Halo Blight

With a name like halo blight, you might expect little cherubs in today's post, but that's not the case. The bacteria responsible for halo blight are no angels. Halo blight is a major bean disease, worldwide, affecting kidney beans, lima beans, snap beans, scarlet runners, and many other bean varieties.

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

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 leaves.


​In common blight, those lesions have wide, lemon-colored borders, and they continue to grow. Bacterial brown spot lesions have narrow light green borders and the centers tend to dry out and look tattered. Halo blight lesions tend to stay small and they 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, while symptoms of the other two diseases are difficult to distinguish from each other, both being the same water-soaked lesions seen on leaves.


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 get water to the roots without creating a potential disease site. This is especially important when temperatures are between 68 and 74°F (20 to 23 °C). Unlike many other blights, halo blight bacteria prefer these slightly cooler temperatures.


Also, be sure to start with certified disease-free seed, and place new plants into quarantine before exposing the rest fo your garden to whatever they may be carrying. Speaking of carrying disease, the bacteria responsible for halo blight can also travel on rain splashes, wind, pet fur, shoes and clothing. If you have been exposed to halo blight, you might want to change your clothing and swap shoes to avoid spreading the disease throughout your garden. Finally, when your bean plants have completed their life cycle, cut them off at ground level, leaving soil microorganisms in place, and add plant debris to the compost pile. Leaving plants to break down in the garden can provide potential overwintering sites.


Fixed copper or Bordeaux mixture may be used to prevent halo blight. Plants infected with halo blight should be removed and destroyed, followed by a 2 to 4-year crop rotation program.​