Normally when you think of
garden pests, insects and animals come to mind.  Fungal diseases have
the capacity to be equally pesky, and powdery mildew is no exception.

Powdery mildew is a
disease afflicting many vegetable crops caused by a variety of strains
of fungus. Westsiders beware: the fungus thrives in humid air between 60
and 80 degrees Fahrenheit.  Unlike other fungal infections, it does not
require water to reproduce; its growth is actually inhibited by the
presence of water.

The
disease usually manifests as fuzzy white spots on the leaves and stems
of fruit and vegetable plants. Gardeners often encounter the disease on
their squash leaves at the end of the growing season.  Because squash
leaves are frequently variegated, it can be easy to
misdiagnose normal growth with the disease. Powdery mildew appears as a
fuzzy growth on the outside of leaves, while variegation causes white
coloration of the leaves themselves.

White Variegation on Squash Leaves

Leaves infected with powdery mildew
frequently turn yellow and die, causing reductions in yields and produce
quality.  While mild infections don’t cause a great deal of harm, it’s
worth controlling the disease early, because it tends to spread quite
quickly.

Powdery mildew can be
prevented by planting resistant varieties, placing plants likely to be
affected in full sun, and planting in a location with good air
circulation.

If
it’s too late to prevent an infection, there are several ways to treat
the disease once it appears. Do your best to avoid agitating areas of
the plant already infected, which can spread spores to nearby leaves or
other plants.  In mild to moderate cases, Farmscape has been successful
in controlling powdery mildew by spraying with neem oil or products such
as Serenade, an organic biological fungicide that uses a strain of bacillus subtilis
bacteria to controls infections.

To combat more severe
cases of powdery mildew, you could consider using an OMRI approved
fungicide product. Be careful if you choose to use such products, as organic fungicides often use copper or sulfur as an
active ingredient, which can be toxic if ingested. Such products may also harm your garden's production if they kill beneficial fungus growth in your soil. Make sure that you carefully weigh the costs and benefits of the use of
such products. In all cases, you should try to use them sparingly and
only in cases of severe infection. If powdery mildew is idenitified and treated when an infection is mild, more powerful control products should be unnecessary.

If you grow food crops in your garden, chances are that you
will eventually encounter powdery mildew. If you learn to identify,
prevent and control the disease, your garden will be healthier and more
productive when the inevitable strikes.

Powdery Mildew image from Jeff Kubina, "Powdery Mildew Fungi on Pumpkin Leaves," August 1, 2010 via Flickr, Creative Commons.