Reports on Plant Diseases

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Symptoms

The disease appears as round, brown spots (about the size of a silver dollar) in closely cut bentgrass and bermudagrass putting greens. As the disease develops, the spots change from brown to the color of straw and become somewhat sunken: hence, the descriptive term "dollar spot." On coarse, taller, lawntype grasses (for example, bluegrasses, fescues, and ryegrasses) the spots may reach four to eight inches in diameter. Dollar spot is distinguished from most turfgrass diseases by the characteristic girdling lesions on the leaf blades of live plants at the margin of the affected areas. The lesions may be up to an inch long, often at or near the leaf tip. They are bleached white to light tan with a dark brown, reddish brown, or purplish border (Figure 1). The lesions usually extend across the blades of Kentucky bluegrass and fine-leaf fescues. On coarser grasses, such as tall fescue, the lesions tend to occur along the margins of the leaves.

Under close mowing (as on putting or bowling greens), the diseased spots rarely enlarge beyond 2 1/2 inches in diameter. If fungicides are not applied, the spots may become so numerous that they emerge and produce large, irregular, sunken areas of straw-colored dead turf (Figure 2). When dew is present on the blades of grass on overcast days or early in the morning and the dollar-spot fungi are active, mycelia (a white cobwebby or cottony growth) of the causal fungi may be seen on the diseased turf (Figure 3). Guttation fluid (dew) increases the process of infection. Some mycelia may persist under the leaf sheaths, but most disappear as they are dried by the wind and sun. The mycelia of the dollar spot fungi are often confused with spider webs. Spider webs, however, are in a single plane, whereas dollar spot is three dimensional. When turf is maintained at a higher cut of 1 to 2 1/2 inches, the diseased areas are noticeably more irregular in outline and larger, with some spots four to eight inches in diameter. The spots may coalesce to give diseased turf a drought-stricken appearance.

The rhizomes, stolons, and roots are not invaded by the dollar spot fungi, however, a fungus-produced toxin affects the uptake of water and nutrients and causes the roots to thicken, stop growing, and turn brown. The replacement roots soon are similarly affected. This is why dollar spot is most severe in dry soils deficient in nitrogen.

Disease Cycle

Dollar spot fungi survive unfavorable growing periods as black, paper-thin flakes (stromata) on foliage surfaces and in soil, and as dormant mycelia in living or dead turfgrass tissues. In spring or early summer, when the temperature reaches 50 to 60 F (10 to 15 C), the mycelia and stromata resume mycelial growth. The fungi enter plants through cut leaf tips and natural openings (stomates) when plant surfaces are wet. Maximum disease development usually occurs between 70 and 80 F (21 to 26 C) in dry soils where there is a buildup of thatch and soil nitrogen and where potassium levels are low. Infected tissue first appears water-soaked and dark, becoming light tan when dry. If the nights are cool and dry soon after infection has occurred, or if cultural and chemical control measures are applied promptly, damage is limited to the leaf blades, and disease turf usually recovers quickly–especially if it is growing rapidly. If nights are warm and damp, if heavy dews persist after infection, and if fungicides are not applied, the dollar-spot fungi rapidly kill plant tissues, and diseased areas may require weeks or months to recover. When turfgrasses are maintained with adequate nitrogen, potash, and water, less dollar spot occurs, and recovery is more rapid. The fungi spread to new areas mostly through transport of infected sod or clippings by waterland wind or on shoes, hoses, mowers, other turf equipment, golf carts, and animals.