Halo Blight

Causal Agent

Halo blight is caused by the bacteria Pseudomonas syringae pv. phaseolicola.

Symptoms

The initial leaf symptoms appear as small watersoaked lesions on the underside of leaves. The lesions quickly turn necrotic and are sometimes surrounded by a halo. In cases of severe infections, the entire plant may turn chlorotic. The chlorosis is due to a toxin produced by bacteria at temperatures between 64 °F and 73 °F. During these temperatures, chlorosis of plants can occur. It will not be noticed at higher temperatures. On bean pods, rust-colored, watersoaked lesions are seen. If infections include sutures, bean seed may be discolored and shriveled.

Disease Cycle

As with other bacterial bean diseases, contaminated seed and leftover plant debris are sources of inoculum. Splashing rain, irrigation water, or soil blown by wind can spread the bacteria to susceptible host plants. The bacteria enter the plant through natural openings like stomates or through wounds and colonize the plant. The disease is most severe during cooler temperatures of less than 73 °F.

Management

Managing halo blight is very similar to managing other bacterial bean diseases. Using clean, disease-free seed and crop rotation for at least 2 years to non-host crops will significantly reduce inoculum. Since the bacteria can survive on the leaves of non-host plants or volunteer beans, good weed control is essential. Allow no cultivation work or field walk-throughs if foliage is wet to avoid spreading bacteria from infected to healthy plants. Follow the label and apply copper-based bactericides in the late vegetative or early flowering state.