How to Manage Pests

UC Pest Management Guidelines

Dry Beans

Halo Blight

Pathogen: Pseudomonas syringae pv. phaseolicola

(Reviewed 1/18, updated 1/18)

In this Guideline:


SYMPTOMS AND SIGNS

Halo blight symptoms first appear as small, angular, water-soaked spots (almost resembling little pin pricks) on the undersurfaces of leaves. As these spots grow and turn brown, a characteristic light green to yellow halo appears around the spots. This halo is due to the action of a toxin produced by the bacteria and is a diagnostic symptom of the disease. In severe infections, the leaves and upper parts of plants turn yellow (chlorotic). On pods, small water-soaked spots, about the size of pinpricks, develop that grow into sunken spots and turn reddish brown. Sometimes a creamy white ooze may be seen inside these spots. Pod symptoms of common and halo blight diseases are virtually indistinguishable (see photo comparison). Pod infections can transfer to the seeds inside, causing them to be shriveled, discolored, or smaller than normal size.

COMMENTS ON THE DISEASE

Halo blight disease occurs worldwide. It can cause extensive losses under humid, moist conditions and moderate temperatures (60° to 73°F, 16° to 23°C). Fortunately, such conditions are unusual in California and thus halo blight is relatively uncommon. Halo blight bacteria can overwinter in infested debris or seed. Infested seed is the most important inoculum source. Use the table below to distinguish the symptoms of halo blight from common 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 halo blight is very similar to the control for common bacterial blight.

  • Plant certified disease-free seed produced in arid regions unfavorable for development of bacterial diseases, such as California and Idaho.
  • Use furrow irrigation.
  • Avoid sprinkler irrigation since this can provide the needed moisture and humidity for halo blight development in California.
  • In fields that have had halo blight problems, deep plow infested debris and practice a 2 to 3-year rotation with a non-legume crop.

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Dry Beans
UC ANR Publication 3446

Diseases

C. A. Frate (emeritus), UC Cooperative Extension, Tulare County
P. Gepts, Plant Sciences, UC Davis
R. F. Long, UC Cooperative Extension, Yolo County

Acknowledgment for contributions to Diseases:
A. E. Hall (emeritus), Botany and Plant Sciences, UC Riverside
R. M. Davis (emeritus), Plant Pathology, UC Davis
R. L. Gilbertson, Plant Pathology, UC Davis
S. R. Temple (emeritus), Plant Sciences, UC Davis

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