Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES
Powdery Mildews on Ornamental Plants
Almost
all landscapes have plants that become diseased with one of the powdery mildew
fungi. Although the fungi that cause
powdery mildew are usually different on different plants, 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.
Symptoms
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 noticed. 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.
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Powdery mildew on begonia leaves. |
Powdery mildew on lilac leaf. |
Hosts
Powdery
mildew fungi infect almost all ornamental plants. They are commonly seen only
on those plants more naturally susceptible to the disease. Susceptible woody
plants include some deciduous azaleas, buckeye, catalpa, 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, kalanchoes,
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.
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Powdery mildew on dogwood leaves. Note the distortion
on affected leaves. |
Powdery mildew on azalea leaves. |
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Environmental Favoring Powdery
Mildews 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 humidity’s. Overcrowding and
shading will keep plants cool and promote higher humidity. These conditions
are highly conducive to powdery mildew development. |
Powdery mildew on maple leaves. |
Control
of Powdery Mildews
Before
using fungicides you should attempt to limit powdery mildews by other means.
The following cultural practices should be beneficial for controlling powdery
mildews.
1.
Purchase
only top quality, disease free plants of resistant cultivars and species from a
reputable nursery, greenhouse or garden center.
Horticulturists in the green industry and extension offices should be
consulted concerning the availability and performance of resistant varieties.
2.
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.
3.
Maintain
plants in a high vigor.
4.
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).
5.
Space
plants for good air circulation. DO NOT
plant highly susceptible plants—such as phlox, rose and zinnia in damp, shady
locations.
6.
Do
not handle or work among the plants when foliage is wet.
7.
Water
thoroughly at weekly intervals during periods of drought. The soil should be moist 8 to 12 inches
deep. Avoid overhead watering and
sprinkling the foliage, especially in late afternoon or evening. Use a soil soaker hose or root feeder so the
foliage is not wetted.
Chemical
Control of Powdery Mildews
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.
For example, lilacs can have powdery mildew each year, with little or no
apparent effect on plant health. On some
plants, powdery mildews can result in significant damage. Thus, fungicides must be used to achieve
acceptable control. For best results
with fungicides, spray programs must begin as soon as mildews are
detected. Spray on a regular schedule,
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. Fungicides generally
recommended for powdery mildew control include: Triadimefon
(Bayleton, Strike); Triforine
(Funginex), Thiophanate-methyl (Cleary’s 3336,
Domain), and Propiconazole (Banner).
Information obtained through the
Ohio State Extension Factsheet HYG-3047 -96
Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES