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Disease Cycle and Epidemiology

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Disease Cycle

Mycelium of S. homoeocarpa can penetrate leaves directly, enter through cut leaf tips, or enter through stomata and cause infection. Hyphae colonize epidermal and mesophyll cells. The fungus secretes enzymes and toxins that result in tissue necrosis. S. homoeocarpa survives as mycelium or stromata in infected plants. When the environment is conducive for disease, mycelium grows from infected tissue and infects nearby plants. The fungus does not produce spores, so movement of mycelium or infected leaf debris by equipment, people, animals, water, or wind aids in dissemination (Figure 16).

Figure 16

Epidemiology

Dollar spot symptoms develop rapidly at temperatures between 15°C and 32 °C (60°F and 90°F). These temperatures, combined with long periods of leaf wetness from dew, rain, or irrigation favor growth of the fungus and infection of leaf blades and sheaths. The disease is most prevalent during the spring and fall.

Low soil moisture enhances the severity of dollar spot. For example, in one study, foliar blighting of Kentucky bluegrass (Poa pratensis) was more than doubled under conditions of low soil moisture. This probably results from drought or nutrient stress imposed on the turf, increasing host susceptibility.

Turfgrasses grown under low nitrogen fertility exhibit more dollar spot than grasses maintained at optimum fertility. Nitrogen-starved turf has been shown to be more susceptible to S. homoeocarpa because there is a greater amount of senescent foliage compared with turf maintained under higher nitrogen fertility. Senescent foliage provides a good food source for the fungus, and can act to increase the spread of S. homoeocarpa to healthy plant tissues.