By Robert Harveson, Extension Plant Pathologist
Pathogen
Xanthomonas campestris pv. phaseoli (Smith) Dye. Pathogen is a gram negative, straight rod that is aerobic and motile by one polar flagellum. It produces a yellow, non-water soluble carotenoid pigment and mucoid growth on nutrient glucose agar. The pathogen's host range in addition to common bean, includes scarlet runner bean, tepary bean, soybean, Dolichos lablab,Lupinus polyphyllus, Stizolobium deeringianum, Strophostyles helvola, and several species of Vigna.
Disease Symptoms
The first symptoms seen with common bacterial blight are small water-soaked spots on the underside of leaves. The spots enlarge and coalesce becoming brown, dry, and brittle. A narrow yellow border surrounding lesions also characterizes these spots. Lesions may be found in both interveinal areas and along leaf margins (Figure 2). Affected pods develop circular, water-soaked areas that may also have yellow masses of bacteria observed on lesions. Later, spots on pods, and become reddish-brown and sunken. Early pod infection results in small, shriveled and discolored seed.
Favorable Environmental Conditions
Pathogen can survive in the field on infected seed and plant debris. Warm weather favors infection and symptom development. Planting next to a field blighted the year before can expose the current crop to potential infection. The bacterium will spread within fields from infected to healthy plants by storms, people, and machinery. Rain and hail can also move the pathogen from field to field.
Management
Genetic Resistance
Use seed that is blight tolerant, and certified to be disease-free.
Cultural Practices
Don't spread old bean straw into fields to be planted with beans. Avoid walking into fields that are wet. Other cultural practices for management include avoiding the reuse of irrigation water, and incorporating infected debris after harvest and rotating out of beans with other crops for at least two years.
Chemical/Biological Control
Treating seed with streptomycin helps to reduce contamination of the seed coat. Use of copper-based sprays at the mid vegetative to flowering stages will also help to provide some control.
