Apple scab development depends on the length of wet spells and the temperature during these spells. The fungus that causes apple scab favors the humid, temperate climate of New Hampshire. Our favorite apple varieties of McIntosh and Cortland are highly susceptible to the fungus. Soon after, these spots become more circular and turn olive green with a velvety grayish-dark surface. When the velvety surface disappears, the spots appear slightly raised and metallic black in color.
Infected fruit has distinct, almost circular scab lesions that are at first velvety and olive green.
In a short time, the fruit becomes darker, scabby and sometimes cracked. Apple scab is a fungus disease that overwinters in dead, infected, fallen leaves. During wet spells the spores are released and are blown or splashed onto the leaves and fruit. The disease is most active from the end of April until late June. Apple varieties that are susceptible to scab injury will need fungicide sprays.