Powdery Mildew

The white coating on Dahlia leaves is powdery mildew

By: Sandy Feather 2008
Penn State Extension

Q. Over the past week or two, Dahlias in a hanging basket have developed a white coating on their leaves. It is easy to rub-off, but I'm wondering what it is, and if it is harmful to our Dahlia plants, especially if something isn't done about it?

A. The powdery white substance you are seeing is a fungal disease called powdery mildew. Powdery mildew affects a wide range of plants, from herbaceous plants like your dahlias to trees and shrubs. There are different strains of powdery mildew that attack different plants. Plants that are frequently infected with powdery mildew include apple and crabapple, azalea, black-eyed Susan, calendula, cucurbits (melons, cucumbers, gourds, pumpkins and squash), dahlia, flowering dogwood, common lilac, monarda, phlox, rose, sycamore, verbena, and zinnia.

Lawn Grasses are also Susceptible

Certain varieties of Kentucky bluegrass are susceptible as well, particularly when they are grown in shade.

powdery mildew on dahlia
The powdery white substance on these Dahlia leaves is a fungal disease called powdery mildew

Although we usually think of powdery mildew as a dry, humid weather problem (water on the foliage actually inhibits the germination of the fungal spores), the high humidity we have had creates a very favorable environment for its development. Powdery mildew is favored by warm days with low relative humidity and cool nights with high relative humidity. Although it not usually life threatening for most plants, it can make them unsightly. Infected leaves may yellow and drop prematurely, and some herbaceous plants turn black. It can kill vegetable crops such as cucumbers and zucchini.


Powdery Mildew 'Dee-fense'

The best defense against powdery mildew is to grow resistant varieties when they are available. Although they can get the disease when conditions are extremely favorable, they are less likely to become infected than varieties that are extremely susceptible. I was not able to find any dahlia varieties listed as resistant, although there are cultivars and species of other plants that are.

Cultural Controls for Powdery Mildew

Cultural controls include spacing the plants carefully, pruning and thinning, and controlling weeds to allow good air circulation and sun penetration. Plants grown in full sun are not affected as severely as those grown in shade. Avoid overhead irrigation late in the day or at night because it elevates the humidity level around susceptible plants. It is best to water early in the morning. Removing infected leaves where practical helps reduce the amount of fungal spores present to continue the infection. Fungicide applications can be made preventatively to keep powdery mildew from becoming established on susceptible plants. Dithane (mancozeb), Spectracide Immunox Multi-Purpose Fungicide (myclobutanil), Bayer Advanced Disease Control for Roses, Flowers and Shrubs (tebuconazole), and Ortho Funginex (triforine). Organic gardeners can use sulfur, but not when temperatures are above 85 degrees.


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