Scab
Venturia inaequalis
66 host plants
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 Apr 3, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,181 | Ascospore maturity (≥95%) | — | — |
| Seattle / UW | 1,171 | Ascospore maturity (≥95%) | — | — |
| Kent / Auburn | 1,111 | Ascospore maturity (≥95%) | — | — |
| Olympia / Tumwater | 1,106 | Ascospore maturity (≥95%) | — | — |
| Bellingham / Whatcom | 1,101 | Ascospore maturity (≥95%) | — | — |
| Tacoma / Puyallup | 1,075 | Ascospore maturity (≥95%) | — | — |
| Sequim / Rain Shadow | 1,070 | Ascospore maturity (≥95%) | — | — |
Source: Gadoury & MacHardy 1982, Phytopathology 72:901-904 (DOI). Probit model relating ascospore maturity to degree-day accumulation base 0C from green tip. [VERIFY] Exact threshold values cited via NEWA (Cornell) implementation and multiple extension programs; original 1982 paper not directly accessed. About GDD₃₂ →
Note: Infection events require a wetting period of defined duration at a given temperature (Mills table). GDD predicts spore readiness; moisture determines whether infection occurs.
No published PNW validation. Model developed in continental NE US (Zone 5). Maritime PNW has wetter springs (Stensvand dry-period adjustment less relevant) but different diurnal temp patterns. Widely applied across North American apple regions including the PNW extension system. PNW Handbooks cite 865 GDD₃₂ for ascospore season end; Pest Prophet models converge at ~900 GDD₃₂. All 7 HFG weather stations passed 900 GDD₃₂ by late March 2026, consistent with observed primary season completion.
Management
Begin treatment in spring at first bud break before symptoms appear.
Primary infection during spring rain events when emerging leaves are wet. Ascospores released from overwintered leaf litter. Revised Mills table (MacHardy & Gadoury 1989): 45-50°F requires 12-15 hours leaf wetness; 51-60°F requires 6.5-11 hours; 61-75°F requires 6 hours (minimum). Western Oregon data (1989-2001): average 8 infection periods per year, each lasting ~28.5 hours. Most Puget Sound spring rain events deliver 10+ continuous hours of wetness at 50-60°F, nearly guaranteeing infection on susceptible tissue. (MacHardy & Gadoury 1989, Phytopathology 79:304-310; PNW Plant Disease Management Handbook)
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.