Dollar 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.
A
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 (See Pics Below). 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 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.
Recent
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.
Damage:
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 relatively resistant.
Cultural Management:
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.
Chemical Management:
Dollar
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.