of Rhode Island GreenShare Factsheets
Powdery mildew is one of the most widespread and easily
recognized plant diseases. Powdery mildews are most severe
when the weather is warm and dry, and they affect virtually
all kinds of plants: cereals and grasses, vegetables, flowers,
weeds, shrubs, fruit trees, and broad-leaved shade and forest
trees. Many plants have been developed to be resistant to or
tolerant of powdery mildew. Succulent tissue is the most susceptible
Even though there are several types of powdery mildew
fungi, they all produce similar symptoms on plants. Powdery
mildews are characterized by spots or patches of white to grayish,
talcum powder-like growth. Tiny, pinhead-sized, spherical fruiting
structures--first white, then yellow-brown and finally black--may
be present singly or in a group. These are the cleistothecia
or overwintering bodies of the fungus. The disease is most
commonly observed on the upper side of the leaves. It also
affects the bottom sides of leaves, young stems, buds, flowers
and young fruit. Infected leaves may become distorted, turn
yellow with small patches of green, and fall prematurely. Infected
buds may fail to open.
The severity of the disease depends on many factors: variety
of the host plant, age and condition of the plant and weather
conditions during the growing season. Powdery mildews are severe
in warm, dry weather because, unlike most fungi, powdery mildew
require a wet leaf surface for infection to occur. However,
the relative humidity of the air does need to be high for spore
germination. Therefore, the disease is common in crowded plantings
where air circulation is poor and in damp, shaded areas. Incidence
of infection increases as relative humidity rises to 90 percent,
but it does not occur when leaf surfaces are wet (e.g., in
a rain shower). Young, succulent growth is usually more susceptible
than older plant tissues. Powdery mildew can seriously impact
yield on flowering crops such as squash, pumpkins, cyclamen
and reiger begonia, but on other plants such as lilac and oak,
the mildew is unsightly but does not severely harm the plant.
Powdery mildews are host specific: they cannot survive
without the proper host plant. For example, the species Uncinula
necator, which causes powdery mildew on grape and linden,
does not attack lilac. Similarly, Microsphaea alni affects
elm, catalpa, lilac and oak but not turfgrass.
Powdery mildews produce mycelium (fungal threads) that
grow only on the surface of the plant. They never invade the
tissues themselves. The fungi feed by sending haustoria, or
root-like structures, into the epidermal (top) cells of the
plant. The fungi overwinter on plant debris as cleistothecia
or mycelium. In the spring, the cleistothecia produce spores
that are moved to susceptible host tissue by splashing raindrops,
wind or insects.
Several practices will reduce or prevent powdery mildews.
Many plants, such as roses, vegetables and Kentucky bluegrass,
are developed to be resistant or tolerant to powdery mildew.
Use resistant varieties whenever possible.
Once the disease becomes a problem:
Avoid late-summer applications of nitrogen fertilizer to limit the
production of succulent tissue (which is more susceptible to infection).
Avoid overhead watering to help reduce the relative humidity or water
in the early morning to let the tissue dry as soon as possible.
Remove and destroy all infected plant parts (leaves, etc.). For infected
vegetables and other annuals, remove as much of the plant and its debris in the
fall. This decreases the ability of the fungus to survive the winter. Do not
compost infected plant debris. Temperatures often are not hot enough to kill
Selectively prune overcrowded plant material to help increase air circulation.
This helps reduce relative humidity and infection.
An alternative nontoxic control for mildew is baking soda combined with
a lightweight horticultural oil. Researchers at the University of Rhode Island
have confirmed that a combination of 1 tablespoon baking soda plus 2.5 tablespoons
oil in 1 gallon of water is effective against powdery mildew on roses. Use of
this combination on other crops is still experimental.
Adapted from the Colorado
State University Cooperative Extension, 1999
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information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program