Halo Blight

With a name like halo blight, you might expect to read about little cherubs, but that’s not the case. The bacteria responsible for halo blight are no angels.

​Halo blight is a global disease of legumes, affecting kidney beans, lima beans, snap beans, scarlet runners, and other bean varieties.

The halo blight bacteria (Pseudomonas syringae pv. phaseolicola) enters plants through tiny wounds, often caused by insect or herbivore feeding or injury, and through natural openings, such as the stoma.

Symptoms of halo blight

Halo blight is frequently confused with bacterial brown spot and common blight. In all three cases, small, water-soaked lesions appear on the leaves. In common blight, those lesions have wide, lemon-colored borders and keep growing. Bacterial brown spot lesions have narrow light green borders, and the centers tend to dry out and look tattered. Halo blight lesions stay small and have prominent light green halos, hence the name.

​Leaves are not the only place damage occurs. Pods can also become infected, making them inedible. Pods infected with common blight have lesions with red or rust-colored borders. Symptoms of the other two diseases are difficult to tell apart, as both have the same water-soaked lesions on the leaves.

Halo blight on mature bean pod (Howard F. Schwartz, CSU, Bugwood.org) CC BY 3.0 US

Managing halo blight

As with other diseases, prevention is the easier way to go. Since moisture is needed for halo blight to develop and spread, avoid overhead watering and save the sprinklers for your lawn. Furrow irrigation will direct water to the roots without creating a potential disease site. Halo blight is most likely when temperatures are between 68 and 74°F (20 to 23 °C). Unlike many other blight diseases, halo blight bacteria prefer these slightly cooler temperatures.

Use these tips to prevent halo blight in your garden:

• Start with certified disease-free seeds.
• Place new plants into quarantine.
• Avoid areas known to harbor halo blight. These bacteria can travel on pet fur, shoes, and clothing. 
• After harvesting, cut bean plants off at ground level, leaving soil microorganisms in place, and add plant debris to the compost pile.
• Employ a two- to four-year crop rotation program.

Fixed copper or Bordeaux mixture treatments may prevent halo blight.