Disease Management

Bean Bacterial Blight

Bacterial Blights of Common Bean

There are three major bacterial diseases of common bean: Bacterial Brown Spot (Pseudomonas syringae pv. syringae), Common Blight (Xanthomonas campestris pv. phaseoli), and Halo Blight (Pseudomonas syringae pv. phaseolicola). These diseases may be difficult to distinguish in the field; isolation and identification by laboratory procedures is often required.

Symptoms of Bacterial Brown Spot first appear as small, circular, brown and necrotic spots, often surrounded by a yellow zone. The lesions may enlarge, coalesce, and fall out giving leaves a ragged appearance. Symptoms on pods are initially water-soaked and circular, becoming brown and sunken. Distortion of pods can occur. The Brown Spot pathogen can survive and grow on the surface of some plants without causing disease (epiphytically); symptoms develop only after favorable conditions occur. The bacteria overwinter in weeds, in bean crop residues, and on seed.

Symptoms of Common Blight initially appear as water-soaked spots that enlarge, lose turgidity, and become necrotic and bordered by a small zone of yellow tissue. Lesions may coalesce lending the plants a burned appearance. Pods develop roughly circular, slightly sunken, red-brown lesions which become covered with bacterial ooze during humid periods. Seeds develop yellow to brown spots, become shriveled, and exhibit poor vigor and germination. Common blight is carried both on and in seed, in bean crop debris, and epiphytically on volunteer beans and perennial hosts. X. campestris pv. phaseoli is a warm climate pathogen, favored by temperatures of 28-32° C and high humidity. It is spread by windblown rain, soil, and plant debris; contact between wet leaves; irrigation water, people, insects, and contaminated equipment.

Symptoms of Halo Blight first appear on leaf undersides as small, water-soaked spots which develop into numerous reddish-brown lesions with green to yellow halos which vary greatly in size. Severe infections can result in the bacteria becoming systemic and cause leaf yellowing and death. Pod symptoms consist of water-soaked, red or brown lesions and may exude bacterial ooze. The disease is favored by temperatures of 18-22° C, high humidity, and free moisture on leaves, stems, or pods. The pathogen is carried in and on seed and infected plant debris; like the other bacterial pathogens, P. syringae pv. phaseolicola is capable of surviving epiphytically. The pathogen enters the plants through natural openings or wounds and is spread by wind-blown rain, soil or plant debris as well as contact between leaves, people, and contaminated equipment.

Management

• Only certified, western-grown, disease free seeds should be planted. Never save your own seed.
• Choose bean varieties with resistance or tolerance to the bacterial diseases.
• Incorporate infected bean residues promptly after harvest and thoroughly cover.
• Avoid pesticide application, harvesting, or cultivation when leaves are wet.
• Rotate out of beans for a minimum of two years.
• Streptomycin seed treatment can reduce surface contamination of the seed, but will not control systemic infections of bacterial diseases.
• Weeds and volunteer beans should be eliminated.
• Use furrow irrigation rather than overhead irrigation where possible.
• Copper-based bactericides can reduce epiphytic bacterial populations and reduce disease severity when applied preventively. However, these materials cannot eradicate the bacteria and if conditions remain favorable, the bacteria will increase and spread rapidly. For this reason, copper-based bactericides are not highly recommended.

References:

Dillard, H. R. and D.E. Legard. 1991. Bacterial Diseases of Beans. http://www.vegetablemdonline.ppath.cornell.edu .

Howard, R.J. et.al. eds.1994. Diseases and Pests of Vegetable Crops in Canada. Canadian Phytopathological Society.554 pp.

Schwartz, H.F., et al, Eds. 2005. Compendium of Bean Diseases. APS Press109 pp.