Dollar spot, caused by Sclerotinia homoeocarpa, is a widespread and very destructive turfgrass disease that can be observed throughout the year in the United States. Dollar spot is known to attack most turfgrass species including annual bluegrass, bentgrasses, fescues, Kentucky bluegrass, perennial ryegrass, bermudagrasses, zoysiagrasses, centipedegrass and St. Augustinegrass.
Conditions: Dollar spot is favored by air temperatures ranging from 60-90 degrees F (optimum 70-80 degrees F) with extended periods of high humidity (greater than 85 percent humidity at night). The disease tends to be most severe under the following conditions: warm days, cool nights, infrequent rain with long dew periods, daily ground fogs that extend leaf wetness periods and low nitrogen fertility. When the fungus is active and leaf surfaces remain wet, a fine, white, cobwebby mycelium covers the infection centers or diseased patches during early morning hours.
Symptoms: Dollar spot symptoms can vary based on the turfgrass species and height of cut. Under close mowing heights, as with intensively maintained bentgrass or annual bluegrass, the disease appears as small, circular, straw-colored spots of blighted turf about the size of a silver dollar. On the coarser textured turf maintained under higher mowing practices, such as Kentucky bluegrass or perennial ryegrass, the blighted areas are considerably larger, straw-colored patches that are 3-6 inches in diameter.
Affected patches frequently coalesce and involve large areas of turf. Grass blades generally die back from the tip with distinct hourglass-shaped lesions that are straw-colored or bleached white. Hourglass bands may not appear on warm-season grasses.
Cultural Control Strategies: Implementing proper cultural practices is crucial to reducing disease severity. Management tactics include the following: maintaining adequate nitrogen when dollar spot is active, making frequent light nitrogen applications, avoiding drought stress, limiting irrigation toward dusk, removing dew by mowing, poling, or rolling, aerifying to reduce compaction and thatch and removing trees or adding fans to increase air circulation.
Chemical Control Strategies: Integrating cultural and chemical control strategies provides the most effective control of this fungal disease. Fungicides are commonly used to manage dollar spot. Since dollar spot is a foliar disease, select spray nozzles and spray volumes that provide good coverage to maximize fungicide activity. Early spring applications of fungicides after the second true mowing can control early dollar spot infections. Preventative applications of DMI fungicides for fairy ring and waitea patch (average soil temperatures of 55 degrees F at a 2-4 inch depth) also provide early dollar spot control. Routine fungicide applications are commonly needed when air temperatures are 70-90 degrees F. Extended periods of temperatures above 90o F may significantly slow dollar spot development.
Resistance: Dollar spot resistance to classes of fungicides can be a significant issue. While resistance to the dicarboximide and DMI fungicides can occur, it is far less common compared to benzimidazole fungicide (thiophanate-methyl) resistance. Preventive applications and rotating fungicide classes, including the use of multi-site fungicides, is important for reducing the risk of resistance.
Dr. Rob Golembiewski is part of the Bayer Green Solutions Team.