Bacterial brown spot is a bacterial disease that causes brown spots on beans, lentils, and other legumes.
The bacterium responsible for bacterial brown spot is called Pseudomonas syringae pv. syringae. This bacteria is already found on the leaves of many different plants. Wind, rain, overhead watering, or coming into contact with infected seeds and plants can cause bacterial population explosions in snap beans, pole beans, peas, lima beans, and other legumes. Bacterial brown spot can also infect orchids, so be sure to wash your hands and sanitize your tools.
The pathogen enters through stoma and points of injury, usually when temperatures are below 85°F, and during the rainy season. Before long, symptoms will appear.
Bacterial brown spot symptoms
Bacterial brown spot is frequently confused with halo blight and common blight. In all three cases, small, water-soaked lesions appear on leaves. In common blight, those lesions have wide, lemon-colored borders, and they continue to grow. Halo blight lesions tend to stay small and they have prominent light green halos, hence the name. Bacterial brown spot lesions have narrow light green borders and the centers tend to dry out and look tattered. The spots may overlap, creating an even more tattered appearance. Pods infected with bacterial brown spot become distorted. Lesions are also seen on stems.
These lesions interfere with photosynthesis, weakening plants and reducing your crop. Rather than allowing infection to spread, it is better to remove infected plants and toss them in the trash.
Preventing bacterial brown spot
Start out with certified disease-free plants and seeds. If you buy plants, put them in quarantine until you are certain that they are healthy.
Overhead watering spreads bacterial brown spot infections, plus it provides the bacteria with needed moisture, so don’t do it. Use soaker hoses, drip lines, or simply hold your garden hose closer to the ground as you water.
At the end of the growing season, remove host plant debris and add it to the compost pile. Crop rotation and fixed copper sprays can also reduce the risk of bacterial brown spot.
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