Apple scab identification & organic treatment
Venturia inaequalis
How to identify apple scab
The most common apple disease in humid climates, producing olive-green, velvety spots on leaves and fruit that later turn dark and corky, sometimes cracking the fruit skin.
Symptoms
- Olive-green, velvety spots on leaf upper surfaces, usually first seen soon after bloom
- Leaves may yellow and drop if heavily spotted
- Fruit develops olive-to-black scabby spots that turn corky, tan, and can crack the skin
- Repeated years of infection can severely reduce fruit set and tree vigor
Host crops
Apple.
How it spreads
Overwinters in fallen leaves on the orchard floor; spores are forcibly discharged during spring rains and carried by wind onto newly emerging leaves and blossoms.
Favorable conditions
Cool, wet spring weather (55-75°F) with extended leaf wetness; the infection period is tied directly to how long leaves stay wet after each rain.
Progression
- Overwintering in leaf litter (Fall through winter) — Fungus develops fruiting bodies in fallen leaves.
- Primary spore release (Bud break through bloom, rain events) — Spores discharge and infect new leaves/blossoms.
- Lesion development (1-3 weeks post-infection) — Olive-green velvety spots appear.
- Fruit infection & secondary spread (Through summer) — Spots spread to fruit; corky scab and cracking follow.
Organic treatment for apple scab
- 1. Rake and remove fallen leaves · medium effort
Clean up all leaf litter under the tree each fall, or mow/shred it finely, to reduce the spring spore source.
When: Fall cleanup
- 2. Copper or sulfur fungicide program · medium effort
Apply an OMRI-listed copper spray at green tip, then sulfur every 7-10 days through petal fall during wet stretches.
When: Green tip through petal fall
- 3. Prune for airflow and sun · medium effort
Open up the canopy so leaves dry quickly after rain, shortening the infection window.
When: Dormant pruning
Prevention
- Plant scab-resistant apple varieties ('Liberty', 'Enterprise', 'Freedom').
- Remove and compost/shred fallen leaves every autumn without fail.
- Space and prune trees for rapid leaf drying.
Easy to confuse with
Cedar-apple rust (orange, not olive-green), Sooty blotch (smudgy, superficial, wipes off with rubbing).