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Slime
molds cause concern among many turfgrass managers when they suddenly
appear after heavy rains or watering in warm weather. Numerous species
may be involved, including the common Physarum cinereum,
and occasionally Mucilago spongiosa and Didymium crustaceum.
Other species of Fuligo, Mucilago, Physarum,
and Stemonitis have also been reported on turfgrass.
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Slime
molds are primitive organisms that lack cell walls and flow
or move like amoebae over low lying objects and vegetation,
such as turfgrasses, strawberries, bedded flowers, ground
covers, alfalfa and clovers, plantains, dandelions, mulches
and growing media, bases of trees, and even sidewalks and
driveways. These organisms are not parasitic but feed on decaying
organic matter, fungi, and bacteria in the straw and soil.
Generally, slime molds do little damage to living plants,
but may cause some yellowing by shading the affected leaves.
Moist, warm weather, and high soil moisture favor the fruiting
of slime molds on turfgrasses.
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Click
on image for larger version
Figure 1. Plasmodium of Physarum Cinereum Forming
Immature Sporangia (Noel Jackson)
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Symptoms
The slimy amoebalike stage appears as watery white, gray, cream
to light yellow, red, violet, blue, green, or greasy purple-brown
masses in round to irregular patches from 1 inch to 2 feet in diameter.
This stage is made up of a naked mass of protoplasm called a plasmodium,
which simply "engulfs" its food. The plasmodium soon "heaps
up," and the crusty (usually gray or yellow) fruiting bodies
envelop individual leaves with numerous small, purplish, blue gray,
gray, black, dirty yellow, or white powdery structures called sporangia.
Slime molds commonly reappear in the same areas each year.
Life
Cycle
Slime molds survive unfavorable conditions as microscopic spores
in the soil and turfgrass thatch. The spores are spread primarily
by air currents, water, shoes, mowers, and other turf equipment.
During or after warm, wet weather or deep watering from late spring
to autumn, the spores absorb water and crack open, and a motile
swarm spore emerges from each. The amoebalike swarm spores feed
on fungi, bacteria, other microorganisms, and decaying organic matter
in the straw mulch while they undergo various changes and numerous
fissions. Finally, they unite in pairs to form zygotes and become
a shapeless, slimy plasmodium that increases in size. The plasmodium
works its way to the soil surface and creeps over vegetation in
round to irregular patches. Here the crustlike fruiting or reproductive
state is formed, which is the only stage that most of us ever see.
The round, pinhead-sized fruiting structures (sporangia) are variously
colored and range from white, gray to creamy white, purplish brown,
bluish gray, tan to orange, brown, or black. They are filled with
dark masses of powdery spores that are easily rubbed off the leaf
or stem.
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Click
on image for larger version
Figure 2. Slime Mold (Physarum cinereum) on
Alta Fescue
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Slime
molds are nonparasitic. They are much more unsightly than
harmful and merely use grass leaves and stems as a means of
support for their reproductive structures. Slight damage may
occur when leaves are smothered or shaded for several days
to a week. The weakened and somewhat yellow grass leaves are
more susceptible to killing by secondary fungi and bacteria.
An abundance of straw mulch favors slime molds by providing
a ready source of organic matter and high populations of microorganisms.
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Control
No control measures are usually considered necessary. If the
molds are abundant, the unsightly spore masses may be broken
by vigorously raking, brushing, poling, or hosing down with
a stream of water. Washing is suggested only after the onset
of dry weather. Mowing the grass usually removes the spore
masses. Although many fungicides might be beneficial, they
are not recommended.
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Click
on image for larger version
Figure 3. Slime Mold Growing on Sundial Near Blueberries
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For
further information concerning turfgrass diseases, consult an Extension
Specialist, Department of Crop Sciences, University of Illinois,
Urbana.
University
of Illinois Extension provides equal opportunities in programs and
employment.
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