Most common on crabapple, apple, pear and many other members of the rose family.
Leaf spots appear in spring as small olive-green lesions on the uppper and lower leaf sufraces. As spores form, lesions develop a velvety brown appearance. Later, lesions are more elongate and often follow veins, but the same velvety brown look is present. As leaves age, they turn yellow (except for the scab lesions) and fall from the tree. In severe years, susceptible crabapples and apples may be defoliated by late June. Flower parts, fruit, and succulent twigs can also be infected. Tissues may become curled or puckered, and fruit and twig lesions may be somewhat raised. Severely infected fruits may crack.
The fungus (Venturia inaequalis
) overwinters in fallen infected leaves. Spores (ascospores, sexual spores) are released in wet spring weather during bloom. Infection occurs under a wide range of temperatures, but wet plant surfaces are necessary. The severity of the primary infection increases with the duration of wetting. The disease is capable of causing severe damage in wet springs. Secondary cycles of infection occur primarily by conidia that move with water splash or run along leaf surfaces. For this reason, secondary infections are more elongate and follow veins.
This disease is so common in areas with wet spring weather that it is expected annually; the question is only how severe the infection will become. For new plantings, there are many choices of desirable resistant varieties and resistance is by far the preferred means of management. Removal of fallen leaves can help decrease the primary inoculum levels, but the fungus is so common and spores so readily dispersed that infection is still likely to occur. Fungicides can be used to control this disease but require multiple applications on a preventive basis, beginning when leaf buds break and repeating until two weeks after petal fall. However, under prolonged wet periods, additional applications may be necessary to maintain high aesthetic value.