Apple scab

Apple scab begins as a lesion-forming infection of young apple and crabapple tissues. The causal fungus, Venturia inaequalis, attacks leaves, flowers, petioles, and fruit, with symptoms varying according to tissue age and weather. Newly infected leaves develop pale olive or velvety spots that commonly become brown and may cause puckering, twisting, or premature leaf fall. Fruit lesions are often olive-green at first, then turn brown and corky as the apple grows.¹

Severe fruit infection can produce cracks, deformity, russet-like scarring, and reduced market value. Infection early in fruit development is especially damaging because expanding tissue can split around the lesion. Later infections may remain as small superficial spots. The disease is usually cosmetic on mature fruit, but repeated defoliation weakens trees and can reduce flowering, yield, and winter hardiness in subsequent seasons.

Apple scab epidemics are driven by infected leaves that remain on the orchard floor after autumn leaf fall. The fungus forms overwintering structures in those leaves; during spring, wetting and suitable temperatures stimulate the release of airborne ascospores, which initiate primary infections.²

Rain is central to transmission because spores require moisture to germinate and infect susceptible tissue. After a latent period, new lesions produce conidia that spread the disease through repeated cycles during rainy weather. Disease forecasting models, including systems based on leaf wetness duration and temperature, help estimate infection periods and improve spray timing. Young expanding leaves and fruit are most vulnerable, while dry conditions interrupt the infection cycle. The fungus is not the same disease as pear scab, although related scab fungi affect both crops.

Apple scab is best controlled with an integrated program that reduces primary inoculum and protects susceptible tissue. Removing or shredding fallen leaves, accelerating leaf decomposition with approved orchard practices, and pruning to improve air circulation can reduce the number and duration of infection opportunities.

Resistant apple and crabapple cultivars are among the most durable long-term tools, although resistance is not always absolute and pathogen populations can overcome particular resistance genes. In commercial orchards, fungicides are applied preventively or during forecast infection periods because they are generally less effective after lesions are established. Products must be rotated among effective modes of action to slow fungicide resistance, and labels, preharvest intervals, and local regulations determine appropriate use.³

Home growers should identify symptoms before spraying: nutrient disorders, insect injury, and other fungal diseases can resemble scab. A resistant cultivar, good sanitation, and regular inspection often reduce the need for chemical control.

Apple scab is shaped by a narrow interaction among host growth, fungal biology, temperature, and leaf wetness rather than by rainfall alone. A brief rain followed by prolonged humidity can be more important than a larger storm followed by rapid drying. Forecasting therefore uses environmental measurements, not simply calendar dates or visible symptoms.⁴

Crabapples can function as important sources of inoculum near apple trees, and ornamental plantings may sustain the disease even where fruit production is not the goal. Conversely, resistant ornamental crabapples have been developed and are often evaluated as part of broader landscape disease-management programs. Resistance breeding has drawn on wild and cultivated apple relatives, but durable resistance requires attention to pathogen diversity and orchard practices rather than reliance on a single cultivar trait.

Scab management also has an ecological dimension: reducing unnecessary sprays protects beneficial organisms and lowers selection pressure for resistant fungal strains. Orchard sanitation, disease forecasting, and cultivar choice can therefore complement one another instead of treating fungicides as the sole defense.

Ascospore

A sexual spore released from overwintering fungal structures and responsible for many primary spring infections.

Conidium

An asexual spore produced in lesions that enables repeated secondary spread during the growing season.

Leaf wetness

The period during which water or sustained moisture remains on plant surfaces, allowing spore germination and infection.

Primary infection

The first disease cycle initiated by spores released from overwintered fungal material.

Resistance management

Practices such as rotating fungicide modes of action to reduce selection for resistant pathogen populations.