Validation of an ascospore release prediction model for apple black spot (Venturia inaequalis)

A computer model, which uses degree days to simulate ascospore maturation and daily rainfall to simulate release of mature ascospores, was tested on observed ascospore release data.The mean number of degree days from 22 August to 50% ascospore maturation for the observed data (653 o C days) was not significantly different from the model value of 534 o C days.The mean difference between modelled and observed percentage release for 79 individual sampling periods was not significantly different from zero, indicating that the model was not biased towards over-or under-prediction.Ninety percent of the modelled predictions were within ±10% of the observed ascospore counts, indicating good precision.The model accurately identified occasions when <5% ascospore release occurred and when substantial release followed dry weather.