Bacterial Brown Spot Garden Beans
» Bacterial brown spot has caused significant losses in snap bean production over the past few years.
» The development of bacterial brown spot is favored by moderate temperatures and frequent rainfall.
» Resistant varieties, the application of bactericides, and inoculum reduction are used to manage the disease.
Bacterial brown spot infects both leaves and pods, reducing
photosynthetic area and lowering pod quality. Yield losses
can range from trace to 100% when severe disease occurs
in the early growth and flowering stages.1 Small (3/25 to 8/25
inch diameter), brown, circular lesions develop on the leaves
(Figure 1). The lesions may be surrounded by a narrow
green-yellow zone.2,3 The lesions can enlarge and coalesce,
and the dead tissues can fall out, giving the leaves a ragged,
shot-holed appearance (Figure 2). Lesions form mostly on
younger leaves, rarely on older tissues.1,2
Lesions on the pods are circular and initially appear water-
soaked (Figure 3a). With time, the lesions develop into
sunken, brown, necrotic spots (Figure 3b). Infected pods can
become twisted and kinked, and a white to cream colored
bacterial ooze may form on the lesions.1,3,4
Bacterial brown spot of beans is caused by the bacterium
Pseudomonas syringae pathovar (pv.) syringae (Pss). Pss has a
very wide host range and can grow on many different plant
species. There are different strains of the bacterium, and
only specialized strains cause disease on beans.2 This
bacterium is commonly an epiphyte on healthy bean leaves,
meaning that it grows on the bean leaves without causing
DISEASE CYCLE AND FAVORABLE CONDITIONS
As mentioned, Pss has a wide host range, and it can be
found on a number of crop and weed species that serve as
sources in inoculum for beans. The bacterium can also
overwinter in infested bean debris and on (and in) infected
bean seed.1,2 The bacterium is spread by splashing rain and
sprinkler irrigation to neighboring plants. It also can be
spread by workers, equipment, and insects moving through
bean fields when plants are wet. The disease is most severe
when temperatures are cool to moderate (below 80 °F) and
relative humidity levels are above 95%. Severe epidemics of
brown spot occur in seasons with frequent rainfall events,
especially hard, driving rains.
The conventional understanding of the development of
brown spot is that the bacterium spreads to a leaf or pod by
splashing water, where it then enters the plant tissues
through wounds or natural openings and then initiates the
disease process resulting in symptoms. Recent studies have
shown that in many cases, the bacterium is already present
on many plants, existing as an epiphyte not causing disease.
When conditions favor the growth of the bacterium,
populations levels pass a certain threshold, and Pss switches
into pathogen mode and initiates the disease process. The
difference between these two concepts of how disease
develops may seem subtle, but they make a difference with
regards to how the disease is managed, (1) by preventing
spread or (2) by preventing the growth of the bacterium to
the point where it becomes a pathogen.5,6 Brown spot is
often most severe after a period of heavy, driving rain, and
it has been shown that this type of rainfall stimulates the
growth of Pss populations on bean leaves.
Recommendations for managing bacterial brown spot
typically include the integration of crop rotation and
sanitation efforts to reduce inoculum levels, planting disease
resistant varieties and pathogen-free seed, avoiding working
in fields when plants are wet, and the use of copper-based
bactericides to protect plants and reduce the spread of the
bacterium. In areas and conditions where the bacterium is
already present on plants, growing epiphytically, the focus
should be on efforts that keep bacterial populations low and
on minimizing conditions that favor disease development.
Sanitation efforts should include the prompt destruction of
bean debris shortly after the final bean harvest. Plant
material should be disked into the soil to promote rapid
decomposition. Manage weed hosts and volunteer beans
that can serve as sources of inoculum. Clean equipment and
tools between use in infested and disease free fields, and
clean harvesting machinery, seed-cleaning equipment, and
storage containers at the end of the season. Rotate to non-
host crops (corn, small grains, non-legume vegetables) for
three to four years. Avoid the use of sprinkler irrigation if
possible, or irrigate at a time (such as during dew formation)
that will not increase the amount of time plants are wet. Do
not reuse irrigation water.1,2,3
Resistance to bacterial brown spot is available in commercial
bean cultivars. Studies have shown that Pss populations on
resistant cultivars are lower than they are on susceptible
cultivars grown under the same conditions. Lower
populations (below the threshold that triggers the switch to
pathogenicity) may be one reason why brown spot is less
severe on resistant cultivars of beans.5,6
Disease management guides usually recommend the use of
certified, pathogen-free seed, and in some cases, treatment
of seed with streptomycin to eliminate any bacterial
contamination on the outside of the seed. These methods
reduce the likelihood that the bacterium will be introduced
into a field. However, in areas where the bacterium is
commonly present on crop and weed hosts, the importance
of seedborne inoculum in the development of a disease
epidemic is not clear because the bacterium can easily
spread to bean plants as they emerge.1,2,3
During the season, growers should regularly inspect plants
for symptoms of bacterial brown spot, especially after
prolonged periods of high humidity and/or frequent
rainfalls. Scout weekly from midseason to harvest. Begin
treatments with bactericides at the first sign of disease.3
Copper-based bactericides can be used to slow the spread
of the bacterium and reduce foliar populations. Example
products registered for use on dry and snap beans include
DuPontTM Kocide® DF Fungicide/Bactericide, Champ®
Formula 2 Flowable Agricultural Fungicide/Bactericide, and
Cuprofix® Ultra 40 Disperss® Dry Flowable Fungicide/
Bactericide.3,7 Apply the products according to label
directions, typically at 7 to 14 day intervals, depending on
weather conditions and label instructions. The bactericides
are most effective if applied as protectant treatments early
in the season during periods of cool to moderate
temperatures and moist conditions. Bactericide applications
to hail damaged plants shortly after the damage occurs can
help protect wounded plants from infection.1 However, the
overall effectiveness of bactericide treatments has been
inconsistent, and bactericides are less effective during
periods of persistently wet weather.3
1 Schwartz, H. 2011. Bacterial diseases of beans. Fact Sheet No. 2.913 Colorado State University Extension.
2 Schwartz, H., Steadman, J., Hall, R., and Forster, R. 2005. Compendium of bean diseases, 2nd Ed. American Phytopathological Society, St. Paul MN.
3 Reiners, S., Wallace, J., Curtis, P., Helms, M., Landers, A., McGrath, M., Nault, B., and Seaman, A. 2018. Cornell Integrated crop and pest management guidelines for commercial vegetable production. Cornell Cooperative Extension.
4 Babadoost, M. 2000. Bacterial diseases of beans. University of Illinois Extension. Report on Plant Disease No. 921.
5 Hirano, S., Rouse, D., Clayton, M., and Upper, C. 1995. Pseudomonas syringae pv. syringae and bacterial brown spot of snap bean: A study of epiphytic phytopathogenic bacteria and associated diseases. Plant Disease 79:1085-1093.
6 Hirano, S., and Upper, C. 2000. Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae—a pathogen, ice nucleus, and epiphyte. Microbiology and Molecular Biology Reviews 64:624–653.
7 Egel, D., Foster, R., Maynard, E., Weller, S., Babadoost, M., Nair, A., Rivard, C., Kennelly, M., Hausbedk, M., Szendra, Z., Hutchinson, B., Orshinsky, A., Eaton, T., Welty, C., and Miller, S. 2018. Midwest vegetable production guide for commercial growers 2018.
For additional agronomic information, please contact your local seed representative. Developed in partnership with Technology Development & Agronomy by Monsanto.
Individual results may vary, and performance may vary from location to location and from year to year. This result may not be an indicator of results you may obtain as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible. ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. The recommendations in this article are based upon information obtained from the cited sources and should be used as a quick reference for information about bean diseases. The content of this article should not be substituted for the professional opinion of a producer, grower, agronomist, pathologist and similar professional dealing with this specific crop. SEMINIS DOES NOT WARRANT THE ACCURACY OF ANY INFORMATION OR TECHNICAL ADVICE PROVIDED HEREIN AND DISCLAIMS ALL LIABILITY FOR ANY CLAIM INVOLVING SUCH INFORMATION OR ADVICE. 180118102826 020918DME
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