Bacterial Brown Spot

Causal Agent

Bacterial brown spot is caused by the bacteria Pseudommonas syrigae pv. syringae.

Symptoms

Small, necrotic lesion with a yellow-green halo develop in young leaves. Once lesions enlarge and merge, the dead tissue can fall out, giving the leaves a tattered appearance. On bean pods lesions have a water-soaked appearance. As lesions age they become brown and sunken. Occasionally, bacteria will ooze from lesions.

Disease Cycle

P. syringae pv. syringae has a wide host range and is found on leaves of many crops and weeds without causing disease.  It can also survive in plant debris and can be found on bean seed (Schwartz, 2011; Schwartz et al, 2005). The bacteria are splashed onto bean plants by rain or overhead irrigation. They enter leaves or bean pods through wounds and natural openings. Bacterial brown spot is most severe at high humidity (95% or higher) and temperatures of less than 80 °F.

Management

To manage bacterial brown spot, use resistant varieties when available and certified disease-free seed. Incorporate bean residue into the soil. The bacteria cannot survive in the soil once the bean residue is decomposed. Crop rotation for at least 2 years to non-legume crops reduces inoculum in the field. Copper-based bactericides can reduce inoculum on bean foliage and reduce the spread to bean pods.