Identification
Apple scab eventually causes defoliation, weakening the affected tree’s defenses and ability to produce fruit. Olive green, velvety lesions will appear on the leaves as well as the stems, blossoms and fruit, eventually darkening to brown. These lesions may initially be mistaken for mold on the fruit or fuzz on the leaves. As the disease progresses, leaves will yellow, becoming curled and distorted. Severe infections will cause the fruit to distort and crack, becoming inedible. These symptoms usually appear in early spring.
Life Cycle
During fall, the fungus overwinters in fallen leaves and any infected fruit remaining on the ground. It remains dormant until late winter and early spring, when the fungus starts its reproductive cycle by producing ascospores that are dispersed by wind and rain to the young, developing leaves of nearby host trees, embedding themselves and marking the beginning of the primary infection phase. In as soon as 10 days, apple scab lesions will start to appear on the infected trees and shrubs. The spread continues over the summer, leading to the potential for secondary infections. The severity of this singular annual reproductive cycle depends on weather conditions, especially humidity and temperature.
Distribution & Habitat
Apple scab emerged in central Asia, with the first recorded reports dating back to 1819 by Swedish botanist Elias Fries. This disease has an extensive geographical distribution, thriving in the same temperate environments across the globe where its primary hosts, apple and mountain ash trees, are prevalent. This fungus favors the cool temperatures and wet spring weather that facilitate the release and germination of its spores. Even so, the fungal spores themselves are not capable of traveling long distances on their own. It is thought that the spread across the globe was caused by human migration.
Damage & Symptoms
The damage from Apple Scab is twofold, impacting both the appearance and the health of trees and shrubs. As shared above, the infected leaves and fruit will become discolored and exhibit scabby lesions. The leaves and fruit will fall from the trees, with the fruit developing cracks and lesions. This disease has a devastating impact on the trees and shrubs it impairs.
Management & Control
Begin by planting varieties of trees and shrubs that are resistant to the disease to offer a proactive defense. Plant in areas that get at minimum six hours of sunlight each day. This will reduce the cool, damp conditions preferred by apple scab. Prune fruit trees to create adequate ventilation and drying conditions will also help.
Well-timed fungicide applications may be necessary during prolonged wet periods in early spring to protect new growth.
Prevention
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