Apple scab is a widespread and highly destructive fungal infection that targets apple and crabapple trees globally. It poses a significant threat to commercial orchards and home gardens by severely reducing the quality and yield of fruit. Unchecked infection can lead to the premature dropping of leaves and fruit, compromising the tree’s overall health. This infection is closely tied to wet and cool weather conditions, making it a recurring problem in temperate regions each spring.
Identifying Symptoms on Apple Trees
The first noticeable signs of infection often appear on the leaves shortly after bud break. Initial lesions present as small, dull, olive-green spots that frequently develop a velvety texture due to spore production. These spots darken over time, becoming more defined, and can cause infected leaves to pucker or twist. Severe infections cause leaves to yellow and drop prematurely, sometimes resulting in significant defoliation by mid-summer.
Symptoms on the developing fruit are often the most concerning, as they directly impact marketability. Fruit lesions begin as small, olive-green patches, but quickly develop into rough, dark brown to black, scabby areas. As the fruit expands, the inelastic scab lesions restrict skin growth, causing the apple to become distorted and often leading to deep cracks. Late-season infection may result in “pinpoint scab,” which appears as tiny, dark spots visible only after the fruit is placed into cold storage.
While less common, the fungus can also infect young shoots and twigs, creating small, blister-like lesions or cankers. These infections serve as minor overwintering sites for the fungus and compromise the integrity of the bark. In severe cases, the affected bark may crack and peel away, weakening the woody tissues and potentially allowing entry for secondary pathogens.
The Fungal Cause and Disease Cycle
The cause of apple scab is the fungus Venturia inaequalis, which relies on moisture to complete its life cycle. It survives the winter as fruiting bodies called pseudothecia, embedded within fallen leaves on the orchard floor. As temperatures rise and spring rains begin, these pseudothecia mature and discharge their sexual spores, known as ascospores, into the air.
This release marks the beginning of the primary infection period, which is critical for disease management. Airborne spores are carried by wind and rain to newly emerging, susceptible tissue, such as young leaves and flower buds. If moisture is present on the tissue for a specific duration—a period that varies based on temperature—the ascospore will germinate and penetrate the host’s waxy cuticle. Infection periods typically last four to six weeks, covering the time from green tip until petal fall.
Once a primary lesion is established, it produces a second type of spore through asexual reproduction, called conidia. Conidia are responsible for the secondary infection phase, which repeats rapidly throughout the remainder of the growing season. They are easily spread by splashing rain and wind to new, healthy tissue on the same tree or nearby trees. Since new generations of conidia can be produced every nine to seventeen days under ideal cool and wet conditions, the disease is considered polycyclic, allowing it to spread exponentially.
Control and Prevention Strategies
Effective apple scab management relies on cultural practices aimed at reducing the source of the initial infection. Since the fungus overwinters in fallen leaves, sanitation is a highly effective non-chemical control method. Raking and destroying or composting all leaf litter in the autumn or early spring significantly reduces the number of ascospores available to initiate the primary infection. Applying a nitrogen-based fertilizer, like urea, to the fallen leaves can also accelerate decomposition, suppressing the development of overwintering fungal structures.
Pruning the tree canopy to improve air circulation is another cultural strategy, as it promotes faster drying of leaves and fruit after rain or dew. By reducing the duration of leaf wetness, the window of opportunity for spore germination and infection is narrowed considerably. Proper pruning helps create an unfavorable microclimate for the fungus, making it harder for both primary and secondary infections to take hold.
Selecting apple varieties that possess genetic resistance to the disease is the most sustainable long-term prevention strategy. Many modern cultivars, such as ‘Liberty,’ ‘Freedom,’ and ‘GoldRush,’ carry the Vf resistance gene, which makes them highly resistant to common strains of Venturia inaequalis. Planting these resistant varieties can dramatically reduce or even eliminate the need for regular fungicide applications.
For highly susceptible varieties, chemical control using fungicides is required, and careful timing is paramount. Applications must begin early in the season, starting at the “green tip” stage, to protect new growth from the primary ascospore infection. Applications are repeated at seven- to ten-day intervals, especially during wet periods, and continued until the “petal-fall” stage to interrupt the infection cycle. Rotating between different chemical classes of fungicides is necessary to prevent the fungus from developing resistance.