| The influence of leaf wetness duration (L) and temperature (T) on dollar spot (Sclerotinia homoeocarpa) severity in controlled environments was determined by inoculating pots of creeping bentgrass (Agrostis palustris 'Penncross') and placing them in cabinets set at 10, 17.5 or 25°C for 0, 12, 24, 36, or 48 h of L. Linear regression analysis was used to develop the model that estimated focus diameter, y (cm), for L and T combinations, where: y = -0.3 + [-1.475 + (2.087 x 10-1)T + (2.497 x 10-2)L + (-4.284 x 10-3)T2 + (-6.485 x 10-4)TL]2. This model explained 83.9% of the variation within the data (P < 1.0 x 10-38, n = 141). Epidemiology studies were conducted during 1996--1998 and disease progress curves were best fit to exponential or logistic models, both of which describe polycyclic diseases. Dollar spot epidemics started when 9--10 days with mean air T > 16°C accumulated after May first. Step-wise multiple regression did not correlate weather variables with dollar spot epidemics. In a two-year study, ten impedance leaf wetness sensors were evaluated for accuracy and precision of monitoring dew onset and dissipation on creeping bentgrass. The Campbell Scientific Model 237 sensor, when placed within a Kentucky bluegrass canopy maintained at fairway height, was the best sensor for monitoring leaf wetness duration (LWD) on creeping bentgrass greens. For the entire dew duration of approximately 15 h, this sensor estimated on average within 1.7 h of the actual condition. The estimates from this sensor may be used in currently available turfgrass disease forecasting models because such variation in LWD measurement does not impact the capacity of the models to predict disease. |
