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There are three major bacterial diseases of common beans: common bacterial blight (Xanthom*onas campestrispv.phaseoli), halo blight (Pseudomonas syringaepv.phaseolicola), and bacterial brown spot (Pseudomonas syringaepv.syringae). In rainy and windy seasons, it is not unusual to suffer a yield loss of 10 to 20 percent or more, but the greatest loss is a reduction in quality due to pod blemishes. While symptoms vary among the three diseases, disease cycles and management are much the same. Because chemical control is often less effective against common bacterial blight compared to halo blight and bacterial brown spot, accurate disease diagnosis is important.
Symptoms
Common bacterial blight
Common bacterial blight affects bean foliage, pods, and seedlings. Early foliar symptoms are small, angular, light green, water-soaked or translucent spots. During warm and wet conditions, these lesions rapidly enlarge and merge. Gradually, the centers of the lesions become dry and brown, and are surrounded by a distinct, narrow zone of yellow tissue. In highly susceptible varieties, the lesions continue to expand until the leaves appear scorched, ragged, and torn by wind and rain. Pod symptoms consist of lesions that are generally circular, slightly sunken, and dark red-brown. In severe cases, entire pods may shrivel and die. Seeds in less severely affected pods may show no symptoms of disease or may be slightly wrinkled.
Halo blight
Halo blight affects bean foliage, pods, and seedlings. Leaf symptoms appear as water-soaked spots on the lower surface. A zone of yellow-green tissue (halo) appears around the infection points. Infection foci generally remain small. In cases of severe leaf infection, plants develop generalized systemic chlorosis. The systemic chlorosis is particularly pronounced at 64-73?F, temperatures that are favorable for production of the non-host-specific phaseolotoxin. Pod symptoms generally consist of red or brown lesions that may also appear water-soaked. As pods mature, and turn yellow, pod lesions may remain green and may exhibit crusty bacterial ooze on the surface. Developing seed may be shriveled or discolored if lesions expand to involve the pod suture.
Bacterial brown spot
Bacterial brown spot affects bean foliage and pods. The lesions are generally circular, brown, and necrotic and are often surrounded by a blight yellow zone. Lesions occasionally fall out, giving the leaves a shot-hole appearance. Water-soaking of the tissue is generally absent or minimal. Stem lesions are occasionally observed when the pathogen develops systemically. Lesions on pods are circular and initially water-soaked. They become brown and necrotic. Infected pods may be twisted or bent where lesions develop. Occasionally, ring sports of lesions occur around a central lesion.
Life Cycle
The disease cycles of all three pathogens are very similar. The bacteria can survive for six to eighteen months in plant residue on the soil surface. Infected/contaminated seed is a source of inoculum. The bacteria can spread from plant to plant and field to field in many ways, including wind-driven and splashing rains, overhead irrigation, surface-drainage water, and farm machinery. Bacteria enter plants through natural openings or injuries caused by insects, or even accompany other diseases such as rust. Once inside the plant, the bacteria may move systemically to the leaves, stems, and pods and into the seed. Under ideal conditions, even a few infected seeds per acre may be sufficient for a severe outbreak of halo blight. Common blight and bacterial brown spot are favored by cloudy, damp weather and temperatures of 82 to 90?F, whereas halo blight thrives under damp, cooler conditions (64 to 72?F).
Management
- Crop rotation and clean tillage (for example, plowing) help reduce the risk of disease by reducing the amount of inoculum in the immediate area. However, plowing may increase soil loss through erosion.
- Plant only certified, pathogen-free seed from reputable suppliers and produced in semiarid regions.
- Do not enter fields to cultivate or handle plants wet with dew or rain.
- Equipment should be sanitized by spraying with a disinfectant before moving from an infected to a blight-free field.
- Sprays provide good control of bacterial brown spot and halo blight but only moderate control of common blight.
Efforts to develop acceptable bean varieties resistant to bacterial blights have not been very successful, partly because of appearance of new races of the organisms
