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Number 2, 2000
Powdery Mildew
Recently, we have received calls from homeowners, golf course
superintendents, and sod producers concerned about a whitish material
covering turf leaves. The cause, powdery mildew, seems to be attacking
turf in Illinois this spring in areas normally not affected.
Powdery Mildew (Erysiphe graminis D.C.) is a disease that
is usually observed in turf predisposed by shade or several consecutive
days with overcast skies (low light intensity), high humidity,
poor air movement and air temperatures between 61-77ºF. However,
powdery mildew is somewhat of an anomaly because the one environmental
condition it does not need to survive and spread is free water.
Spores of the pathogen germinate in the absence of free water.
A rainless period with high humidity compounded with low light
intensity (common to this spring) have increased incidences of
powdery mildew. One of the most susceptible hosts of powdery mildew
in the turfgrass community is Kentucky bluegrass.
Symptoms:
Powdery mildew is first seen as isolated colonies of fine grayish-white
mycelia on the leaves of the plant. Diseased areas of turf can
be several inches to more than ten feet in diameter with diffused
borders. Older or lower leaves on the individual plant are often
times more severely infected than the upper, younger leaves. The
powder-like material is not uniformly distributed on the leaf
surface and is difficult to scrape off with a fingernail. In severe
infestations, leaves appear to be dusted with flour or ground
limestone. Powdery mildew infestation can become severe enough
to turn leaves yellow, then tan or brown as they die.
Causal Pathogen and Cycle:
Powdery Mildew (Erysiphe graminis D.C.) is an obligate
parasite that infects the leaves and shoots of grasses. It survives
as cleistothecia (black, fungus-fruiting bodies) via mycelium
during unfavorable growth periods. The powdery appearance is due
to the production of enormous numbers of microscopic spores called
conidia. The conidia are easily disseminated by air currents and
mechanical means causing new infections within two hours after
landing on a leaf surface. If environmental conditions remain
favorable (61-77ºF, cloudy) spores can be continually produced
for 7 to 14 days until the host plant dies. The fungus colonizes
the epidermal cell walls and then feeds by a special structure,
the haustorium, that extends into the lamea of the epidermal cell
walls. Powdery mildew fungi live mostly on the outer surface of
the host plant.
Control:
Because powdery mildew is often associated with shaded conditions,
the first method of controlling the disease is to increase light
penetration and improve air circulation. Current reports of powdery
mildew in open areas this spring can not be attributed to shade
but to the low light intensities produced by cloud cover. So,
increasing light penetration in a normally sunny site is not an
option this spring.
Cultivar selection is the second cultural control used to ward
off infestation of powdery mildew. More shade tolerant cultivars
of Kentucky bluegrass have been reported to be more tolerant to
powdery mildew infestation. Data from Maryland suggests variability
among specific cultivars in powdery mildew proneness.
Cultural practices that reduce stress can aid in the recovery
of turf infected by powdery mildew. Moderate fertilization and
increased mowing height (2 inches or greater) and frequency are
recommended. Fungicides can be used to control powdery mildew,
both preventatively and curatively, on high value turfs where
powdery mildew infestations repeatedly occur. Fungicides labeled
for the control of powdery mildew include Banner, Bayleton, Eagle,
Golden Eagle, Lesco Turf Fungicide, Rubigan, Sentinel, and Twosome.
However, proper cultural practices will usually eliminate the
need to use a fungicide to control powdery mildew. As always,
read, understand, and follow labels for appropriate pesticide
use.
There are no models available to predict the growth of powdery
mildew, but the weather conditions of this atypical spring have
been ripe for outbreaks of this fungus. Powdery mildew will rarely
kill a host plant but it should be monitored closely. Plants that
have been infected by powdery mildew will be more susceptible
to other stresses encountered within the turfgrass environment
(i.e. weed invasion, other fungi, drought stress). Proper cultural
practices, including increased mowing heights, adequate fertility
and increased air circulation will aid in the control of powdery
mildew
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(Luke Cella, Tom Voigt and Hank Wilkinson)
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