Will 2011 be a white mold year?

Late July rains and wet soil conditions in some parts of Michigan have raised concerns about the potential of white mold outbreaks in dry bean fields.

White mold can severely reduce yields of dry beans. It is not a problem every year, but only when there is a prolonged period (five to six days) of wet soil at or before flowering. The wet soil conditions allows the overwintering bodies, sclerotia, to germinate and produce mushroom-like structures called apothecia. The apothecia then shoot out spores that can infect dry bean plants through the flowers. The white mold infection moves from the flowers to the stems. Infected stems above the point of infection usually die, producing little or no marketable yield depending on the time of infection.

The good news is that even in years with ample rainfall at flowering time, other factors may still keep white mold to a minimum. White mold infection prefers moderate temperatures in the 70 to 85 degree range. Hot, 90-degree days provide a much less favorable environment and infection is limited. Many of our current varieties, such as Zorro black and Medalist and Vista navy, have a tolerance to white mold infection. A listing of variety characteristics including white mold tolerance can be found in the 2010 Dry Bean Research Report.

Additional factors that favor white mold are narrow row widths of 22 inches or less, lush vine growth, and susceptible varieties. If those factors match your situation and your fields have been wet, your dry beans are beginning to bloom and moderate temperatures are forecasted, then a preventative fungicide treatment may be warranted. The window is small. Time the treatment when the dry beans are at 100 percent bloom. This is when every plant has one open flower. To be effective, the fungicide needs to cover the bottom 1/3 of the plant. This might mean using 60 psi or more of pressure with multiple nozzles over the rows or twin jet type nozzles. Covering the lower 1/3 of the plant is difficult to accomplish with aerial application.

There are several fungicides that provide significant protection against white mold. The results of the 2010 white mold fungicide trial at the Montcalm MSU AgBioResearch Center compares the effectiveness of six products applied at various rates and combinations. This trial is also found in the 2010 Dry Bean Research Report.

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