Long periods of wetness are required for sporulation of Stemphylium botryosum f. sp. lycopersici in tomatoes and of Alternaria porri f. sp. solani in potatoes.  Several short wet periods interrupted by dry ones were able to replace long wet periods.  Under these conditions, sporulation of S. botryosum f. sp. lycopersici was somewhat lower, and of A. porri f. sp. solani much higher, than under continuous periods of equivalent wetting duration.  Both pathogens produced more spores when the wet period was extended from 8 to 16 hours per night and the night/day temperature was elevated from 10/20 to 15/25 or to 20/30 C.  In A. porri f. sp. solani only some of the spores formed each night dispersed during the following day.  The number of spores produced each night increased during a week-long period.

Additional key words: epidemiology.