Table 1. Approximate wetting period required for leaf infection by ascospores of Venturia inaequalis.  Times represent the minimum hours of continuous leaf wetness required for infection at each temperature.

^a When rain begins after sunset, leaf wetness should be assumed to begin at sunrise.  For all other events, times should be computed from the start of rain.  Note that night-released ascospores could cause significant disease in orchards with poor scab control the previous year (i.e., greater than 1% leaf infection at leaf fall).  However, in low-inoculum orchards (most commercial orchards) when rain begins at night, ascospore release, and infection periods, should be assumed to start at dawn.  When alternating wet and dry conditions exist, add the wet hours together, unless the dry intervals are 8 hours or longer.

References:

MacHardy, W. E. and Gadoury, D. M. 1989. A revision of Mills' criteria for predicting apple scab infection periods.  Phytopathology 79:304-310.

Gadoury, D. M., Seem, R. C., Stensvand, A., and Falk, S. P.  1998.  Recent developments in the epidemiology of apple scab.   (Personal communication from D. M. Gadoury - full text available from the author upon request).