There are three distinct bacterial diseases that have been found on snap and dry beans in New York: Bacterial brown spot, caused by Pseudomonas syringae pv. syringae, common bacterial blight, caused by Xanthomonas campestris pv. phaseoli, and halo blight, caused by Pseudomonas syringae pv. phaseolicola. These diseases can cause serious yield losses in heavily infected bean fields.
Brown spot is no longer considered an economic problem in most years in New York. It is common in wet, cool years in snap and dry beans. Common blight and halo blight are rarely observed nowadays presumably due to the routine use of certified seed.
Symptoms and Signs
- Bacterial brown spot. The initial foliar symptom of bacterial brown spot is small water-soaked spots that develop into distinctive necrotic brown spots about 3-8mm in diameter, often with a narrow, diffuse yellow margin (Figure 1). These lesions may enlarge, coalesce, and fall out giving the leaves a tattered appearance. Sunken brown spots can form on the pods (Figure 2). If infection occurs early in pod development, the pod may become bent or twisted at the infection site (Figure 3).
- Common bacterial blight. Leaf symptoms initially appear as water-soaked spots that become necrotic, light brown lesions of irregular shape with distinct, bright yellow margins (Figure 4). These lesions enlarge to 10 mm or greater (Figure 5) and may kill the leaflet. Similar water-soaked spots form on the pods and enlarge into reddish brown lesions (Figure 6). In humid weather, a yellow bacterial exudate may be present on the pod lesions. Infected developing seeds may abort or shrivel and discolor as they mature (Figure 6).
- Halo blight. Symptoms of halo blight initially appear as small water-soaked spots on the underside of the leaflets, eventually developing into numerous small, reddish-brown lesions on the leaves (Figure 7). Greenish-yellow halos, highly variable in size, subsequently develop around these spots (Figures 7 and 8). During severe infections the disease may become systemic and cause yellowing and death of new foliage. At temperatures above 80°F halos are very small or absent. Pod symptoms first appear as small water-soaked spots and streaks on the pod surface (Figure 9). The water-soaked areas enlarge and are sometimes surrounded by a narrow reddish zone. Light, cream-colored bacterial exudate may be present on the lesions under moist conditions.
Pod symptoms of common and halo blight are very similar; therefore, these diseases should be identified by leaf symptoms or the bacterium. Leaf symptoms of halo blight may be confused with those of brown spot because of the variability in margin and halo size with temperature.
Disease Cycle
- Bacterial brown spot. The bacterium Pseudomonas syringae pv. syringae can cause diseases on several kinds of plants, but only a unique form of this bacterium causes that known as bacterial brown spot. These bacteria can grow on the surface of some plants, including snap and dry beans, without causing disease. Bacteria that exist this way are called epiphytes. Bacterial brown spot on beans often occurs after large epiphytic populations of the bacteria develop. Since severe infection may not develop until after a major rainstorm, an absence of symptoms does not mean that the bacteria are not present.
Hairy vetch and other weeds have been reported as over-wintering sources of the pathogen in Wisconsin. Recent research in New York, however, suggests that weeds are not an important source of the pathogen but that bean crop residues may be a significant source of inoculum. The pathogen spreads from overwintered bean stem and pod pieces to the current bean crop by rain and overhead irrigation. The bacteria may also be spread by equipment used in contaminated fields, and by people or animals walking through the field.
Brown spot bacteria can survive on bean seed, but recent assays of certified snap bean seed-lots used in New York indicated that the pathogen either was not present or was below the detectable level. Noncertified seed lots of snap or dry beans may carry infections that can introduce the bacteria to new locations. In New York, seed borne bacterial brown spot most likely serves to introduce the disease into an area, whereas infested crop residues are the source of bacteria for continuing epidemics.
- Common bacterial blight pathogen can survive on or in the seed, and contaminated seed is the primary source of this pathogen. Bean plant residues can also be a source of the bacterium. The pathogen survives better in residues on the soil surface than residues that have been worked into the soil. In Michigan, the common blight bacterium was found as an epiphyte on several weeds, including lambsquarters and pigweed. Common bacterial blight is introduced to new regions via contaminated bean seed. Once introduced, secondary spread can occur by wind-blown rain, overhead irrigation, contaminated equipment, or people and animals. Common bacterial blight epidemics are favored by high temperature and humidity. Yield is affected most if blight develops before the pod-fill stage.
- Halo blight. Infected bean seed is the most important source of the halo blight bacterium. The pathogen can survive more than 4 years in bean seed, and a single contaminated seed in 16,000 is sufficient to cause a severe epidemic under favorable weather conditions. The pathogen can also survive in bean residues from previous seasons. Halo blight bacteria can be spread by wind-driven rain, overhead irrigation, equipment, or people and animals. Severe outbreaks of halo blight often occur after heavy rainstorms. Although disease can develop rapidly from 60° to 80°F, temperatures above 80°F inhibit development of the characteristic yellow halos and systemic chlorosis.
Control
Cultural Control Practices
The best control of bacterial bean diseases is prevention. Only certified, western-grown, pathogen-free seeds should be planted. The chance of acquiring seedborne bacteria is significantly reduced by using certified bean seed produced in the arid climates of the western United States. Since moisture is required by these pathogens for reproduction and spread, seeds are more likely to become contaminated with bacteria when seed production fields have been exposed to summer thunderstorms, cool autumn temperatures and rains, and overhead irrigation.
Growers should never save their own seed, and should avoid using non-certified seed. If non-certified seed must be used, those fields should be isolated from fields planted with certified seed to reduce disease spread from contaminated fields. Because minor pod lesions on snap beans can result in rejection by processors and fresh-market buyers, snap bean fields in particular should be isolated from dry bean crops sown with non-certified-seed.
Choose resistant bean varieties to the bacterial diseases that have occurred in the past. Varieties differ greatly in their susceptibility to different bacterial diseases. In some varieties, the leaves are very susceptible to one or more of the pathogens, but pods may be resistant. In other varieties, the pods may be very susceptible, but few lesions develop on leaves. Seed companies continue to develop and release varieties with resistance to bacterial pathogens.
Tractors and other equipment used in fields with bacterial diseases should be thoroughly cleaned and disinfected before moving to disease-free fields. To prevent spread of the bacteria on equipment or in spray water, avoid making pesticide applications or cultivating when the leaves are wet.
All three bacterial pathogens survive best in crop residues on the soil surface. Infested bean residues should therefore be incorporated into the soil shortly after harvest and thoroughly covered to promote decomposition. These bacteria are unable to survive in soil without bean residues. Bean residues left on the soil surface do not decay quickly, allowing the bacteria to survive longer.
Fields where bacterial diseases are a problem should not be used for beans again until the crop residues are completely decomposed. To achieve this, follow a minimum 2-year rotation out of beans. Good rotation will also help reduce buildup of root-rotting organisms, white and gray molds, and anthracnose.
Fungicides
Seed treatment with the antibiotic streptomycin can help reduce contamination of the surface of the seed coat. Streptomycin seed treatment will not provide complete control, but it has been very effective against surface contamination. Streptomycin seed treatment will not control systemic bacterial infections. Antibiotics must be commercially applied by licensed seed treaters; they may not be applied to seed by growers.
Copper-based fungicides will reduce epiphytic populations of bacterial pathogens on bean foliage, and also reduce disease severity when applied as a preventative. They cannot eradicate pathogens from infected plants. Under persistent wet weather, bacterial populations can increase very quickly making control difficult even with frequent applications of copper fungicides. Bacteria can develop resistance to copper.
For labeled fungicides see current Cornell Integrated Crop and Pest Management Guidelines for Commercial Vegetable Production.
More information:
Margaret Tuttle McGrath
Associate Professor
Long Island Horticultural Research and Extension Center (LIHREC)
Plant Pathology and Plant-Microbe Biology Section
School of Integrative Plant Science
College of Agriculture and Life Sciences
Cornell University
mtm3@cornell.edu
Originally prepared for VegetableMD Online website by Helene Dillard and Daniel E. Legard. Edited and updated May 2021 by Margaret Tuttle McGrath.