A Model for Weather-Based Forecasting of Anthracnose on Annual Bluegrass

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Abstract
A multiple regression equation relating hours of leaf wetness and temperature to the incidence of disease incited in annual bluegrass by Colletotrichum graminicola was developed from 2 yr of field data. The model is ASI = 4.0233 - 0.2283LW - 0.5308 T - 0.0013LW2 + 0.0197T2 + 0.0155 (LW .times. T), in which ASI = anthracnose severity index, T = average daily temperature (.degree.C) for a 3-day period 10-12 days preceding symptom expression and LW = average hours of leaf wetness per day for the same period. ASI values of 1, 2, 3, 4, 5 and 6 were equal to < 10, 11-20, 21-30, 31-40, 41-50 and > 51% of the turfgrass area diseased, respectively. The accuracy of the model was tested with data from 3 locations in 1982. The model successfully predicted 14 of 16 periods of disease increase when an ASI value of 2 was taken as the minimum condition for infection. Average daily ASI values predicted from temperature and leaf wetness data were related to rate of disease increase according to the Gompertz transformation.