Pear Scab
Venturia pirina
0 host plants · Fungal
Last updated
Quick Reference
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of May 7, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,933 | Ascospore maturity (~100%) | — | — |
| Kent / Auburn | 1,920 | Ascospore maturity (~100%) | — | — |
| Seattle / UW | 1,905 | Ascospore maturity (~100%) | — | — |
| Olympia / Tumwater | 1,863 | Ascospore maturity (~100%) | — | — |
| Tacoma / Puyallup | 1,831 | Ascospore maturity (~100%) | — | — |
| Bellingham / Whatcom | 1,814 | Ascospore maturity (~100%) | — | — |
| Sequim / Rain Shadow | 1,756 | Ascospore maturity (~100%) | — | — |
Source: Spotts, R.A., Cervantes, L.A., and Niederholzer, F.J.A. 2000. Pear scab: Components of potential ascospore dose and validation of an ascospore maturity model. Plant Disease 84:681-683 (DOI). Regression model: ln(1/(1-Y)) = -0.00797 + 0.00415X where Y = proportion mature asci, X = accumulated GDD base 32F from bud scale separation. Foundation: Spotts, R.A. and Cervantes, L.A. 1994. Factors affecting maturation and release of ascospores of Venturia pirina in Oregon. Phytopathology 84:260-264 (DOI). About GDD₃₂ →
Note: Ascospore maturity predicted by GDD; actual infection events require rain or prolonged leaf wetness at susceptible temperatures.
Developed in Oregon. Strongest regional fit of any disease GDD model in the HFG system. No separate Western Washington validation, but climate is more comparable than continental models. Maritime Puget Sound may accumulate GDD more slowly in early spring than Hood River.
Management
From bud scale separation through early summer (duration of ascospore release). Most critical during rapid leaf expansion.
Primary infection requires rain or prolonged leaf wetness during the ascospore release period. Optimal infection temperature range 15-25C (59-77F). Longer wetting periods required at cooler temperatures.
Cultural Controls
- Remove and destroy fallen leaves in autumn PNW Plant Disease Management Handbook
- Apply nitrogen (5% urea) to fallen leaves in autumn PNW Plant Disease Management Handbook
- Prune infected twigs showing scab cankers Spotts & Cervantes 1994
- Select resistant cultivars when available PNW Plant Disease Management Handbook
Pear scab is a significant disease in western Oregon and Washington pear production. The ascospore maturity model was developed at the Mid-Columbia Agricultural Research and Extension Center in Hood River, Oregon, making it the most regionally appropriate GDD disease model in the HFG system. Western Washington's wetter springs may extend the primary infection season relative to the Mid-Columbia.
Sources & References
Primary: Spotts, R.A., Cervantes, L.A., and Niederholzer, F.J.A. 2000. Pear scab: Components of potential ascospore dose and validation of an ascospore maturity model. Plant Disease 84:681-683.
- Spotts, R.A. and Cervantes, L.A. 1994. Factors affecting maturation and release of ascospores of Venturia pirina in Oregon. Phytopathology 84:260-264.
- PNW Plant Disease Management Handbook: Pear (Pyrus spp.) - Scab