The fungus that causes scab on apples and flowering crabapples is a serious disease that results in premature defoliation and a reduction in the number of flowers the following year. The weakening effects of the disease can predispose the tree to winter injury and other diseases.
The apple scab fungus actually infects susceptible trees in early- to mid-April and continues for five to nine weeks. To develop, the fungus needs moist conditions on the leaf surface for a period of time, which varies according to the surrounding temperature. In cool weather the leaf must remain wet for longer periods of time in warm weather. The first symptoms appear 9 to 17 days after infection. Given favorable weather conditions, the disease can reinfect the tree throughout the summer.
Apple scab usually first appears on the undersides of leaves as small, irregular lesions that are light brown to olive green in color. As the infection progresses to involve the entire leaf, the lesions become more circular, turn dark-green to brown, then dark brown to black. The infected leaves thicken, which causes the top side of the leaf to bow upwards. Additionally, leaves may curl and scorch at the margins. Severe infections will cause leaves and fruit to drop early.
To control apple scab: