Symptoms. The symptoms for all three diseases are very similar, and it generally takes an experienced diagnostician to differentiate them. On the leaves, spots begin as small watersoaked areas. As the spots expand, the center dries and the edges often have a narrow bright yellow band, called a halo. The halo may be present in all three diseases, but is usually wider in Halo Blight. The halo may be small or absent in all three diseases as well, especially if the temperature is above 80 F. On a susceptible variety, the spots will continue to expand until they merge, and can take up large portions of the leaf. The unaffected portions of the leaf can survive without drying. The dry centers of the spots may tear and fall out. In Halo Blight, very severe infection may cause defoliation, wilting, and death of the plant. In Common Blight, dead leaves generally stay on the plant. Common Blight may also cause water-soaked spots to appear on the stems at any time during the season, usually at the lower nodes. These may kill the plant if they girdle the stem, or may weaken it so that it breaks off in a storm.

On pods infected with Common Blight or Fuscous Blight, small round watersoaked spots appear which grow to large irregular patches that may have a reddish border or even be entirely reddish. The spots become brown as they age. During very humid weather, there may be a yellowish crust of bacteria on the surface of the spots. Halo Blight causes similar symptoms on pods, although the spots are smaller, sunken, and brown. The bacterial ooze is white. In severe infections with any of these diseases, the entire pod may shrivel, and the seeds may either not develop or be shriveled. In less severe infections, especially if the suture of the pod is not affected, the seeds develop normally although they may be slightly wrinkled or have a yellowish polished appearance, which may be blotchy or follow veins. Seedlings grown from infected seed have the characteristic spots on stems, cotyledons, and first true leaves. Symptoms may appear later if weather is unfavorable for disease development. In Halo Blight, an older plant grown from infected seed will have yellow areas on the leaves between the veins, with the veins remaining dark.

Similar Diseases. Pod symptoms are similar to anthracnose, which causes a tan to salmon-colored ooze to form in the spots, while the bean blights cause white or yellow ooze.

Prevention.

• These pathogens survive in diseased seed, it is important to use certified disease free seed. One or two infected seeds in a lot are enough to cause a severe outbreak in the field.
• Use seed that has been treated with antibacterial chemicals as well, since dust from processing may contain the pathogen.
• A rotation of 2 to 3 years with no beans is recommended.
• Avoid working in fields with wet leaves.
• Control weed hosts, which include goosefoot, pigweed, and amaranth.
• Plow after harvest to bury debris; the pathogen may survive in debris in mild winters.
• Clean equipment between fields.
• Sanitize equipment if blight has been a problem on a farm.
• Resistance is available for halo blight, but there are not many resistant varieties for common or fuscous blight.
• See current recommendations for chemical control measures.

By Pamela S. Mercure, IPM Program Assistant, University of Connecticut

References

Saettler, A. W. Common Bacterial Blight. pp. 29-30 in Compendium of Bean Diseases. R. Hall, ed. APS Press, St Paul, MN. 1991.

Saettler, A. W. Halo Blight. p. 30 in Compendium of Bean Diseases. R. Hall, ed. APS Press, St Paul, MN. 1991.

Sherf, A.F. and A. A. MacNab. Vegetable Disease and Their Control. John Wiley and Sons, New York. 1986.

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