The unusual May snowstorm and the cool wet weather in June created ideal conditions for this fungus.   The apple scab fungus overwinters on fallen diseased leaves. In spring, the fungus forcibly ejects spores into the air from the fallen leaves. Rake up and destroy fallen leaves before the first snowfall to eliminate locations where the fungus can survive to re-infect the crab tree next spring. Fall lawn fertilizer applications will encourage the breakdown of the crabapple leaves. In mulched areas, urea can be applied to chopped leaf litter to encourage decomposition. http://www.extension.umn.edu/distribution/horticulture/M1235-3.html http://www1.extension.umn.edu/garden/yard-garden/trees-shrubs/managing-apple-scab/index.html