Why Do My Apple Leaves Have Black Scab Marks?

Dark, olive-brown to black scabby spots or patches on apple leaves, often causing the leaf to yellow and drop early, combined with similar rough, corky, cracked or black patches on the developing fruit skin, is apple scab — one of the most widespread and damaging apple diseases in temperate climates. Caused by the fungus Venturia inaequalis, apple scab thrives in cool, wet spring weather and is a persistent problem in many UK and northern European gardens.

The scab disease cycle

The scab fungus overwinters in fallen infected leaves on the ground. In spring, as temperatures rise and leaves open, the overwintered spores are released in wet conditions and infect emerging leaves and developing fruitlets. The earliest infections establish and produce secondary spores that spread the disease further throughout the canopy during wet weather. New infections continue as long as wet conditions persist. The key infection window is from bud burst through about six weeks after petal fall — keeping this period dry or protected is the focus of management.

Damage to leaves and fruit

On leaves: dark olive-brown spots that may cause the leaf to yellow and fall. Severe early infection causes defoliation by midsummer, significantly weakening the tree. On fruit: scab causes rough, corky, cracked, or black patches on the skin. Lightly scabbed fruit is perfectly edible after peeling; heavily scabbed fruit may crack, deform and become susceptible to secondary rots.

Organic management

Rake up and remove (compost or dispose of) fallen leaves in autumn — this removes the primary overwintering source. Apply a fixed copper or sulphur spray from green tip stage through six weeks after petal fall. Improve canopy air circulation through pruning to reduce the humid conditions that favour infection. Avoid overhead irrigation during the infection season.

Resistant varieties — the permanent solution

Many modern apple varieties carry genetic resistance to scab, making fungicide treatment unnecessary. In the UK, varieties like Scrumptious, Fiesta, and Greensleeves have good scab resistance. In North America, the Freedom, Liberty and Pristine series are highly resistant. If scab is a persistent severe problem, replacing susceptible varieties with resistant ones is the most sustainable solution.