Some orchards may have an abundance of other leaf spots that are of no concern at this time of year. Scab lesions can be distinguished from leaf spots caused by other fungi because the latter usually appear as round lesions with distinct margins and necrotic tissue that is visible on both the upper and lower leaf surfaces. Mites, including rust mites, can also cause the entire undersurface of leaves to appear brown or bronzed in some orchards. Mite damage on the underside of leaves should not be confused with the more distinctive spots caused by scab. Continuous wetting periods of at least 48 hours are required to initiate infections on fruit during the preharvest interval.
Fruit gradually become more resistant to apple scab as they mature.
With extremely high inoculum levels, a few fruit infections might occur with wetting periods as short as 30 hours, but economic damage is not likely unless wetting periods exceed 48 hours. Very severe infections could be expected if we should encounter continuous wetting periods of more than 96 hours. Drying periods as short as two hours in the middle of longer wetting periods will significantly reduce the amount of infection that occurs. Apparently infections occurring the during the last week before harvest are not sufficiently well established to allow further development during cold storage. Wetting periods required for storage scab have been investigated in various production regions around the world.