Agriculture: Dry Beans Pest Management Guidelines

Common Bacterial Blight

Symptoms and Signs

Symptoms of common bacterial blight first appear on leaves as small, water-soaked spots, light green areas, or both. As these spots enlarge, the tissue in the center dies and turns brown. These irregularly shaped spots are bordered by a lemon yellow ring, which serves as a diagnostic symptom of common bacterial blight. The spots or lesions develop on the edges or interveinal areas of leaves. The spots may grow together, resulting in the death of the entire leaf and defoliation of the plant.

Infected pods will have small, water-soaked spots that develop into large, dark red irregular spots, sometimes with a yellow slimy ooze. Pod symptoms of common and halo blight diseases are virtually indistinguishable (see photo comparison). Pod infections can transfer to the seeds inside. White-colored seed may show butter-yellow spots when infected. Heavily infected seed may be shriveled and germinate poorly.

Comments on the Disease

Like most bacterial diseases, common bacterial blight is favored by high moisture and humidity. Because of dry summers and the predominance of furrow or drip over sprinkler irrigation, this disease is uncommon in California. Although common blight bacteria can overwinter in infected debris, survival and dissemination in association with seed is more important. In fact, bean seed for other states is often produced in California because environmental conditions are unfavorable for the development of bacterial diseases. Use the table below and photo comparison to distinguish the symptoms of common blight from halo blight and bacterial brown spot.

Common blight Halo blight Bacterial brown spot
Leaves
  • Large irregular lesions with about 1/4 inch wide lemon colored borders.
  • Water-soaked spots are prominent on the undersurface of the leaves.
  • Lesions may coalesce and leaves may fall off.
  • Begins as small necrotic water-soaked lesions that become surrounded by prominent light green ‘halos' which are diagnostic.
  • Later, the newly emerging leaves take on a light green to yellow color due to the systemic spread of the toxin produced by the bacterium.
  • Heavily infected plants may be defoliated.
  • Bacterial brown spot begins as small water-soaked lesions with narrow light green borders.
  • The lesions expand from round to oval and then centers become tattered or drop out.
  • The tattering of the leaves is often diagnostic.
Pods
  • Pods show water-soaked spots that eventually have red-rusty borders.
  • Pods show water-soaked spots that are difficult to distinguish from symptoms of common blight.
  • Pods may have small necrotic lesions but these are typically much smaller and less prominent that those of common or halo blight.

Management

Control of common blight is very similar to the control for halo blight. Plant certified disease-free seed that was produced in arid regions unfavorable for development of bacterial diseases, such as California and Idaho. Avoid the use of sprinkler irrigation, especially for seed crop production, since sprinklers can provide the needed moisture and humidity for common bacterial blight to develop in California. In fields that have had common bacterial blight problems, deep plow infested bean debris and practice a 2- to 3-year rotation with a non-legume crop.

There are no commercially available resistant varieties, although some tolerant varieties are available (e.g., Great Northern Harris). Pesticide sprays are generally not effective for controlling common bacterial blight.

Text Updated: 01/18