Apple Scab
Venturia inaequalis is the most widespread apple disease in temperate climates. It overwinters in fallen leaves on the soil surface and releases spores in spring during wet weather, infecting young leaves and fruitlets. Infections in cool, wet springs can be severe enough to defoliate trees and render the entire crop unmarketable. Warm, dry summers significantly reduce infection pressure after initial spring infection.
Symptoms to Look For
- Olive-green to brown, velvety or powdery spots on the upper surface of young leaves in spring
- Yellow halos around spots on older leaves as infections expand; heavily infected leaves yellow and drop early
- Dark, sunken, or corky scabby patches on fruit skin, sometimes cracking as the fruit expands
- Fruit deformed or cracked where early infections occurred on the fruitlet
- Severe early defoliation leaving trees bare by midsummer in a bad infection year
Affected Plants
Organic Solutions
Lime Sulphur Spray
Apply lime sulphur at the green-tip to pink stage in spring, before first infections establish. This is the most effective organic fungicide timing for scab. Follow with sulphur-based sprays every 7 - 14 days during any wet period through petal fall. Lime sulphur can cause phytotoxicity in warm temperatures - do not apply above 24 degrees.
Sulphur-Based Fungicide
Wettable sulphur or sulphur dust applied every 7 - 10 days from green tip through the end of the primary infection period (6 weeks after petal fall) provides good protective coverage. Apply before rain events if possible, as spore release and infection require wet conditions. Sulphur is approved for organic use.
Rake and Remove Fallen Leaves
The fungus overwinters entirely in fallen leaves. Raking up and removing or composting leaves in a hot pile in autumn removes the majority of the overwintering inoculum and dramatically reduces the following spring's infection pressure. This is the single most impactful cultural practice available.
Copper Hydroxide Spray
Copper-based sprays applied at bud swell and again at tight cluster provide early-season protective coverage. Copper is toxic to the scab fungus at the spore germination stage. Use at the minimum effective rate to avoid copper accumulation in soil over many seasons.
Prevention
- Rake up and remove or hot-compost all fallen leaves in autumn - this single step reduces the following season's primary inoculum by 70 - 90%
- Choose scab-resistant varieties where possible - Redfree, Liberty, Freedom, Pristine, and Enterprise have strong genetic resistance and require little or no spray programmes in most locations
- Prune to open canopy structure to improve airflow and accelerate leaf drying after rain - a well-pruned open-centred tree is significantly less susceptible than a dense unpruned one
- Apply the first protective spray at green tip before any symptoms appear - early protective sprays are far more effective than curative sprays after infection has occurred
Garden, by Willowbottom recommends only organic, wildlife-friendly solutions. No synthetic pesticides, no harmful chemicals - ever.