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Symptoms and signs
Symptoms
Dollar spot is a foliar disease of turfgrass. Affected grasses exhibit white to straw-colored lesions that progress downward from the leaf tip or laterally across leaf blades. A brown border usually surrounds each lesion (Figure 1). 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 (Figure 2). 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 (Figures 3-5). 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 (Figures 4-6).
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Individual dollar spots may be less distinct on higher mowed turfgrasses (Figure 7-9). 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 (Figures 7-12). Dollar spot-affected turfgrass areas often become thinned of foliage and invaded by weed species. 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 (Figure 12), 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.
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Signs
Grayish-white, cottony mycelium often forms on infected grass blades in the early morning hours when dew is present (Figures 13 and 14). Aerial mycelium produced by S. homoeocarpa is similar in appearance to mycelium produced by Pythium aphanidermatum, which causes Pythium blight, and Rhizoctonia solani, the causal agent of brown patch. However, the latter pathogens produce foliar symptoms that are distinct from those produced by S. homoeocarpa. Foliar mycelium of S. homoeocarpa emerges from the white to straw-colored lesions, whereas abundant mycelium of Pythium and Rhizoctonia typically emerges from brown, blighted leaves. Even though the aerial mycelium of the three pathogens is similar in appearance, microscopic observation of hyphae reveals major differences. The most notable difference between S. homoeocarpa (Figure 15A), P. aphanidermatum (Figure 15B), and R. solani (Figure 15C) hyphae is the right and acute angle branching produced by Rhizoctonia hyphae. The branch hyphae appear slightly constricted at the origin of each branch, and a septum is present near a branch origin. Presence of hyphal septa can distinguish S. homoeocarpa hyphae from that of P. aphanidermatum which produces aseptate (nonseptate) hyphae (Figure 15B) except for the occasional wall formation as cells die back. Sclerotinia homoeocarpa also produces hyphae that are generally larger in diameter than hyphae of R. solani (Figure 15).
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Copyright © 2005
by The American Phytopathological Society