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Leaf Blights of Dry Bean
Common blight usually is not prevalent until the pods are beginning to fill. Typically, common blight produces large irregular areas of brown leaf tissue. Many of the brown areas are formed near the leaf margins. The brown tissue is usually surrounded by a narrow lemon-yellow border. Common blight usually does not spread in dry weather.
Copper fungicides are not effective against common blight in our area. A laboratory culture may be required to confirm field identification of a bacterial disease and to distinguish between the bacterial blights.
Thunderstorms could result in sudden and dramatic appearance of bacterial diseases. They may spread rapidly and become severe following thunderstorms that cause leaf injury from hail, sandblasting, or wind whipping, followed by rain. The rain is essential to spread of the bacteria and the wounds assist the bacteria in entering the leaf. The bacterial blights can also be spread if plants are cultivated when they are wet. The most common bacterial diseases in North Dakota are: halo blight and common blight.
Halo blight may occur any time during the cropping season. Typical symptoms are small brown spots surrounded by a large light green or yellow halo. The halo ranges from the size of a dime to the size of a quarter. The halo is caused by a toxin produced by the halo blight bacterium. This toxin is produced only when the temperatures are below 70 degrees for at least part of the day. In hot weather, halo blight will resemble bacterial brown spot.
Bacterial brown spot, usually a minor disease, was severe in 1994 due to extended periods of cool wet weather. Symptoms are similar to halo blight except that no halo is formed. Brown spot caused severe defoliation in many fields in 1994.
Copper fungicides help to slow down the spread of brown spot and halo blight but are only moderately effective and must be applied early in the season and continued through multiple applications to provide control. Application after disease is a serious problem is not likely to provide much disease control.
For more information see http://www.ext.nodak.edu/extpubs/plantsci/pests/pp622w.htm
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