The Dollar Spot disease is caused by the pathogenic fungal species Sclerotinia homoeocarpa and infects the foliar of warm and cool season turfgrasses.
Affected grasses show white to straw-colored lesions that progress downward from the leaf tip or laterally across leaf blades. A brown border usually surrounds each lesion. Older lesions on higher mowed grass frequently appear hourglass-shaped, being narrower in the middle than at the top or bottom. Individual leaf blades may contain many small lesions or one large lesion or the entire leaf blade can become blighted . Infected leaves become blighted, turning white to straw-colored as lesions expand and coalesce. Blighted leaves are formed in aggregates that appear as circular, sunken patches, measuring from < 1 to > 10 cm (< 0.5 to > 4 inches) in diameter . On golf putting greens and other closely mown areas, the patches appear as white to straw colored spots that are similar in diameter to a silver dollar, hence the name dollar spot. See pictures (Source: http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/pages/dollarspot.aspx ).
Dollar spots may coalesce into large straw-colored areas of blighted turf measuring 15 cm – 3 meters (6 inches to roughly 10 feet) in diameter. Dollar spot-affected turfgrass areas often become thinned of foliage and invaded by weed species.
Be aware that symptoms of dollar spot, Pythium blight, and brown patch may be similar at certain stages of disease development. Usually, dollar spot is not associated with a rapid kill of turfgrass plants as are Pythium blight or Rhizoctonia brown patch. The fungi that cause dollar spot and brown patch often produce distinct lesions on infected leaves , but Pythium blight does not. Even though dollar spot symptoms are confined to aerial parts of turfgrass plants, Sclerotinia homoeocarpa produces a metabolite that is toxic to bentgrass roots. The toxin causes roots to thicken, cease to elongate, and become devoid of root hairs.
grewish- white mycelium in the morning
Biology and Disease Cycle
The fungal pathogen penetrates leaves directly by forming an appressoirum or by entering through stomata or cut leaf tips. The hypae grow and colonize the epidermal and mesophyll cells. The fungal pathogen also secrets enzymes and toxins, which result in necrostic tissues. The fungus survives as mycelium or stromata in infected plants, while spores are not produced.
Conditions for the disease
- optimal temperatures between 15°C and 32°C
- long periods of leaf wetness from dew, rain, irrigation
- most prevalent during springtime and fall.
- low soil moisture (which stress the plant and makes it more susceptibility to the pathogen)
- turfgrass under low nitrogen fertility show mowre dollar spot than onces with optimum fertility.
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