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University
of Rhode Island GreenShare Factsheets
Powdery
Mildew on Shrubs
Almost all landscapes have plants that become diseased
with one of the powdery mildew fungi. Although the fungi that
cause powdery mildew differ from plant to plant, all of the
powdery mildew diseases are similar in appearance. In most
cases, prompt recognition and control actions can prevent severe
damage to plants from powdery mildew diseases.
Powdery mildews, as the name implies, often appear as
a superficial white or gray powdery growth of fungus over the
surface of leaves, stems, flowers or fruit of affected plants.
These patches may enlarge until they cover the entire leaf
on one or both sides. Young foliage and shoots may be particularly
susceptible. Leaf curling and twisting may be noted before
the fungus is evident. Severe powdery mildew infection will
result in yellowed leaves, dried and brown leaves and disfigured
shoots and flowers. Although it usually is not a fatal disease,
powdery mildew may hasten plant defoliation and fall dormancy
and the infected plant may become extremely unsightly. On roses,
uncontrolled powdery mildew will prevent normal flowering on
highly susceptible cultivars.
Powdery mildew fungi infect almost all ornamental plants.
The most susceptible woody plants include some deciduous azaleas,
cherry, a few of the flowering crabapples, dogwood, English
oaks, euonymus, honeysuckle, horse chestnut, lilac, privet,
roses, serviceberry, silver maple, sycamore, tulip tree, some
viburnums, walnut, willow and wintercreeper. Powdery mildews
are also common on certain herbaceous plants, such as chrysanthemums,
dahlias, delphiniums, phlox, Reiger begonias, snapdragons and
zinnias. Remember that each species of powdery mildew has a
very limited host range. Infection of one plant type does not
necessarily mean that others are threatened. For example, the
fungus that causes powdery mildew on lilac does not spread
to roses and vice versa.
Most powdery mildew fungi produce
airborne spores and infect plants when temperatures are moderate
(60 to 80 degrees
F) and will not be present during the hottest days of the summer.
Unlike most other fungi that infect plants, powdery mildew
fungi do not require free water on the plant surface in order
to germinate and infect. Some powdery mildew fungi, especially
those on rose, apple and cherry are favored by high humidities.
Overcrowding and shading will keep plants cool and promote
higher humidity†both highly conducive to powdery mildew development.
The following cultural practices should be beneficial
for controlling powdery mildews.
• Purchase only top-quality, disease-free plants of resistant cultivars
and species from a reputable nursery, greenhouse or garden center.
• Prune out diseased terminals of woody plants, such as rose and crabapple,
during the normal pruning period. All dead wood should be removed and destroyed
(preferably by burning). Rake up and destroy all dead leaves that might harbor
the fungus.
• Maintain plants in a high
vigor. o Plant properly in well-prepared and well-drained
soil where the plants will
obtain all-day sun (or a minimum of 6 hours of sunlight daily).
• Space plants for good air
circulation. DO NOT plant highly susceptible plants--such
as phlox, rose and zinnia--in
damp, shady locations.
• Do not handle or work among
the plants when the foliage is wet.
• Water thoroughly at weekly
intervals during periods of drought. The soil should be to
a depth of 8 to 12 inches.
Avoid overhead watering and sprinkling the foliage, especially
in late afternoon or evening. Use a soaker hose or root feeder
so the foliage is not moistened.
In many cases, powdery mildew diseases do little damage
to overall plant health, and yearly infections can be ignored
if unsightliness is not a major concern. Lilacs, for example,
can support powdery mildew every year with little or no apparent
effect on plant health. On some plants, however, powdery mildews
can result in significant damage and fungicides may be necessary
to achieve acceptable control. For best results with fungicides,
spray programs must begin as soon as mildews are detected.
Spray on a regular schedule, spraying more often during cool,
damp weather. Use a good spreader-sticker with the fungicides.
Be sure and cover both surfaces of all leaves with the spray.
Use only fungicides registered for powdery mildew control.
Adapted from Stephen Nameth
and Jim Chatfield, Ohio State University Extension, 2000
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
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.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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