Common Bacterial Blight of Snap Bean in Florida
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Common Bacterial Blight of Snap Bean in Florida

   

Common Bacterial Blight of Snap Bean in Florida 1

Ken Pernezny and Jeffrey B. Jones2

By far, the most frequently encountered bacterial disease of snap bean in Florida is common bacterial blight caused by Xanthomonas campestris pv. phaseoli. This disease is a major problem of both snap beans and dry beans in most areas of the world where beans are grown, but is particularly troublesome in warm, humid climates with significant rainfall. Two other bacterial diseases of bean are well known in many bean production areas. These are halo blight and brown spot, caused by Pseudomonas syringae pv. phaseolicola and P. s. pv. syringae, respectively, which tend to cause problems in cool weather. However, in our experience, outbreaks of these latter two diseases are unusual in our state.

Symptoms

Symptoms on leaves first appear as small, water-soaked spots ( Fig. 1 ), usually more evident on the underside of the leaves. These lesions become larger and develop into dry, brown spots with distinct, rather narrow, yellow halos ( Fig. 2 ). Spots may begin to coalesce, and the yellowing of leaves becomes more general ( Fig. 3 ).

CREDITS: Courtesy Howard Schwartz
Figure 1. Initial symptoms associated with common bacterial blight on underside of bean leaves consist of water-soaked lesions.
Figure 2. More advanced lesion of common bacterial blight on top surface of bean leaf showing beginning of chlorotic (yellow) halo around lesion.
Figure 3. Numerous lesions of common bacterial blight on bean leaf.

Symptoms on pods are quite striking. Rather large, water-soaked areas develop ( Fig. 4 ). These areas are more or less circular and slightly sunken. Older pod lesions can become distinctly brick-red in color and more deeply pitted ( Fig. 5 ). When highly humid conditions occur, pod lesions become covered with a yellowish bacterial ooze that can dry to a yellowish, crusty mass. It is interesting to note that we have seen several fields in southern Florida where significant pod symptoms have appeared with little foliar damage.
Figure 4. Striking water-soaking symptoms of common bacterial blight on bean pods.
Figure 5. Advanced symptoms on bean pods, with appearance of characteristic brick-red areas in center of lesions.

Cause and Disease Cycle

Common bacterial blight is caused by the bacterium Xanthomonas campestris pv. phaseoli. This organism is related to those that cause devastating bacterial diseases of tomato, pepper, citrus, lettuce, and many ornamental crops in Florida. X. c. pv. phaseoli has a limited host range, affecting snap bean, dry beans, scarlet runner bean, and a few other members of the legume family.

Contaminated seed is probably the major source of bacteria introduced into new bean crops. Cells of X. c. pv. phaseoli from seed may invade emerging seedlings producing watersoaking symptoms in young plants or may colonize the surface of shoot tissues, inciting disease later when conditions are more favorable.

It is possible that X. c. pv. phaseoli may survive in crop debris in the soil from season to season. However, in southern Florida, the non-cropping period occurs in the hot, wet summer, when decomposition of crop debris is rapid and thorough. It is unlikely that significant populations of X. c. pv. phaseoli would survive beyond six weeks under such conditions.

Abundant scientific evidence exists that X. c. pv. phaseoli can survive and multiply as an epiphyte or resident on the shoot surfaces of weed hosts, primarily members of the legume family. Symptomless, alternative weed hosts may serve as sources of inoculum for this pathogen in Florida.

Common bacterial blight causes great damage at warm temperatures of 82-90 degrees F (28-32 degrees C) for daytime highs. Rainfall or overhead irrigation (as from "rainbird" machinery in Homestead) is usually needed for disease development and progression in the field. The major mode of dispersal of the bacterium is by splashing water. Cells of X. c. pv. phaseoli enter bean plants through stomates (breathing pores) in leaves and other plant organs and hydathodes (vein endings) at margins of leaves. Research has shown that wind-blown soil particles create wounds that are very important as portals for ingress of blight bacteria. Bacteria are also readily transmitted mechanically, especially when field plants are wet. There is some evidence that insects, in particular, whiteflies, leafminers, and beetles may transmit the bacterium. X. c. pv. phaseoli can become systemic in bean; that is, it may invade and travel within the vascular (water-conducting) tissue of the plant.

Control

Plant pathogen-free seed, preferably from plants grown in dry environments, under furrow or similar irrigation. AVOID MOVEMENT THROUGH AND WORK IN FIELDS WHEN PLANTS ARE WET. This simple cultural practice can greatly reduce disease development and spread. Promptly destroy crops and disc in residues. Control leguminous weeds that may serve as inoculum reservoirs. Insects should be conrolled, particularly whiteflies. Sprays of fixed copper bactericides (fungicides) may provide some control once disease appears. Consult the University of Florida, IFAS, Plant Disease Management Guide and current labels for specific legal uses of pesticides for common bacterial blight control.

Footnotes

1. This document is PP-62, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: June 2002. Reviewed: June 2008. Please visit the EDIS Web site at http:/edis.ifas.ufl.edu

2. Ken Pernezny, professor, Plant Pathology, Everglades Research and Education Center, Belle Glade, and Jeffrey B. Jones, professor, Plant Pathology, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.


Copyright Information

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