Common Bacterial Blight

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.

This particular pathogen (Xanthomonas campestris pv. phaseoli) is attracted to legumes, such as beans, and fenugreek.

Common bacterial blight on bean leaf (Howard F. Schwartz, CSU, Bugwood.org) CC BY 3.0 US

Common bacterial blight symptoms

Common bacterial blight looks like halo blight and bacterial brown spot, but there are differences.

Common bacterial blight starts with insignificant angled, light green, water-soaked areas that start out on the underside of leaves. These areas expand and merge. You may also see a yellowish bacterial ooze or a dry crust on affected plants. As plant cells die, these areas turn brown and dry. You may be able to 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 bacterial brown spot lesions merge into large ovals.

Infected stems have sunken, water-soaked areas that develop red streaks. These stems often become girdled, causing them 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 expansion, merging, and drying pattern. These damaged areas may or may not affect the seeds within. You wouldn’t want to plant or eat them, though.

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. Beetles, leaf miners, and whiteflies can also carry common bacterial blight.

Use these tips to prevent common bacterial blight in your garden:

• Plant tolerant varieties, such as Great Northern Harris dry beans. 
• Space plants for good airflow. 
• Remove any weeds from the legume family. 
• Avoid overhead watering.
• Do not work around plants when they are wet.
• Throw infected plants in the trash. 

Some fungicides may treat common bacterial blight, but these chemicals come at a price since bacteria can evolve much faster than we can.

Fixed copper or Bordeaux mixture may be applied while plants are in the vegetative growth and flowering stages, but preventing disease is much easier and more effective.