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Friday, July 29, 2016

Clubroot of cabbage

Clubroot symptoms on mustard plants.
M. Grabowski, UMN Extension
New sightings of an old foe

M. Grabowski, UMN Extension

Club root is an unusual disease that results in swollen distorted roots of plants in the cabbage family. Although a few reports from the 1950’ and 60’s indicate that clubroot occurred in Minnesota at one time, this disease has not been common in recent years. To determine just how wide spread this disease is in Minnesota, the Minnesota Department of Agriculture has been looking for clubroot as part of their pathways survey for new and emerging invasive plant pests. The disease was identified in Ramsey county on edible mustard greens in the 2016 survey. 

Firm white galls from clubroot infection of a turnip root.
G. Holmes, CA Polytech State University
What is clubroot?
Club root is a plant disease caused by Plasmodiophora brassicae, an interesting microorganism that can only grow and reproduce within the roots of an infected plant. The clubroot pathogen infects members of the cabbage family including broccoli, cauliflower, radish, turnip, Brussels sprouts, rutabaga, and mustard greens. Although the pathogen can infect and survive on the roots of some grasses and weeds, only members of the cabbage family develop swollen distorted roots.





How to recognize a plant with clubroot
Cabbage plant wilting because of clubroot.
G. Holmes, CA Polytech State University
Plants infected with clubroot may be stunted and have small leaves. Because the roots are not functioning properly, infected plants wilt with only slight drought stress and may turn yellow. Once plants are dug up the infection is quite obvious. Roots are swollen and distorted into large clubs. Smaller bulbous galls may be seen on root branches off the side of larger clubbed roots, or coming off a large taproot like a turnip. Infected roots are white and firm early in the growing season but turn dark and begin to decay by the end of the season. As these galls decay, resting spores of the pathogen are released into the soil. These thick walled spores are capable of surviving in the soil for over 20 years.

Managing clubroot in the vegetable garden
Although the clubroot pathogen is very difficult to get rid of once it is found on a site, there are several strategies a gardener can use to reduce damage from this disease.

  • Bury infected plants at the same site where the disease was discovered. Moving infected plant material to a compost pile or a different part of the garden will spread the pathogen to new areas.
  • Do not plant any members of the cabbage family in the garden for 5 to 6 years. This will not eliminate the pathogen but will reduce it to a less damaging level. Consider planting members of the cabbage family in pots, in a community garden plot, or in a flower bed to allow the infested area of the garden to remain free of susceptible plants for the full rotation period.
  • Have a soil test done. If the soil pH is lower than 7.0 add lime to the soil to increase the pH above 7.0. This will not kill the pathogen but will create conditions that are unfavorable for disease.
  • Clean garden tools and equipment before using them in other areas of the garden. Do not move soil from the infected area of the garden, as the soil will contain clubroot spores.
  • Look for varieties that have resistance to clubroot. Several cabbage varieties with resistance to clubroot are available. Check seed catalogs for resistant varieties of other members of the cabbage family.


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