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VEGETABLE CROPS

Bacterial Diseases of Beans

Fact Sheet Page: 729.50 Date: 10-1991

COOPERATIVE EXTENSION • NEW YORK STATE • CORNELL UNIVERSITY


Helene R. Dillard and Daniel E. Legard
Department of Plant Pathology, NYS Agricultural
Experiment Station at Geneva, Cornell University

There are three distinct bacterial diseases found on snap and dry beans in New York State: 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.

Brown spot was not considered an economic problem in New York until the early 1980s. It is a recurring problem that can cause serious losses in snap beans. Common blight is often seen in drybean fields, but usually does not pose a significant threat to the crop. Halo blight has caused serious losses in both dry and snap beans in New York in some years, but because of the increased use of certified seed, it now occurs only sporadically. All three bacterial diseases can cause serious yield losses in heavily infected snapand dry-bean fields.

Symptoms and Signs

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

Hairy vetch and other weeds have been reported as overwinteringsources 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 byequipment used in contaminated fields, and by people or animals walking through the field.

Brown spot bacteria can survive on bean seed, but recentassays of certified snap bean seedlots used in New York indicated that the pathogen eitherwas not presentorwas belowthe detectable level. Noncertified seed lots of snap or dry beans may carry infections that can introduce the bacteria to new locations. I n 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.

Cultural Control

The best control of bacterial bean diseases is prevention. Only certified, western-grown, disease-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 noncertified seed. If noncertified 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 noncertified-seed dry beans.

Choose bean varieties with tolerance or resistance to the bacterial diseases that occur frequently in the growing area. 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.

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.

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.

Fields where bacterial diseases are a problem should not be used for beans again until the crop residues are completely de-composed. 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.

Chemical Control

Seed treatment with the antibiotic streptomycin can help reduce contamination of the surface of the seed coat. Streptomycin seed treatment will not provide 100 percent control, but it has been very effective against surface contamination. Streptomycin seed treatment will not control systemic infections of bacterial diseases. Antibiotics may not be applied to seed by growers, but must be commercially applied by licensed seed treaters.

Copper-based bactericides will reduce epiphytic populations of bacterial pathogens on bean foliage, and also reduce disease severity when applied as a preventative. These compounds, however, cannot eradicate the pathogens once the plants are infected. If wet weather is persistent, bacterial populations can increase very quickly and are difficult to arrest unless several applications of copper-based bactericides are made. For these reasons, copper-based bactericides are not highly recommended.

Refer to the current Cornell Pest Management Recommendations for Commercial Vegetable Production for an updated list of available bactericides and their proper use.

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Acknowledgments

Appreciation is extended to Jim Hunter, John Ludwig, and Jim Steadman for suggestions and assistance, to Joe Ogrodnick for photographic assistance, and to Howard Schwartz for the photograph of common bacterial blight (fig. 6). The New York State Snap Bean Research Association funded portions of the research leading to these results.

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