Halo blight

Halo blight of beans is found worldwide, but particularly problematic in temperate and subtropical regions. The disease is reported in North America, South America, Europe, Africa and Asia, especially in regions with high humidity and moderate temperatures (16-28°C).

Biology

Pseudomonas syringae pv. phaseolicola is a seedborne bacterium that spreads through infected seeds, rain splash, irrigation and mechanical contact. The bacterium survives in crop debris and on contaminated plant surfaces. It enters plants through natural openings (stomata, hydathodes) or wounds and prefers cool (16-24°C) and humid conditions for optimal growth and infection.

Symptoms

Halo blight of beans initially manifests as small, water-soaked lesions on the leaves, which are often surrounded by a characteristic yellow-green halo. These lesions may expand and coalesce, leading to large necrotic areas that can cause significant leaf damage. As the disease progresses, irregular brown necrotic spots develop, further weakening the plant. Infected pods exhibit dark, sunken lesions with a greasy appearance, which can reduce the marketability of the beans. Additionally, seeds from infected plants may be discolored and shriveled, impacting seed viability. In severe cases, the disease can lead to extensive defoliation and reduced plant vigor, making the plants more susceptible to other environmental stresses and infections. In Idaho, plant quarantine measures involve field inspections, certified seed tagging and strict regulations on imported seed to prevent disease introduction and spread.

Damage

The impact of halo blight on bean crops can be severe, leading to significant economic losses. The disease reduces seed quality and yield, making it a major concern for both commercial and subsistence farmers. In fields with high disease incidence, crop losses can be substantial, sometimes leading to complete failure. The weakened plants are more vulnerable to secondary infections, further exacerbating yield reductions. Additionally, the presence of Pseudomonas syringae pv. phaseolicola in seed lots can result in quarantine restrictions, limiting international trade and affecting market access for growers.

Management

Managing halo blight requires an integrated approach combining cultural, chemical and genetic strategies. Cultural control focuses on prevention through certified disease-free seeds, crop rotation, avoiding overhead irrigation and proper field sanitation. Chemical control involves using copper-based bactericides to suppress disease spread and treating seeds to prevent early infection. Genetic resistance is a long-term solution, with resistant varieties reducing reliance on chemicals. Implementing these strategies together helps minimize disease incidence, protecting both yield and seed quality.

Further reading

• Laboratory guide for identification of plant pathogenic bacteria
• Pseudomonas syringae pv. phaseolicola — Data sheet on quarantine pests

Pesticide warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.