When is the right time to treat for apple scab?
You are correct, early spring is the most important time to apply controls for apple scab. However, in some cases additional applications are beneficial. The following excerpt from a University of Minnesota bulletin explains why:
"When should you start spraying?
To protect leaves and fruit from scab, most home apple growers need to spray fungicides in spring. Sprays should start at the growth stage known as "half-inch green tip," when the leaf buds have swollen and begun to open so that about half an inch of leaf tissue is visible.
How often should you spray?
Scheduling fungicide sprays for scab after the first application at half-inch green can involve a little guesswork. In warmer weather, leaves grow quickly, and newly-exposed tissue will be unprotected. In cooler weather, growth slows or stops
Check fungicide labels for the recommended spray interval. Most labels offer a range of days to wait before spraying again. (E.g. seven to ten days after spraying, you will need to spray again). In plantings where there was a severe scab infection the previous year, use the shortest interval. In plantings where scab has not been a problem, a longer interval will probably give adequate protection. In addition if the weather is dry the longer interval is acceptable.
When can you stop spraying for scab?
In mid-June, examine the leaves on your trees for scab lesions. Be very thorough, checking upper and lower leaf surfaces, leaves on the interior and exterior of the canopy, leaves close to the ground and those higher in the tree. If you find no or very few apple scab leaf spots, you need not spray fungicide again. If you find scab lesions, or if there are unsprayed trees in your neighborhood with scab lesions, you should continue to spray, because the lesions on the leaves will release more scab spores all summer long.
If scab has been a problem in your apple planting, it may take a year or two to get it under control. If you continue with appropriately timed sprays that cover all leaf and fruit tissue, and practice excellent sanitation of fallen leaves, and if outside sources of fungal spores are few or distant, you should find in the second or third year that you only need to spray from half-inch green tip to mid-June."
Go here to read the complete bulletin: