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Bacterial blights on dry beans appearing (06/30/16)

Bacterial blights are already appearing in dry edible beans. The amount of damage that we are observing this early is rare and is cause for concern.

Bacterial blights on dry beans appearing

Bacterial blights are already appearing in dry edible beans. The amount of damage that we are observing this early is rare and is cause for concern. The onset of bacterial blight is directly related the severe storms that moved through the region over the last two weeks. The storms damaged leaf tissue, providing entry points for bacterial pathogens and the rainfall provided a situation where the bacteria is able to infect and to spread. A perfect storm, so to speak. This is particularly true in areas like Walsh County, where sustained winds were over 50 mph for periods of time and many inches of rain fell. Many fields have plants damaged badly enough that growers are wondering where the plants will be able to survive.

We have little experience with this level of bacterial blight this early, but can expect some things as the season progresses.

Given the level of damage we are seeing in the hardest hit fields, it is likely that pathogen inoculum will be present for the rest of the season. However, that doesn’t necessarily mean that the disease is going to immediately spread to new-growth or healthy leaves. For the disease to continue to spread we will need more events to damage new tissue that will provide entry points for new infections (high winds, wind-driven rain) and rainstorms that give the pathogen an opportunity to spread and infect. If you have a long period of limited wind and no rain it is likely the plants will look better as new growth emerges. Conversely, frequent storms are going to exacerbate the problem.

Managing bacterial blights is difficult due to limited management tools. One of the most important things growers can do is to stay out of fields when they are wet. Moving through a field in the early part of the day when dew is on the plants will damage tissue and spread bacteria while there is water on the leaves; a recipe for new infections. Moving in a dry field may still damage tissue, but the bacteria are less likely to be moved or cause new infections.

Fungicides for bacterial blights are not commonly recommended in our area, however, some chemicals are occasionally applied to dry beans in Western Nebraska and Eastern Colorado. In those environments, multiple preventative applications may be used throughout the course of a season. According to our colleagues from the region, Cupric Hydroxides are often the most effective, but other products are being evaluated. These products act as protectants, and help protect the plant tissue from new infections, and are often applied 2-4 times. Unfortunately, common blight, which is what we are currently seeing in the region is less responsive to chemicals than halo blight and bacterial brown spot. Commonly applied fungicides for white mold or rust are not recommended because they are unlikely to have any effect on bacteria.

The one up-side to having the disease pressure early, if there actually is an upside, is that most canopies are still open. This will allow the plants to dry quicker and give the bacteria a narrower window to cause new infections.

 

 

Sam Markell

Extension Plant Pathologist, Broad-leaf Crops


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