By Katherine Gathje, U of M Interim Extension Educator
Apple scab is the most common disease among apple trees in many states. It is caused by a fungus that affects the appearance of the leaves and fruit. Here are some ways to identify apple scab and manage the disease.
Apple scab creates noticeable spotting on the leaves and fruit. Spots on the leaves are round, olive-green, and located along leaf veins. They can be up to a half-inch in length and have velvet-like borders. As the fungus progresses, the spots turn brown and black and can grow together. Some trees with cases of apple scab can drop the leaves during mid-summer. On the fruit of the apple tree, olive-green spots will also form; however, they become corky over time. If the apple is infected young, it may cause the fruit to become deformed and crack.
The fungus that causes apple scab is Venturia inaequalis. This disease can infect apples, crab-apples, mountain ash, pears, and Cotoneaster trees. The strain is specific to each genus, meaning an apple tree infected will not transfer to a pear tree. The fungus lives on fallen diseased leaves in the winter, and shoots spores in the air during spring. The spores are carried by wind to newly developing twigs, leaves, flowers, and fruit. With moisture, the fungus can start new infections. The new infections will develop into spots and produce their spores in 9-17 days. These spores will spread throughout the tree and to neighboring trees of the same species. The life cycle continues with moisture and can happen many times throughout the growing season. The ideal condition for apple scab is warm and rainy weather. With many years of the disease, a tree will become weakened resulting in early leaf loss, decreased growth, and a higher susceptibility to winter injury.
The best way to prevent apple scab is to plant disease-resistant varieties. Many trees are either disease-resistant or completely immune to apple scab. Examples of apple trees very likely to be infected include McIntosh and Honeygold. Honeycrisp is resistant to the disease. Redfree and Liberty are two varieties immune to apple scab. When planting new trees, consider their resistance. Make sure to properly space the trees with others by considering their mature size. More information on tree varieties can be found at http://extension.umn.edu/meeker.
Practices can be done to manage the disease as well. Cleaning up leaves in the fall before the first frost occurs is the best way. Rake together and destroy the leaves by burning or composting. Leaves can also be chopped with a mulching mower, and a fall lawn fertilizer will help break the leaves down further. Another way to help the tree is to create proper airflow. Pruning the tree to have an open canopy aids in drying leaves when there are high levels of moisture. Fungicides are another consideration to be used on specific varieties of apple trees.