Climatic transferability and regional validation of a PAT-based mechanistic model for apple scab ascospore maturation under temperate Himalayan conditions.

The seasonal availability of airborne ascospores of Venturia inaequalis is a key determinant of primary apple scab infections, and mechanistic models describing ascospore maturation are widely used in disease forecasting systems. However, the applicability of such models under climatic conditions different from those for which they were originally developed remains insufficiently evaluated. This study presents a field evaluation of the climatic transferability of the Proportion of Ascospores capable of becoming Airborne (PAT) based ascospore maturation model proposed by Rossi et al. in 2000, under the temperate Himalayan conditions of Jammu and Kashmir, India. The original model structure and equations were retained and applied using hourly meteorological data (air temperature, rainfall, relative humidity and leaf wetness) collected over six consecutive seasons (2020-2025). Model outputs describing ascospore maturation and discharge dynamics were evaluated against independent field observations of airborne V. inaequalis ascospores obtained using a volumetric spore trap. Validation was conducted using event-based biological validation and statistical goodness-of-fit metrics. The model consistently reproduced the timing of ascospore availability, the accelerated maturation phase, and the exhaustion of the primary ascospore pool. Approximately 90% of observed ascospore release events occurred within model-predicted release-favourable windows. Statistical evaluation of maturation trajectories showed excellent agreement between predicted and observed dynamics (R