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Bacterial Blight Symptoms
Blight can be observed as small, greasy green (water-soaked) spots on leaves, stems, or pods. On leaves, the water-soaked spots enlarge and are rapidly replaced with dead tissue. If halo blight is the pathogen, the dead tissues is surrounded by a diffused light green zone or halo. Halos are not produced at temperatures above 70 degrees Fahrenheit (21 degrees Celsius.) Sometimes the halo-causing toxin is transported to the upper, young leaves which become yellowed. When halos are not formed, the necrotic spots resemble the symptoms of brown spot, so named because the lesion remains a small chocolate-brown spot with little halo formation.
Leaves infected with common or fuscous blight show large dead areas and sometimes appear as if they have been burned with a torch. Common blight on younger green foliage can be recognized by the distinctive yellow band separating apparently healthy and dead tissue.
All of the blight organisms form water-soaked, greasy-looking lesions on pods. With age, the water-soaked spots become sunken and develop a reddish margin. Masses of bacteria often exude from the centers of the spots. When fresh, the masses are yellowish in the case of common and fuscous blights and cream colored in the case of halo blight and brown spot. They dry to form a flaky surface which glistens in sunlight.
Stem infections occur. Generally the affected areas are dark red. If the area encircles the stem near joints (nodes), the plants can break.
White bean seed heavily infected with common blight turn a butter-yellow color. Pinto or other colored beans generally do not show this color. Infected seed often is shriveled. Heavily infected seeds produce stunted and internally infected plants. Even a few infected plants can cause an epidemic in favorable weather.
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