Bacterial Diseases Widespread in Panhandle Dry Beans - UNL CropWatch, Aug. 4, 2011

Bacterial Diseases Widespread in Panhandle Dry Beans - UNL CropWatch, Aug. 4, 2011

Bacterial diseases of dry beans

Figure 1. Characteristic foliar symptoms of dry bean bacterial diseases.  Clockwise from upper left: brown spot, common blight, halo blight, and wilt.

August 4, 2011

Due to recent thunderstorms with hail and the higher temperatures, growers are experiencing widespread bacterial disease problems in dry beans throughout the Panhandle. Following is an update of the current situation.

Four major bacterial diseases affect Nebraska dry bean crops: common blight, halo blight, brown spot, and wilt (Figure 1). Each disease is caused by a distinct pathogen, and favored by environmental conditions involving high levels of moisture.

These diseases are enhanced when plants are wounded by storms, hail, or any physical damage from humans, animals, or farm equipment. Bacteria cannot infect plants by themselves, but instead require either natural openings in leaves or plant wounds to initiate infection. Storms with high winds and hail provide the perfect opportunity for bacteria to become established as they can cause wounding and will move the pathogen and/or infected plant residues between and within fields.

Bacteria can survive in crop residues until environmental conditions favorable to infection occur. Daily temperatures that promote development of each respective disease vary slightly:

  • halo blight — less than 80°F
  • brown spot — less than 85°F
  • common blight and wilt — greater than 85°F

Disease Epidemics in 2011

Due to the high frequency of storms this summer, bacterial problems are beginning to appear throughout the Panhandle. The arrival of very hot temperatures over the past several weeks also has contributed to increased disease development. These observations are currently following the same pattern as Goss’s wilt of corn, another bacterial disease that functions similarly to the bacterial diseases in dry beans.

Over the last month, I have visited and documented disease incidence on several dozen fields stretching from Box Butte County to Cheyenne and Kimball counties. Additionally eastern Scotts Bluff and western and central Morrill counties are also strongly affected by these issues. The Panhandle Disease Diagnostic Clinic has received 40-45 samples from fields across the area with suspect bacterial issues.

Brown spot appears to be the most commonly identified disease throughout the area. We have noticed an increased presence of this disease for the last several years, as confirmed by a two-year bacterial disease survey (2009-2010) funded by the Nebraska Dry Bean Commission. This study revealed that over that two-year period, wilt was found in the highest number of collected samples, 23%, followed by brown spot and common blight, both at 17.5%, and halo blight at 9%. The high incidence of brown spot was a surprise and has caused us to begin developing new resistant cultivars with Carlos Urrea, UNL dry bean breeder, and the dry bean breeding program.

Disease Management

Disease development and recommended management are similar for all these diseases. Unfortunately, bacterial diseases are not easy to manage and good control options are limited.
Chemical control using copper-based products is variable, depending on pathogen, weather, and disease pressure. Increased economic returns have been realized for infections with halo blight and brown spot with copper sprays 40 days after emergence and then every 7-10 days for a total of three applications. However, control of common blight has not been as successful as control of halo blight and brown spot, and currently it is unknown how copper sprays affect wilt. Although it may be beneficial to preventatively apply copper after severe storms, it will still not be a rescue treatment.

Genetic tolerance is available in a few cultivars, but complete disease resistance packages are difficult to obtain. Several popular cultivars have good levels of resistance to common and halo blights, but are more prone to infection by rust or white mold. Other cultural practices that can aid in management include:

  • Avoid planting infected seed or beans near fields infected the previous year.
  • Implement some form of tillage to accelerate decomposition of infested tissues, which limits bacterial survival.
  • Avoid unnecessary wounding of plants. Any type of mechanical damage can open the door for bacterial pathogens to infect plants. Even walking through infected fields in the morning when wet with dew provides an opportunity for spreading disease. It is often better to wait until afternoon when the foliage has dried to enter fields for scouting or field operations.

Correct Disease Identification Critical

Making the correct diagnosis also can be critical to control. Because the efficacy of several control measures depends on the particular pathogen involved, the first and most important step is to accurately identify the pathogen in order to make the right management decision. We are happy to provide this service for anyone who may be interested.

For any questions or problems with bacterial disease in beans, please contact Kathy Nielsen at (308) 632-1286 or Bob Harveson at (308) 632-1239 or rharveso@unlnotes.unl.edu.

Robert Harveson, Extension Plant Pathologist
Panhandle REC, Scottsbluff