Bacterial Blight
Pseudomonas syringae pv. syringae
93 host plants
Last updated
This profile synthesizes data from multiple published sources. Expert field review is in progress.
Bacterial blight shows up as water-soaked lesions on leaves and shoots, typically during wet spring weather when plants are just leafing out. You'll notice affected foliage may curl, wilt, or drop prematurely, particularly on forsythia, Magnolia, or mock orange. The damage is most visible in years with prolonged wet springs, when high moisture and cool temperatures create ideal infection windows. Shoots and branches may have dark, sunken lesions with a greasy appearance.
This disease thrives in overhead watering, poor air circulation, and crowded plantings. Since the bacterium (Pseudomonas syringae) survives on infected plant tissue, your best management is spacing plants properly to allow air flow, avoiding overhead irrigation, and promptly removing infected branches and shoots. Maintaining healthy, vigorous plants through good nutrition helps them resist infection better. Avoid unnecessary pruning or handling of plants when wet to reduce wound entry.
Quick Reference
Management
Fall through spring. Fall leaf drop creates entry points through leaf scars (October-November). Late winter through spring (February-May) is the peak infection window when cool wet conditions and frost events coincide with new growth. Young shoots and flower clusters are most susceptible during spring flush." # Source: PNW Plant Disease Management Handbook (multiple host entries)
Cool wet weather (40-65°F with rain) from fall through spring. Frost events are critical triggers — P. syringae produces ice nucleation proteins that promote frost damage at 28-31°F, creating wounds for bacterial entry. Mild, moist weather favors rapid bacterial multiplication. Rain splash spreads bacteria to new tissues. Late frosts in maritime PNW are common and drive spring outbreaks." # Source: PNW Plant Disease Management Handbook (multiple host entries); lifecycle stages documented in this profile
Cultural Controls
- Maintain proper plant nutrition. Healthy plants resist disease better. source: PNW Plant Disease Management Handbook
- Avoid overhead irrigation to reduce leaf wetness and bacterial spread. source: PNW Plant Disease Management Handbook
- Space plants properly and prune to provide good air circulation. source: PNW Plant Disease Management Handbook
- Avoid injuring plants to reduce possibility of infection. Prune and destroy infected tissues as soon as they are noticed. source: PNW Plant Disease Management Handbook
- Prune in dry summer weather when bacterial activity is lowest. Disinfect pruning tools between plants. source: PNW Plant Disease Management Handbook
- Plant disease-resistant species such as Syringa perkinensis, S. reticulata, and S. josikaea for lilac plantings. source: PNW Plant Disease Management Handbook