Dollar spot, a disease of turfgrasses caused by the fungus Sclerotinia
homeocarpa, attacks most turfgrasses grown in the South. Bentgrass,
hybrid bermudagrasses and zoysia are most susceptible to dollar spot. The
disease occurs from spring through fall, and is most active during moist
periods of warm days (70-85°F) and cool nights (60°F) in the spring,
early summer and fall. The disease is spread from one area to another by
water, mowers, other equipment or shoes.
Symptoms. On fine textured and close-cut turf, the disease appears
as round, brown to straw-colored and somewhat sunken spots approximately
the size of a silver dollar; thus, the common name "dollar spot".
In coarse textured grasses maintained at taller cutting heights, the dead
spots are larger and more diffuse. Under these conditions dollar spot can
be confused with brownpatch, R. solani. Dollar spot is readily distinguished,
however, by characteristic lesions on the leaf blades of live plants near
the border of the affected area. Lesions are light tan with a reddish-brown
border, and usually radiate from the margins of the leaf blade. On fine
bladed grasses such as bentgrass, the lesions usually girdle the leaf blade.
If the turf is examined when the disease is active early in the day before
the dew dries, cobweb-like mycelium of the fungus can be seen growing on
affected areas. During early stages of the disease, affected plants may
appear water-soaked and wilted, but spots quickly fade to a characteristic
straw color.
Disease Development. Several factors influence the occurrence and
severity of dollar spot. Bentgrass, hybrid bermudagrass and zoysia are most
susceptible; while St. Augustine and centipede are less frequently attacked
by dollar spot.
Low soil moisture has been reported to enhance dollar spot activity, but
moisture from dew, light rain or irrigation must be present on the foliage
for the disease to develop.
The dollar spot fungus is capable of growth over a wide range of temperatures
(50° to 90°F), but disease development is greatest at temperatures
between 70° and 80°F. The dollar spot fungus survives unfavorable
temperature and moisture conditions in plant tissue and thatch as dormant,
compact masses of mycelium, called sclerotia.
Low nitrogen and potassium levels in the soil have been reported to increase
the severity of dollar spot. Some rather severe outbreaks of dollar spot
have been brought under control by the application of soluble nitrogen fertilizer.
However, the beneficial effect of nitrogen is thought to be due to rapid
recovery of the grass during periods of reduced disease activity. Research
has shown that nitrogen increases the susceptibility of grass to dollar
spot.
Control. Cultural practices that promote healthy turf help to reduce
the occurrence and severity of dollar spot.