew patches in July, for example, it may be time to reach for the fungicide, since the crop needs to stay healthy for a couple more months.

Many powdery mildew fungicides tend to be systemic in activity. That is, they are taken up by the leaf and can fight the fungus inside as well as outside the plant. If you consider using fungicide sprays, make sure the fungicide you use is labeled against powdery mildew and lists the plant you are growing.

In most late-season garden situations, however, the recommended response to a powdery mildew invasion is to do nothing. This option is much cheaper and more relaxing than spraying fungicides. Besides, frost will take care of your plants - and the powdery mildew - soon enough. And if you squint hard enough at those powdery mildew patches, they almost look decorative.

-30-

Editors: No photos are available for this week's column.

ml: isugarden


Extension programs are available to all without regard to race, color, national origin, religion, sex, age, or disability.

News Menu | ISU Extension