There are two primary types of bacterial blight found on dry edible beans in North Dakota, halo blight and common blight. Each is caused by a specific bacterial pathogen. Since bacterial blight is seed borne, all bean seed eligible for certification in North Dakota must be tested for the presence of the pathogen. North Dakota Field Seed Certification Standards specifically state that growers are required to submit seed samples of the “harvested seed of each field or seed lot of dry field beans” for bacterial blight testing. Bacterial blight has traditionally been the greatest factor contributing to bean fields failing field inspection.

Symptoms
Field symptoms of common bean bacterial blight include water soaked lesions on leaves and pods that later develop into larger brown necrotic areas on leaf surfaces and into circular brownish lesions on pods. Severely infected leaves may have a “burnt” appearance. Infected seed may be shrunken, shriveled, and discolored. Halo blight symptoms on leaves give the appearance of a yellow ring around each lesion. Pods develop water soaked brown lesions similar to common bacterial blight. Halo blight also causes seed to become shriveled and discolored.

Control
Effective control measures include:

• planting disease-free seed
• avoiding fields with a history of the disease
• resistant varieties

Testing
The bacterial blight test, known as the dome test, has been used by the department for a number of years to detect bacterial blight in dry edible beans. The dome test provides a blight rating score for each sample, permitting an easy comparison of seed lots. Lower dome scores equate to lower amounts of blight in the seed. Dome scores of four or less are considered acceptable for certification.

The dome test actually measures the symptoms of blight in the form of water soaked lesions on the undersides of primary bean leaves. These results are calculated on the basis of the number of lesions observed on the primary leaves of 13-day-old plants. The average lesion number is determined on 30 plants and samples are typically run in duplicate. The average lesion number corresponds to the average area of the leaf surface covered by lesions.