Root rot on dry beans common under cool, wet growing conditions

The 2014 summer will be remembered as cool across northern Michigan, with late dry bean planting caused by a wet June.

Late-planted black beans with root rot symptoms. Photo by Jim Isleib, MSU Extension
Late-planted black beans with root rot symptoms. Photo by Jim Isleib, MSU Extension

Dry bean root rots are usually caused by Rhizoctonia, Fusarium and Pythium pathogens. These organisms can survive for years in infested soil. When the right conditions are present, disease develops. In 2014, heavy rains were experienced in some dry bean growing regions in Michigan, with serious root rot problems following. Then, cool and dry conditions slowed crop development, adding to the problem. Conditions that favor root rot development include high soil moisture, soil compaction, poor drainage and frequent bean crops.

Above-ground symptoms of root rot on dry beans may include:

  • Seedling death (damping off).
  • Stunted growth.
  • Yellowing of lower leaves.
  • Early leaf drop.
  • Brownish streaks in vascular tissue up to first node.

Below-ground symptoms may include:

  • Sunken, reddish-brown lesions on roots or hypocotyl.
  • Death of primary root, sometimes secondary root.
  • Development of lateral roots above infection site.

If root rot is suspected, dig up a few yellowed plants and inspect the roots carefully. This will be the most dependable in-field diagnostic tool.

Yield losses depend on disease severity, which is affected by environmental conditions from planting through flowering. Bean plants can recover from root rot if conditions improve and soil moisture remains adequate for the reduced root system to access needed moisture and nutrients. Losses can range from slight to 100 percent.

Consider the following management suggestions from Michigan State University Extension for controlling root rot:

  • Avoid fields with a history of root rot problems.
  • Plant beans on well-drained soils. If necessary, take steps to include soil drainage with drain tile, sub-soiling to reduce soil compaction, or include crops that will improve soil structure.
  • Plan a careful rotation. Avoid growing dry beans behind other susceptible crops. Plants resistant to root rot pathogens include barley, corn, alfalfa and wheat. A minimum rotation of three years is recommended.
  • Incorporate crop stubble from previous year and give it time to decompose.
  • Plant timely. Wait until soils warm adequately and aim for proper plant stand and seeding depth.
  • Avoid over-fertilization, especially with nitrogen.
  • Use a seed treatment labeled for root rot and damping off.

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