Bacterial brown spot symptoms on leaves appear as oval spots that have dead (necrotic) tissue in the center surrounded by a narrow zone of light-green to yellow tissue. Water-soaking and slimy ooze is rarely seen with this disease. The spots may grow together and the centers of the spots often fall out, resulting in an overall tattered appearance of the leaf. On pods, small dark-brown spots develop that result in distorted pod growth.
Comments on the Disease
Bacterial brown spot occurs sporadically and rarely in California. It is readily distinguished from the more serious common and halo blights based on leaf symptoms. Overhead irrigation (sprinklers) increases the risk of infection. Use the table below 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 is similar to that for common and halo blights:
Plant certified disease-free bean seed.
Rotate with other crops such as corn, cucurbits, onions, and tomatoes.
Clean crop and other host debris from the field after harvest to help prevent outbreaks of brown spot.
