Trees can withstand a mild or moderate infection, says Watt, but heavy infections tend to weaken trees and make them more vulnerable to winter injury and drought. Over a period of years, the infections can kill the trees. If a tree loses most of its leaves during the spring, it may produce an entirely new set of leaves in the same season, he adds. The best preventive maintenance for homeowners, he says, is to remove fallen leaves from around the trees. The fungus overwinters in the leaves, and spores can re-infect trees the following spring.
Spores can also blow in on the wind from other infected areas.
Fungicide sprays applied early in the growing season may protect trees from the disease, but once homeowners notice an infection on an apple or crabapple tree, it is generally too late to treat it, Watt says. Several varieties of crabapples and apple trees are resistant to apple scab disease and should be considered in future plantings.