Development and Evaluation of an Infection Model forStemphylium vesicariumon Pear Based on Temperature and Wetness Duration

Nonwounded fruits of pear cultivar Passe Crassane and plants of cultivar Conference were inoculated with conidia of Stemphylium vesicarium at temperatures ranging from 5 to 30°C and wetness durations from 0 to 24 h. Plants and fruit were maintained for expression of symptoms of brown spot at 20°C and 80 % relative humidity. Polynomial regression models using the log 10 transformation of disease severity as the dependent variable were developed for fruit and leaf infection, and described well the effects of temperature (T) and duration of wetness (W). The coefficients of determination for the fruit and leaf infection models were 0.88 and 0.92, respectively. Optimal conditions for infection were > 24 h of continuous wetness at 22.6°C for fruits and 21.1°C for leaves. The leaf model was evaluated in 40 field trials in Spain and Italy under a wide range of wetness duration periods and temperature conditions, and was validated in relation to its capacity to predict infection periods. The coefficient of determination for the regression of observed disease severity on the predicted disease severity was 0.83 (P = 0.0001) and the slope was not different from I (P = 0.44), but the intercept was -0.08, indicating that the model overestimated infection risk. However, the leaf model under-predicted disease when high relative humidity periods with low wetness duration occurs. The leaf and fruit models also were relatively accurate in identifying likely infection periods in the field. During 2 years of a study in a commercial orchard, the leaf model accurately predicted six, and indicated broadly the other four, of ten infection periods