Halo blight

Halo blight of bean
The black spots are necrotic spots or dead tissue due to the disease Pseudomonas syringae. The yellow halos surrounding the spots are chlorotic or yellowing tissue.
Common names	bacterial blight of bean grease spot of bean
Causal agents	Pseudomonas syringae pv. phaseolicola
Hosts	bean
EPPO code	PSDMPH

Halo blight of bean is a bacterial disease caused by Pseudomonas syringae pv. phaseolicola. Halo blight’s pathogen is a gram-negative, aerobic, polar-flagellated and non-spore forming bacteria.This bacterial disease was first discovered in the early 1920s, and rapidly became the major disease of beans throughout the world. The disease favors the places where temperatures are moderate and plentiful inoculum is available.

Common beans in moderate temperature regions are victims of halo blights. Main hosts are lima beans, red kidney bean, cranberry yellow eye field beans, snap beans, scarlet runner, kudzu vine and common P.vulgaris. Halo blight is affected by environment factors and enter through plant injuries or natural openings. The development of the Halo blight is highly favored by cool temperature (such as 20–23°C), unlike other common bacterial blights. In warm temperatures (over 24 °C), the production of phaseolotoxin decreases and symptoms become less obvious. Phaseolotoxin is a toxin produced by Halo blight pathogen which causes systemic chlorosis. Halo blight causes small water-soaked spots on leaves. These spot progressively turn dark brown and are surrounded by a wide greenish yellow halo. The necrotic spots remain small unlike that of common blight. Similar to foliage symptoms, halo blights causes water-soaked spots on vegetative pods. It also causes streaks along pod sutures. If lesions becomes severe on the pods, it may penetrate the pod walls or expand into the pod sutures which causes the seed to be wrinkled and discolored (yellow patches on the seed coat). Systemic infections are not common, but occur more favorably in some dry bean varieties. If the disease develops a systemic infection, it will cause curling, yellowing and death of young leaflets.

The seed is a source of inoculum and pathogen survives in seeds from the previous year. The pathogen of halo blight can overwinter in previously infected bean debris, contaminated seeds, weed hosts or volunteer beans. Halo blight can be dispersed by contact between wet leaves, rainfall, irrigation or people and animals moving through infested fields. The Pathogen can enter in either plant injuries or the natural openings in plants such as stomata and hydathodes during periods of high humidity or when the foliage is wet. Then the pathogen survives from defense mechanisms in intracellular spaces and obtain nutrients from the host. After 6–10 days of infection, bacteria oozes from lesions which causes secondary infections.

Toxin phaseolotoxin is produced, which acts as an irreversible inhibitor of ornithine carbamyltransferase (OTC), an essential enzyme involved in the conversion from ornithine to arginine, an amino acid which is utilized in the biosynthesis of proteins in plants. With presence of 30 pmol phaseolotoxin, it is able to reduce OCT activity to less than 20% of the one of unaffected OCT within, leading to arginine starvation and subsequently prohibiting protein synthesis. As a result, disease symptoms appear within 2 days, where chlorotic lesions appear as yellow halos surrounding black necrotic spots on the infected plants.