Managing powdery mildew in the nursery and landscape

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.      

(The following article was revised from an article that appeared in the Sept.5, 2003 Landscape CAT Alert written By Diane Brown-Rytlewski)

Powdery mildew has been showing up on a number of shrubs, trees and perennials, including some coreopsis, lilacs, red maples magnolias, roses, monarda, sedum and others over the past few weeks. Powdery mildew may be more noticeable this year, with our cooler temperatures this summer. Many powdery mildews, like those that cause powdery mildew on roses and lilac, are host specific, while some species may affect a wide range of ornamental trees and shrubs as well as perennials. Common symptoms include distorted or stunted foliage, shoots, flowers or fruit, chlorosis, browning and premature death of leaves and slower growth.

Typically, the fungus appears as a white to grayish powder on the upper surface of the leaf. However, on succulent plants such as sedum, in addition to the powdery areas on upper leaf surface, corky brown patches appear on the leaves and stems that look very different (see photo).

The best approach to avoiding powdery mildew is to select plant cultivars that are resistant to the disease. The Chicago Botanical Gardens have done a number of trials on powdery mildew resistance on perennials their results can be found at:

As growers it is important to make plant decisions with the final user in mind and more and more customers are requesting low maintenance trees, shrubs and perennials. Having to spray fungicides on a weekly basis is not ones idea of low maintenance. Select with disease resistance or at the least disease tolerance when selling plant material.

Sanitation and good cultural practices are important components of managing powdery mildew, especially late in the season. Rake up and discard infected leaves to reduce the amount of inoculum overwintering on leaf litter. Although rain inhibits spore germination, high humidity and dew tend to make the disease more severe. Watering should be done early in the day so plants can dry before nightfall. Plants that are overcrowded or in shaded areas may be more prone to mildew. Rain inhibits spore germination, but high humidity and dew may make the disease more severe.

Many plants that develop powdery mildew late in the season, mid-August and beyond, do not need treatment with fungicides. Fungicides applied after the symptoms have appeared will protect new growth, but will not cause the powdery substance on the leaf surface to disappear. Fungicides are best applied earlier in the season, as protectants, before symptoms appear. However, if you have plants that are still actively growing in the fall when they are put into hoophouses, they could be treated with a fungicide. The following fungicides are effective on powdery mildew, products containing azoxystrobin, chlorothalonil, kresoxim-methyl, triflumizole, myclobutanil ,trifloxystrobin or triadimefon. It is especially important to avoid rotating with another stobilurin fungicide if you have applied a srobilurin fungicide the previous application. Be sure to read the label for restrictions and application rates.

For more details, be sure to review MSU Extension Bulletins E-2782 and E-2783 as they contain specific pesticide recommendations for ornamentals.

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