Bacterial Blight
Pseudomonas syringae, Xanthomonas spp., Erwinia spp.
Overview
Bacterial blight is a catch-all for Pseudomonas, Xanthomonas, and related bacteria that race through wet foliage, turning leaves, shoots, and blossoms brown to black. Infection enters wounds and natural openings; splash and tools move bacteria bed to bed. Beans, stone fruit, and many ornamentals show similar patterns. Bacterial Blight enters through wounds and natural openings, then rides water films, insects, and contaminated tools. Wet foliage and hail or pruning injuries often precede outbreaks. Causal agent: Pseudomonas syringae, Xanthomonas spp., Erwinia spp.. In regenerative gardens, prioritize sanitation, airflow, and unbroken soil cover over calendar spraying, and intervene early when symptoms appear.
Symptoms
Water-soaked leaf spots that enlarge and turn brown or black; shoot tips wilt and die; blossoms brown; stem cankers; sticky bacterial ooze in humid weather. Damage often follows rain, hail, or pruning in wet conditions. Compare several plants and note whether symptoms spread after rain, insect feeding, or wet foliage. Photograph undersides of leaves and cut stems when wilt is present to refine the diagnosis. Rule out simple drought, fertilizer burn, and herbicide drift before assuming a pathogen - abiotic injury often mimics disease.
Environmental Conditions
Humid
Prevention
Start with clean seed and transplants. Space for airflow. Prefer drip irrigation. Apply copper preventively only where warranted. Avoid nitrogen-driven soft growth that tears easily in wind and rain. Mulch to reduce splash, clear season-end debris, and avoid working plants when wet. Design beds for airflow and diverse plantings that support beneficial insects without creating permanent humid pockets around susceptible crops.
Management
Remove infected shoots in dry weather and disinfect tools between cuts. Copper bactericides can reduce spread on some crops but will not cure systemic infection. Stop overhead irrigation. Rotate annual crops and destroy debris. Do not work plants while wet. Prefer drip irrigation, morning watering, and open spacing so leaves dry quickly. Disinfect pruners between cuts on woody plants. Use copper, sulfur, or biological products only as labeled protectants when pressure is high - they rarely reverse advanced infections alone. Destroy (do not casually compost) highly infectious debris.