A fungal disease of apple and crab apple trees
Apple scab is a fungal disease that infects the leaves of apple and crabapple trees. The disease does not kill its host but causes leaves to fall from the tree prematurely. The fungus survives the winter in leaves that have fallen to the ground.
Infected trees require chemical treatment for control
Many types of apple and crabapple trees are susceptible to apple scab and will require fungicidal sprays to prevent the disease.
What to look for
• Velvety brown to olive colored spots on leaves; later these spots turn black.
• Heavily infected leaves may turn yellow and drop from the tree.
• Fruit may also display lesions similar to leaves:
– Lesions become brown and corky.
– Infections cause fruit to mature unevenly and crack.
Treatment of Apple Scab
A number of conditions need to be in place for the apple scab fungus to infect the tree:
• Leaf tissue has begun to grow
• Leaves must be wet for at least 6 hours
• Temperatures in the mid-60s or above
• Spores of the fungus need to be in the vicinity
Weather that is the most favorable for leaf infection by apple scab fungus is also the least favorable for applying fungicide to trees. Nonetheless, the spray needs only 15 minutes of contact time on the leaves to achieve effective control. Under the best conditions, we aim to deliver an 80-90% reduction of symptoms on treated trees. Loss of fruits and inner leaves is still common, even on treated trees. We can never achieve 100% control
of any fungal disease.