What is Apple Scab?
Apple scab is a common fungal disease that affects apple and crabapple trees. It is caused by the fungus Venturia inaequalis, which thrives in cool, wet spring weather. While it rarely kills trees, it can significantly reduce fruit quality and weaken overall plant health.
Apple and crabapple trees are the primary hosts, but the fungus can also affect related species such as hawthorn and mountain ash.
Life Cycle
The apple scab fungus survives the winter in fallen leaves on the ground. In early spring, as temperatures warm and rains become frequent, the fungus releases spores that travel short distances through wind and splashing rain. These spores infect newly emerging leaves and young fruit.
The disease spreads throughout the growing season as secondary spores develop on infected leaves and fruit. Cool, humid conditions—especially extended leaf wetness—favor rapid infection cycles.
The fungus persists year after year unless fallen leaves are removed or decomposed, as these leaves are the main overwintering site.
Symptoms
Early Stage
- Small, pale, olive-green spots on young leaves
- Slight leaf puckering or distortion near infected areas
- Early lesions may be difficult to spot without close inspection
Middle Stage
- Spots enlarge and darken, becoming velvety or brownish
- Fruit starts developing similar scabby spots
- Leaves may yellow and drop prematurely
Late Stage
- Heavily infected leaves fall off, causing thinning canopies
- Fruit becomes misshapen, cracked, or severely scabbed
- Overall tree vigor declines due to repeated defoliation
How to Control Disease Progression
If you notice signs of apple scab, taking quick action can slow its spread and limit damage.
- Remove and dispose of fallen leaves to reduce spore sources.
- Prune trees to improve airflow, which helps leaves dry faster after rain.
- Avoid overhead watering during the growing season.
- Thin fruit clusters to improve air circulation around developing apples.
- Apply preventive fungicide sprays if the disease is recurrent and conditions favor infection.
Treatment Options
Organic Methods:
- Use horticultural oils or sulfur-based fungicides early in the season before symptoms appear.
- Encourage leaf decomposition in fall by mulching leaves or applying a nitrogen-rich fertilizer that speeds breakdown.
- Plant resistant apple or crabapple varieties to reduce long-term issues.
Chemical Options:
- Conventional fungicides (such as those containing myclobutanil or captan) can help prevent new infections when applied during bud break and early leaf development.
- Follow label directions carefully, repeating applications as recommended during wet spring weather.
Step-by-Step Treatment:
- Start with a dormant-season cleanup—remove old leaves and prune for light and airflow.
- Apply the first fungicide spray at green tip (when leaf buds just begin to open).
- Repeat sprays every 7–10 days during wet spring weather until conditions dry out.
- Continue monitoring throughout the season and remove infected leaves as possible.
Expected Recovery Timeline: Trees typically show improvement within one growing season if managed consistently. Full control often takes 1–2 years of proper sanitation and preventive treatment.
Prevention Strategies
- Choose apple or crabapple varieties bred for scab resistance.
- Plant trees in sunny areas with good airflow to help leaves dry quickly.
- Practice good fall cleanup by removing and composting or disposing of leaves.
- Mulch properly to avoid splashing soil and spores onto lower branches.
- Prune annually to maintain an open canopy and healthy structure.
- Monitor weather in early spring—wet, cool conditions may call for preventive fungicide sprays.
