Scab
Venturia spp. — multiple host-specific species: V
67 host plants · Fungal
Last updated
Scab starts as tiny, pinhead-sized pale spots on new spring leaves, then darkens to olive or brownish black over a week or two. The spots enlarge irregularly, often showing a velvety black center with white halos. On fruit, you see raised brown or black circular areas that later develop a corky, cracked appearance. Infected leaves curl, become distorted, and drop early, sometimes defoliating the tree by mid-summer. This disease is especially problematic in areas where spring rains provide the moisture scab fungus needs to spread rapidly.
Scab overwinters on fallen leaves and requires at least 9 hours of leaf wetness at 59-77°F to infect. Cool, wet springs create ideal conditions; you may see 8 or more infection periods per season in such climates. Plant resistant cultivars like Liberty, Akane, or Chehalis apples when possible. If you already have susceptible trees, rake and destroy fallen leaves in fall, shred leaves to speed decomposition, and reduce overhead watering to keep foliage dry. Starting fungicide treatments at bud break (before visible symptoms) is necessary for disease management on susceptible varieties.
Quick Reference
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of May 13, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 2,098 | Ascospore maturity (≥95%) | — | — |
| Kent / Auburn | 2,089 | Ascospore maturity (≥95%) | — | — |
| Seattle / UW | 2,063 | Ascospore maturity (≥95%) | — | — |
| Olympia / Tumwater | 2,025 | Ascospore maturity (≥95%) | — | — |
| Tacoma / Puyallup | 1,993 | Ascospore maturity (≥95%) | — | — |
| Bellingham / Whatcom | 1,972 | Ascospore maturity (≥95%) | — | — |
| Sequim / Rain Shadow | 1,906 | Ascospore maturity (≥95%) | — | — |
Source: Gadoury & MacHardy 1982, Phytopathology 72:901-904 (DOI). About GDD₃₂ →
Note: Infection events require a wetting period of defined duration at a given
No published PNW validation. Model developed in continental
Management
Begin protectant treatment in spring at first bud break, before
Primary infection during spring rain events when emerging
Cultural Controls
- Some apple cultivars have shown good resistance and good quality.
- These cultivars are Akane (Tokyo Rose), Chehalis, Liberty, Prima, and Tydeman Red.
- Intermediate resistance apple cultivars are Jonagold, Macoun, Melrose, Spartan, and King.
- Cultivars Pristine and Enterprise have both powdery mildew and scab resistance.
- Apply nitrogen (5% urea) plus an adjuvant to leaves (on or off the tree) in fall to enhance decomposition of fallen leavses and make them more palatable to earthworms.
- The organic equivalent would be the application of Organic Wet Betty (yucca extract) or 30% yeast extract.
Apple scab is particularly common and destructive west of the Cascade Range in Oregon, Washington, and coastal British Columbia. Western Oregon data (1989-2001) shows an average of 8 infection periods per year lasting about 28.5 hours each. In more arid districts (central Washington, Hood River), the disease is primarily a problem when overtree irrigation is practiced. OSU Extension emphasizes that Western Oregon/Washington's wet spring months create near-ideal conditions for primary infection, and recommends beginning protectant sprays at bud break before symptoms appear, with cultural controls (pruning for air movement, raking or mowing fallen leaves) as the IPM foundation. Dormant-season lime sulfur or copper applications reduce overwintering inoculum.