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Symptoms: 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. 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.
Season: 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. Disease Cycle: 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.
Adapted
from the UMass Cooperative Extension, 2000.
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