spot disease of turfgrass is named for the straw-colored, silver
dollar-sized spots that appear on golf greens. The disease causes
larger spots on higher cut turfgrass. Dollar spot disease of lawns
usually appears in somewhat irregular circles 3 to 6 inches in diameter.
If the spots coalesce, a more general blight may be observed.
more certain diagnosis of this disease can be made by examining
individual leaves. The typical dollar spot lesion is straw colored
and girdles the entire leaf blade. The top and bottom borders of
the lesion are usually dark reddish-brown. The lesion often has
an hourglass shape. If the infected grass is examined in the morning
while dew is still present, the white web-like mycelium of the fungus
may be observed.
|Dollar spot fungus in thatch
||Close-up of fungal infection on grass blades
Dollar spot is most common in early summer, late summer, and early
fall in humid weather with warm days and cool nights. Dollar spot
is less prominent in very hot weather.
studies have suggested that although dollar spot has traditionally
been attributed to the one fungus, Sclerotinia homoeocarpa, there
may indeed other fungi involved in the disease complex. They survive
as resting mycelium and as linear black mycelial inclusions (stromata)
in the thatch. In humid weather, the grass plants exude sugars and
other nutrients during the night in guttation droplets. This provides
a nutrient source for the fungi to begin to grow on the surface
of the leaf blades and initiate infections. Turfgrasses appear to
be more susceptible to infection when nutrient-deficient and when
the soil is dry, probably because both factors reduce the growth
of the grass plants.
Usually, only the leaf blades become infected. Dollar spot rarely
causes severe damage on lawn grasses. All turfgrasses are susceptible
to dollar spot disease, but some Kentucky bluegrass cultivars are
Dollar spot disease usually occurs on lawns that are nitrogen-deficient.
Application of fertilizer will stimulate growth, so the infected
leaf blades may be mowed off. Sufficient water should be applied
when rainfall is sparse. Thick thatch stresses the turfgrass and
increases dollar spot. If dollar spot is present in the lawn, it
is best to mow only when the grass is dry to prevent spreading the
fungus, and to remove the clippings. Mow the grass high and do not
remove more than 1/3 of the leaf blade at a single cutting to avoid
stress. Water deeply and infrequently. Light sprinklings, especially
late in the day, will increase disease. Increase air movement by
pruning trees and shrubs so the grass will dry more quickly.
spot disease responds readily to fungicides, but application of
fertilizer and attention to watering practices is usually sufficient
to minimize the disease. Dollar spot rarely causes sufficient injury
to lawns to justify the use of fungicides.
from the UMass Cooperative Extension, 2000.
Photos from the University of Wisconsin-Madison Turfgrass Diagnostic