Primarily a superficial disease. However, apples infected with the fungus cannot be sold and large lesions can be points of infection for more serious diseases. Extreme infections of this can also weaken the tree.
The fruit develops dark spots that turn green which can become cracked and corky. The leaves develop round greenish-brown blotches that become necrotic and blister which can lead to premature leaf drop. The twigs develop blistered swellings which the burst during the spring to reveal greenish-brown pustules.
Greenish-brown blotches or blisters on leaves.
Premature leaf fall.
Blistered swellings on twigs
Greenish-brown pustules on twigs in the Spring
Dark spots or cracked and corky blisters on fruit.
Removing fallen leaves from diseased plants can interrupt the disease cycle. Pruning out infected branches and burning them.
In foliage outbreaks, spraying with an approved fungicide once a fortnight from when the buds first break until the blossoms drop. If the wood is infected also, then the spraying will need to continue until harvest. Rigorous spraying can eliminate the apple scab by the third growing season.
Spores land on the plant and infect the wood where they multiply and spread. The fungi can overwinter inside the wood or in dead debris on the ground.
General hygiene around plants eg. Removing dead material from the base of the trees etc. Taking care of the tree, apple scab is often a sign of neglect.