Apple and crabapple trees can beautifully enhance our landscape. Unfortunately, their aesthetic value is threatened by apple scab, a disease caused by the fungus Venturia inaequalis.
When are my trees at risk & what should I look for?
The fungus survives the challenging winter months on infected fallen leaves, or, rarely twigs of the tree. Spores, which infect new leaf tissue in the spring, are produced on fallen leaves during warm rains in April and May.
Late Spring - Summer
From May to early June, olive-green or brown spots develop on leaves. As apple scab develops, tree leaves become chlorotic, causing them to turn a pale yellow and fall prematurely. In mid-summer, a tree could be completely defoliated with only a few leaves remaining for the rest of the season.
Fruit is similarly affected with circular brown spots and black margins. Heavily infected fruits may become deformed. Overall, apple scab is less aesthetically pleasing and reduces the vigor of the tree, making it prone to other diseases.
Prevention & Management:
The best defense against apple scab is providing your tree with properly timed fungicide applications to help reduce the severity of the disease. Fungicide treatments are most effective in the spring because they will help keep the fungus from overpowering and causing early defoliation.
Management for apple scab requires removing infected areas of the tree. In dormant season, branches should be properly pruned to open the tree canopy and improve air circulation. When diseased branches are pruned, they should be disposed of to prevent further spread of the disease. Also be sure to remove any rotted fruit from the tree.
Watch this episode of Talking Trees to learn more about the benefits of dormant pruning.
To reduce potential for infection the following spring, rake and dispose of leaves by burning, burying or completely composting. Infected leaves should not be used as mulch near the orchard.