These areas become thickened and puckered, causing leaves to curl and severely distort. The thickened areas turn yellowish gray and velvety as spores are produced on the surface by the leaf curl fungus. The loss of leaves and the production of a second set decrease tree growth and fruit production. Defoliation in spring may expose branches to sunburn injury. The leaf curl pathogen also infects twigs and shoots.
Affected shoots become thickened, stunted, distorted, and often die.
If leaf curl infection builds up and is left uncontrolled for several years, the tree may decline and need to be removed. It takes about 2 weeks after leaves emerge from buds before leaf symptoms appear. The fungus grows between leaf cells and stimulates them to divide and grow larger than normal, causing swelling and distortion of the leaf. Red plant pigments accumulate in the distorted cells. Cells of the fungus break through the surface of distorted leaves and produce elongate, saclike structures called asci.