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Friday, July 29, 2016

Keep Irrigation Water Off of Plant Leaves to Reduce Powdery Mildew

Q. I have powdery mildew on lower branches on my shrubs. Should I remove the dying leaves? Since the temperature will be 114 tomorrow, should I wait until Fall?
Powdery mildew on Japanese euonymous. These plants are famous for mildew.

A. Powdery mildew on these plants is probably left over from this wet spring. Powdery mildew disease does not require high humidity. As far as diseases go, it probably has the lowest requirement for humidity or moisture.
            The usual reasons for powdery mildew besides cloudy, rainy weather is shade, poor air circulation and poor plant health. If you can reduce the shade with some light pruning that may help.
Make sure there is no splashing water from an irrigation system. This will spread the disease from leaf to leaf or plant to plant.
Sulfur dusts as well as neem oil have been known to work as preventive sprays. Otherwise use traditional fungicides that list powdery mildew control on the label. But with these high temperatures the disease should subside. It is a cool temperature disease.
If the leaves look pretty bad, then go ahead and remove them. At least then you are removing disease inoculum. But consider some light pruning instead. You won't hurt anything with light pruning this time of year and you may give the lower parts of the canopy more sunlight.

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