Today we observed apple scab lesions already producing conidia (above). This lesion appeared on the underside mid-rib of a non-protected Golden Delicious leaf, likely from the infection period March 28-29 (click on "older posts" link at the bottom of the page). Finding lesions now has important implications for possible secondary infection with showers tomorrow. As indicated below, fire blight infection should not occur with the cool temperature but it could be warm enough for cedar-apple rust and quince rust, depending on the length of wetting by showers.
It is quite unusual for scab lesions to appear in just 14 days from the first infection period. I recall this happening only one other time in my 34 years at Winchester. But averaging daily high and low temperatures during this 14-day period gives a mean of about 60 degrees F. At this temperature the incubation period would be about 11 days (see Mills Table, page 88 in the 2010 Spray Bulletin). But the straight numerical average of highs and lows is too high because it doesn't take into consideration that the growth of the fungus would have been inhibited by temperatures above 82 F which occurred four days during the incubation period. This finding does show that the scab fungus is able to survive the 93 F which was recorded Apr 7 in a sheltered weather station 20 ft. from where the lesion was located.
