Apple scab on apple trees — symptoms and treatment

Severity: medium

What is Apple scab

Apple scab is a fungal disease caused by Venturia inaequalis. It causes dark, scabby lesions on leaves and fruit, and it is the single most common reason home-grown apples come out blemished and cracked. The fungus rarely kills a tree outright, which is why its severity is rated medium — but it returns season after season and steadily drains the tree through repeated early leaf loss. Fruit from a scabbed tree is still safe to eat once the affected skin is cut away; the damage is cosmetic and structural rather than toxic.

Symptoms

Scab shows first on leaves as dark olive-green spots with a soft, velvety look and diffuse edges, often on the underside or along the midrib. As the spots age they darken toward black and the leaf tissue around them yellows. Heavily spotted leaves drop early, and a tree can shed much of its canopy while the fruit is still developing. On the fruit, infection produces scabby, corky lesions that crack open as the apple expands; late infections may appear only as small black flecks at picking or in storage.

• Early signs: dark olive-green, velvety spots on leaves; faint flecking on young fruit
• Advanced signs: black lesions, yellowing leaves, premature leaf drop, scabby and cracked fruit
• Confusable with: sooty blotch, which wipes off the fruit surface; scab lesions are sunken into the skin and cannot be rubbed away

Causes and conditions

The fungus survives on fallen infected leaves and releases spores in spring, which is why autumn leaf litter is the key to the whole cycle. Spores are carried onto opening buds and young leaves by wind and rain splash, and infection needs the leaf surface to stay wet — so prolonged spring rain, heavy dew, fog and dense, poorly ventilated canopies all favour the disease. Once the first lesions are established they produce further spores that spread within the tree through the growing season, giving repeated waves of infection in a wet year.

Treatment

Sanitation and pruning — cultural

Timing: autumn and dormant season. Remove and destroy fallen leaves in autumn. Prune to improve air circulation. This is the highest-value action available to a home grower, because it removes the material the fungus overwinters on.

Bacillus subtilis (Serenade) — biological

Timing: pre-bloom through harvest. Apply as a preventive spray before infection periods. Can be combined with reduced-rate fungicides.

Mancozeb (Dithane) — chemical

Timing: green tip to petal fall. Apply fungicide spray at green tip stage and repeat every 7-10 days through petal fall. Pre-harvest interval: 28 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Rake up and destroy fallen leaves every autumn; do not compost them near the tree
• Choose scab-resistant apple varieties when planting a new tree
• Prune to open the canopy so leaves dry quickly after rain
• Water at the base rather than over the canopy, and keep trees adequately spaced
• Start protective sprays before symptoms appear — scab is far easier to prevent than to cure

Frequently asked questions

Can I eat apples with scab? Yes. The lesions are on the skin; cut away the scabbed and cracked areas and the fruit underneath is fine. Cracked apples do not store well, so use them first.

When should I start treating? At green tip, when buds first break — the earliest infections happen on the youngest leaves. Waiting until you can see spots means the cycle is already running.

Does it spread to my other trees? It spreads readily between apple trees and to crabapples. It will not infect unrelated plants such as tomatoes or beans.