CORVALLIS, Ore. — Wet spring weather increases the risk of apple scab, a common fungal disease that can weaken trees and damage fruit, according to Oregon State University Extension Service experts.
The apple scab fungus overwinters on fallen leaves and fruit. In spring, moisture triggers the release of spores, which travel through the air and infect young apple leaves and developing fruit. Unopened fruit buds are especially vulnerable, while mature fruit becomes more resistant later in the season.
How to spot early signs of apple scab
Early symptoms appear on young leaves as pale, water-soaked spots about the size of a pinhead. These spots enlarge, darken to a smoky color and eventually turn brownish black. They may be circular or irregular.
Infected leaves may:
- Curl
- Become distorted
- Drop prematurely
Severe infections can cause significant defoliation.
On fruit, scab appears as:
- Small, raised brown or black circular spots
- Velvety brown or black tissues that rupture as lesions age
- Brown, corky patches as fruit enlarges
Practical strategies to manage apple scab
Ross Penhallegon, a retired horticulturist with OSU Extension, recommends the following steps to help prevent and control apple scab:
Plant scab-resistant apple varieties. Recommended cultivars include:
- ‘Akane’
- ‘Chehalis’
- ‘Liberty’
- ‘Prima’
- •Tydeman Red’
Enhance leaf decomposition.
- Apply nitrogen — such as 16-16-16 fertilizer or a liquid fish solution — to fallen leaves in fall to speed up breakdown and encourage earthworms.
Shred fallen leaves.
- Use a mower to chop up leaves in autumn to accelerate decomposition.
Prune for better air flow.
- Thinning the canopy improves light penetration and reduces humidity that favors fungal development.
Avoid wetting the foliage.
- Water at the tree’s base rather than overhead.
Apply dolomitic lime in fall.
- After leaves drop, apply lime to raise soil pH and help reduce spring fungal spores.
Use fungicides when needed.
- If weather conditions favor infection — temperatures above 60°F and wet leaves or blooms — apply fungicides such as Bonide Captan, wettable sulfur, summer lime sulfur or Spectracide Immunox. Follow all label directions.
Regional considerations for Oregon gardeners
Oregon’s varied climate affects apple scab pressure and the strategies gardeners need to prioritize.
Western Oregon:
- Highest apple scab pressure due to prolonged spring moisture
- Resistant cultivars and timely fall cleanup are especially important
- Spring fungicide timing is more critical because rains are frequent
Willamette Valley:
- Similar to Western Oregon, with long stretches of leaf wetness
- Daily irrigation that wets foliage can worsen scab — use drip or soaker hoses
- Expect annual management needs unless resistant cultivars are planted
Oregon Coast:
- Very high humidity and low air flow increase scab risk
- Extra emphasis on pruning for air circulation
- Choose scab-resistant cultivars whenever possible
Southern Oregon:
- Warmer, drier springs reduce scab pressure
- Occasional wet periods can still trigger infection — monitor forecasts
- Orchard sanitation alone may be sufficient for many backyard trees
Central and Eastern Oregon:
- Low humidity and fewer rainy spring days significantly reduce scab incidence
- Overhead irrigation can introduce scab conditions — avoid wetting foliage
- Resistant varieties perform very well with minimal intervention
High-elevation areas:
- Cool nights may delay leaf drying, increasing risk despite drier overall climate
- Combine pruning, sanitation and careful irrigation management
Learn more
For additional information, consult the OSU Extension publication Apple Scab.
Previously titled Prepare now to protect apple trees from apple scab
