In the Garden:
Western Mountains and High Plains
September, 2007
Regional Report

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Powdery mildew rears its ugly head in early fall.

Mildew on the Attack in Early Autumn

It's back! The appearance of a light, grayish white coating on the leaves of lilacs, zinnias, Shasta daisies, chrysanthemums, dahlias, crab apples, and many more garden plants usually means that powdery mildew is on the attack. This fungus disease may appear at any time during the growing season when conditions are favorable, but it's most common in late summer and fall. It proliferates best when weather is relatively cool, the foliage is dense throughout the landscape, and humidity is higher at night. Just walking through my garden, I've noticed how much more prominent this disease is during my favorite time of year.

Luckily, mildew's appearance at this time does not warrant extreme control measures. It is typically too late to get effective control and the foliage will soon be shed from deciduous trees and shrubs. Annuals will be stopped by frost, and perennials will ripen and prepare for winter dormancy. However, you can take some common sense steps now to keep mildew from becoming a frequent visitor in your garden. While the weather is so comfortable, get out there and clean up the garden.

Taking Preventative Action
Since plants vary in their susceptibility to powdery mildew, some plants will need more attention than others. Some of the rose bushes in my garden that were inherited from a previous gardener are heavily covered with the powdery stuff. This might be the excuse I've longed for to justify removing the bushes. A fragrant mock orange would look great in that location.

Sanitation is by far one of the most effective ways to reduce the source of infection for next spring and summer. Promptly remove any annual flowers and ground covers that are badly infested and dispose of them. If you're thinking about adding them to the compost pile, forget about it! Traditional composting methods in the home garden do not get hot enough to kill the mildew spores. So you could be potentially spreading it around if you throw in infected leaves and plant parts. I remember from childhood when my grandmother would pile up the diseased plant refuse and burn it. It was a nice warm garden therapy session on a cool fall evening.

Don't forget to increase air circulation throughout your landscape as you go about your fall cleanup. Pruning, thinning out dense vegetation, and shaping plants can be done now to increase air movement and reduce higher humidity around the foliage.

You can make powdery mildew less obtrusive if you take steps now to keep your garden less hospitable to it.


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