The influence of temperature and leaf wetness duration on infection of perennial ryegrass by Rhizoctonia solani

Controlled environment experiments were conducted to determine the influence of temperature and leaf wetness duration on infection of perennial ryegrass by Rhizoctonia solani. Infection of grass plants raised in pots and exposed to mycelium of R. solani was evaluated at various combinations of temperature and leaf wetness duration. Temperatures included 15, 18, 21, 24, and 27 degrees C. Leaf wetness periods were 9, 12, 15, is, and 24 h. Disease was most severe (more than 50 leaves with brown patch lesions per pot) when plants were in contact with the inoculum source for 24 h of leaf wetness at 24 degrees C. The least amount of disease (0 leaves with lesions per pot) occurred at 15 degrees C and a 9-h wet period. The data were subjected to analysis of variance with orthogonal polynomial contrasts. Significant effects were included in a regression model that described the response of infection to temperature and wetness duration. The polynomial model included linear and quadratic terms for temperature and wetness duration. The adjusted coefficient of determination for the fitted model was 0.93, indicating an excellent fit to the data. The model is intended for use in an improved brown patch warning system for perennial ryegrass in the midwestern United States.