Dollar spot (Sclerotinia homoeocarpa) is a major disease of turfgrasses that is problematic for golf course superintendents throughout most of the United States. Creeping bentgrass (Agrostis stolonifera) is a commonly grown turfgrass species on golf course fairways in many regions of the United States. The disease appears on close-cut turf as spots usually 5.0 cm or less in diameter (Monteith and Dahl, 1932). Individual leaves of infected creeping bentgrass plants shrivel, curl, and turn a bleached white or straw color (Monteith and Dahl, 1932). In advanced stages, spots may become sunken and turf develops a pitted surface. During periods of high humidity when turf foliage is covered with dew, white, cottony mycelium may be present on infection centers.
Hall (1984) developed a dollar spot prediction model based on air temperature and precipitation in Ontario, Canada. He referred to dollar spot as occurring in a series of “steps.” He defined a step as a point at which a decline in the epidemic rate was followed by an increase in epidemic rate. The model stated that a step in a dollar spot epidemic occurred after two consecutive wet days (days in which precipitation occurred) if the average air temperature for the period were 22 °C or greater or after three or more consecutive wet days if the average temperatures for the period were 15 °C or greater. Hall (1984) tested the accuracy of the model by applying a fungicide curatively after infection conditions of the model were met. Using the model, he achieved equal control to that of a preventive program and reduced the number of fungicide applications. Burpee and Goulty (1986) tested Hall's (1984) model in Ontario, Canada, and found that acceptable disease control was not achieved because the model underestimated the number of infection periods.
Based on field observations in Ontario, Canada, Walsh (2000) developed a model to predict the onset of dollar spot using mean air temperature. This model stated that dollar spot would appear after 9 or 10 d of mean air temperatures exceeding 16 °C after 1 May. Growth and development of pathogens and turfgrass hosts are greatly dependent on temperature. Growing degree-day models measure accumulated heat units, which are useful in predicting growth and development of many organisms (Ritchie and NeSmith, 1991). Growing degree-day models have been used to predict the emergence of some weeds and dead spot (Ophiosphaerella agrostis Dernoeden, Camara, O'Neill, van Berkum, et Palm) development in turfgrasses (Brosnan et al., 2010; Fidanza et al., 1996; Kaminski and Dernoeden, 2006). However, we are unaware of any studies using GDD to predict the onset of dollar spot.
Smith et al. (1989) observed that the incidence of dollar spot in cool-season grasses is seasonal and that one epidemic occurs in the spring or early summer and a second occurs in the fall. Hall (1984) and Walsh (2000), however, stated that there was only one annual dollar spot epidemic in Ontario. Although it is well known that dollar spot can be active between spring and fall (Smiley et al., 2005), we are unaware of any research-generated descriptions of the seasonal development and progression of dollar spot epidemics in the United States. A study of the epidemiology of dollar spot is thus warranted given the economic importance of this disease. Furthermore, it is unknown if steps in dollar spot epidemics vary in their incidence based on cultivar grown. Thus, the primary objective of this study was to describe the progression and severity of dollar spot epidemics in six creeping bentgrass cultivars in Maryland. A second objective was to determine if there were GDD relationships with the advent of dollar spot symptoms in spring.
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