Venturia inaequalis (Cke.) Wint.
Occurrence and importance
Scab is one of the most important diseases of apples. It occurs in all apple-growing sections of the United States where there is appreciable rainfall during the growing season. In the cooler, humid producing areas of the north central and northeastern States, the disease is so destructive that a commercial crop of apples cannot be produced without following a rigid and costly control program. The widespread acceptance of such a program accounts for the low incidence of scab observed on the market today. The most susceptible commercial varieties are McIntosh, Delicious, Fameuse, Wealthy, Baldwin, Rome Beauty, Rhode Island Greening, and Oldenburg (Dutchess).
The leaves and fruits are principally attacked by the disease, which can also infect twigs under certain conditions. Fruits may be infected under moist conditions any time during their development on the tree. In the infection of young fruits, scab lesions at first appear as small, irregularly circular, more or less raised, dark spots. As the spots enlarge they assume an olive green to dark brown velvety appearance resulting from the growth of the fungus through the ruptured cuticle of the fruits. Enlargement of the spots continues slowly as the fruits develop, but the fungal growth on the surface weathers away. Old lesions become brown and corky (top photo). A fringe of dark fungal growth may persist for some time at the margin. The spots are usually most numerous around the calyx end but can occur anywhere on the fruit. When infection is severe, the spots may coalesce and form irregular lesions 1 inch or more across. Badly diseased fruits are often misshapen because of dwarfing on the side where the infection occurs. Scab lesions developed at midseason (second photo) often remain active and may enlarge very slightly in storage.
Late-season infections that are incipient at harvest time can produce scab spots on fruits in transit or in cold storage. Commonly called storage scab (third photo), the spots are small, generally under 1/4 inch in size. They may be brown or jet black, depending on the variety of apple, and are often shiny because of the intact cuticle. They are circular and darker in color than orchard lesions and have definite borders. Late-season scab visible at harvest is practically indistinguishable from storage scab. Such lesions often remain viable in storage and enlarge by forming a fringe of olive-green mycelia around the margin of the scab spot. These active lesions are generally included with storage scab in judging changes in the grade of apples during storage.
Apple scab is caused by the fungus, Venturia inaequalis. The fungus overwinters mainly in fallen apple leaves in the orchard. From numerous fruiting bodies in the dead leaves, enormous numbers of spores are discharged during rainy periods in the spring to start the disease on newly developing fruits and leaves. The disease is then spread by spores produced in these and ensuing infections. Cool and moist weather conditions are most favorable for infections, which can take place any time during the growing season. The resistance of fruits increases with maturity, therefore late-season infections do not develop as rapidly as earlier ones.
Scab must be controlled in the orchard through effective orchard sprays. Recommendations of the State agricultural experiment station or extension service should be followed.