Symptoms: Olive green, velvety spots 1/4 inch or more in diameter, appear on leaves of Crabapple (Malus) and Apple trees. The leaves may also turn yellow and drop; often the entire tree may defoliate by mid-summer. The fruit and twigs also develop circular, rough-surfaced olive green spots that eventually turn corky and black. The fruit is often deformed. Analysis: This plant disease is caused by a fungus (Venturia inaequalis). Apples and Crabapples are affected especially during wet springs with temperatures ranging in the 60 to 70°F. The fungal spores overwinter on dead leaves and infect new growth by blowing in the wind and landing on new leaves and buds. The fungus develops if there is water on the leaf surface, and spores are splashed onto new leaves every time it rains. The fungus becomes less active in dry hot summers. Control: Since fungus' are better prevented rather than controlled once established, it is imperative to begin treatment of protective sprays as soon as bud growth begins in the spring. Repeat applications 5 to 8 times, at 7 to 10 day intervals. Once the trees are severely infected by early summer, sprays are ineffective. Read and follow all label directions, there are many products labeled for use in controlling apple scab. Sprays containing Daconil 2787® (Chlorothalonil) are labeled for use on flowering crabs, and sprays containing Immunox™ are labeled for use on Apple trees. Rake up and destroy all leaves and fruit in the fall; do not put them in your compost bin. Plant resistant varieties when selecting new trees.  Return to Gardening Tips |
