Penn State Fruit Research and Extension Center in Biglerville have sent out an alert for apple growers. The first mature spores of the disease were detected on March 24, 2010. The numbers were high for March, being more typical of those observed by mid-April in most years. The findings indicate the growing season is 3 weeks early in Southeastern Pennsylvania. Copper spray applications no later than ¼ inch green tip are highly recommended to manage scab (dormant rate depending on the label). Warm temperatures early in the season will allow large numbers of ascospores to mature and be available for infection at the next wetting periods.
You can purchase a comprehensive guide Fruit Production for the Home Gardener from the office for only $12.00, or view the same information on-line here. The section on Apple Scab can be found here.
Picture and diagram are from Cornell's Fact Sheet.