Powdery mildew is one of those obligingly named diseases that evokes an accurate picture of the dusty white blotches that appear on shrubs, trees, flowers, vegetables and fruit plants. The fungal disease can go on to cause browning of foliage, then severe leaf loss and can even damage your edible fruits. Treating both the causes of powdery mildew and the disease itself is your best bet for keeping it from doing damage to your prized plants. Powdery mildew spores most often proliferate in overly dry and shady conditions, as well as when poor air circulation exists among plants.
If some of your perennials or garden bed plants have exhibited a problem with powdery mildew in the past, take early steps to remove some of the common exacerbators of fungal disease. Powdery mildew is an airborne disease, rather than one with much of a "shelf-life" underground, so most of your efforts should be concentrated on methods above the soil line.
Purchase disease-resistant varieties of plants. Look for ones that are labeled "fungal resistant" or that are specifically marked as resistant to powdery mildew.
Prune away overhead branches. Shade is a prime cause of powdery mildew. If some of your bushes or trees themselves show signs of the fungal disease, remove some of the heavier limbs in order to open up interior sections to sunlight and air circulation. Of course, this is also a good time to remove and dispose of infected plant parts, such as branches, foliage or stems.
Set new plants at the recommended distance apart, away from sunlight-obstructing plants and structures. Good air circulation and sun are important when combating powdery mildew.
Mulch heavily in the spring, after setting in new plants or after perennial plants emerge. Mulch will help to suppress any lingering powdery mildew spores that may have overwintered in the soil.
Choose a low-nitrogen plant food when fertilizing your susceptible plants. The rapid foliage growth associated with excess nitrogen can encourage powdery mildew.
The University of California's Integrated Pest Management program recommends neem oil for the control of powdery mildew, once it appears. Mix 2 tablespoons of the concentrated product for every 1 gallon of lukewarm water, and apply it to the tops and bottoms of foliage with a spray bottle or compression sprayer. Midsummer is the ideal time to begin a spray schedule in most instances, according to one manufacturer, but you can also begin at the first sighting of the disease. Continue the neem oil treatment every one to two weeks.
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