Apple scab is a disease that affects nearly all apple and crabapple tree species (Malus) and is caused by the fungal pathogen Venturia inaequalis. Although the main targets of this disease are apple trees, some other trees and shrubs are susceptible, including hawthorn, mountain ash, pyracantha, and many members of the rose family.
This disease has a serious impact on the appearance of leaves, flowers, and fruit. Any time foliage is impacted by disease, the tree or shrub will also be weakened and/or susceptible to other conditions, pests, and diseases.
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Apple scab is common most anywhere its host trees are found. Seasonal temperatures and rainfall play a crucial role in development of the disease. A cool, wet spring will generally give rise to a high rate of infection. This is because moisture is essential for the germination of fungal spores in newly developing leaves.
As its name implies, the main identifying symptom of this disease is the development of scab-like lesions on the leaves, blossoms, petioles, and fruit of the tree. Leaf spots start as slightly discolored, roughly-circular patches, typically along leaf veins. Eventually, the spots will turn darker and velvety. Severely impacted leaves may pucker or develop a cupped appearance. Infected fruit will be spotty and deformed and may crack as it grows. As the disease progresses, the tree will drop fruit and lose leaves.
When timed appropriately, effective fungicide treatments can help control the disease. This should begin as soon as leaf buds start to appear. Once spots are readily visible on leaves, the most effective window for treatment has already passed so it is important to treat preventatively, particularly for plants that have shown symptoms of apple scab in previous years.
As the fungus is spread through the fallen leaves at the end of winter, properly collecting, and disposing of leaf litter will have a significant effect in reducing the spread of apple scab. These efforts are best carried out earlier in the winter or fall, well before leaf buds emerge.
During winter, it is recommended to clean or lightly thin the crown of the tree to improve air circulation. Proper fertilisation has been shown to reduce the severity of the symptoms of apple scab. If you are considering planting new apple or crabapple trees, look for species that are more resistant to the disease.