Physiological Leaf Spot
Physiological Abiotic disorder
Last updated
This profile contains basic abiotic disorder data. Regional field notes and expert review are in progress.
What Causes It
A non-pathogenic leaf spotting documented on Photinia and a few other ornamentals in western Washington. The exact cause is unknown but appears to involve cool temperatures, low light, and cold-air pooling. Small red to purple spots form on the leaf blade but never develop the dark fungal fruiting centers or chlorotic halos that define true leaf spot diseases. The condition is common on susceptible species but causes only minor damage. (Source: PNW Plant Disease Management Handbook, Photinia - Physiological Leaf Spot.)
Quick Reference
Symptoms
Small red to purple spots on leaves that resemble early fungal leaf spot infections. The spots do not develop dark fruiting centers and do not expand or coalesce the way true fungal leaf spots do. Some leaf drop may occur but the plant is not significantly damaged. Symptoms are most severe on plants in low-lying areas, shady sites, and cold-air pockets. (Source: PNW Plant Disease Management Handbook.)
Red to purple spots without dark centers or concentric rings. Absence of fungal fruiting bodies under a hand lens. Concentration on plants in shaded, low-lying, or cold microsites. Spots do not spread through the canopy the way a pathogen would.
Timeline: Symptoms appear during cool wet weather, typically spring, and may persist on affected leaves through the growing season. Not progressive like a pathogen.
Triggers & Conditions
Cold temperatures, low-lying cold-air pooling sites, and shady conditions are documented contributing factors. Exact cause is unknown. (Source: PNW Plant Disease Management Handbook.)
Cool wet spring weather on susceptible species.
Photinia davidiana and some Photinia hybrids planted in cold low-lying Puget Sound sites develop this spotting reliably. Plants in sunnier, well-drained locations rarely show it. The condition is cosmetic and does not warrant fungicide treatment.
Management
Prevention
- Plant Photinia in full sun
- Avoid low-lying cold-air pocket sites
- Space plants for air circulation
- Maintain general plant vigor
Place Photinia on well-drained sunny slopes, not in low frost pockets or under shade trees.
Plant Tolerance
Photinia is the primary documented host. A similar condition is reported on a few Rhododendron cultivars but the etiology may differ.
More Sensitive
- Photinia davidiana
- Photinia fraseri in cool shaded sites
Site exposure and air drainage are the main determinants. Full sun and well-drained slopes reduce incidence.