Unlike most blights, which are fungal in nature, common bacterial blight is caused by bacteria. And blight is not a disease per se. It is a symptom. But many plant diseases with blight symptoms include the word blight in the name. I know, it gets confusing. The important thing here is to be on the lookout for signs of disease.
This particular pathogen (Xanthomonas campestris pv. phaseoli) is attracted to legumes, such as beans, fenugreek, and lupines. [Sadly, I could not find a licensable photo.]
Common bacterial blight symptoms
Common bacterial blight looks a lot like halo blight and bacterial brown spot, but there are differences. Common bacterial blight starts out looking like insignificant angled, light green, water-soaked areas often seen first on the underside of leaves. These areas expand and merge. As plant cells die, these areas turn brown and dry. If you look closely, you can see a distinct yellow halo around these lesions. If a plant is infected with halo blight or bacterial brown spot, that halo would be pale green. Also, leaves infected with halo blight tend to turn pale green, while the lesions from bacterial brown spot tend to merge into large ovals.
Infected stems have sunken, water-soaked areas that develop red streaks. These stems often become girdled, allowing the stem to break and fall over, exposing the bacteria to wind, garden tools, and your pant leg.
Infected pods exhibit round, sunken, water-soaked spots that follow the same pattern of expansion, merging, and drying. These damaged areas may or may not affect the seeds within. You wouldn’t want to plant or eat them, though.
You may be able to see a yellowish bacterial ooze or a dry crust on affected plants. Throw these plants in the trash.
Common bacterial blight management
This disease commonly occurs when contaminated seeds are planted. Hot, humid weather compounds the problem, especially when temperatures reach daytime highs of 82°F–90°F. Common bacterial blight can also be transmitted by beetles, leaf miners, and whiteflies.
To prevent common bacterial blight from occurring in your garden, try to plant resistant varieties. There aren’t any yet, that I know of, but that can change at any time and there are some tolerant varieties, such as Great Northern Harris dry beans. Space plants properly for good airflow and remove any weeds from the legume family. Avoid overhead watering and working around plants when they are wet.
If any infected plants appear, remove them immediately and completely. There are some chemicals that may treat common bacterial blight, but these come at a price. Bacteria can evolve a lot faster than we can.
Fixed copper or Bordeaux mixture may be applied while plants are in the vegetative growth and flowering stages, but prevention is a lot easier and more effective.
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