A host growth based model to time fungicide sprays against Venturia inaequalis.

Ever since apples are cultivated, apple scab caused by Venturia inaequalis was present and caused serious damages to the crop. Therefore, the host-pathogen relationships, ecology of V. inaequalis and epidemiology of apple scab were studied systems were developed. Almost all of these management tools are 'pathogen oriented', in other words, based on the estimation of the occurrence or amount of mature ascospores and on the predicted severity of ascospore infection. Considering that the decisive time for spraying fungicide against apple scab is during the period of rapid leaf emergence, a more 'host oriented' approach was considered. The system consists of three criteria to time fungicide sprays against primary infections: the number of leaves unprotected, the availability of ascospores, and the infection risk based on the modified Mills table. Assessment of infection risk occurs daily, although hourly weather data are required to calculate degree-days and identify the beginning of rainfall event, duration of surface wetness and temperature. This approach was evaluated from 2008 to 2010 for the scab susceptible cultivar 'McIntosh'. This approach enables better timing of fungicide sprays compared to the standard spray program with minimum usage of at-risk fungicide, and much lower environmental impact of the spray program.