Clubroot

During initial stages of clubroot, aboveground symptoms may be absent. When present, foliar symptoms consist of stunting, yellowing, wilting, and other signs of a dysfunctional root system. Extensive galling, swelling, and distortion of the roots and hypocotyl are the main symptoms of the disease. Galled and clubbed roots are often invaded by secondary rot organisms such as soft rot bacteria; this results in the rapid decay of roots, further decline of infected plants, and release of additional inoculum into the soil.

Clubroot infects all of the cole crops, as well as many weeds in the mustard family. The fungus persists in soil as thick-walled resting spores that can remain viable for 10 years or longer. Infection is favored by acid soils with adequate moisture, but infections do occur above pH 7.0. In the presence of host plant roots, these resting spores germinate by releasing swimming zoospores. Such zoospores infect and colonize root hairs. Later, a second type of zoospore appears that can infect the main roots. Infection and colonization by this second zoospore causes the galling and clubbing of roots. Additional resting spores are formed inside the galled roots and are released into the soil when roots decay. The fungus is dispersed from field to field by the use of diseased field-grown transplants and movement of infested soil on machinery and surface water.

Once in the soil, clubroot fungus remains viable for many years. There is no economical way to eliminate it. Rotation with nonhost crops generally does not provide effective control; however, a 2-year rotation away from crucifer crops and into cereals may be helpful in some instances.

Minimize the spread of the pathogen by using pathogen-free transplants. It is preferable to use transplants that are produced in soilless rooting mixes in trays. However, if field-grown transplants must be used, then grow transplants in fumigated plant beds; young plants can be infected for some time without indicating infection and cannot always be detected at transplanting.

Restrict the movement of contaminated soil (on farm implements) from infested to noninfested fields. Do not use tailwater from contaminated fields to irrigate noninfested fields because the fungus can be transported in water.

Where fields are already infested with the clubroot pathogen, applying lime to infested fields can help create soil conditions unfavorable for spore germination. In general, apply lime if soil pH is lower than 7.2. Annual applications are usually necessary. Not all soils respond favorably to this treatment.

Crop rotation, proper handling of transplants and irrigation water, and liming the field are acceptable management tools in an organically certified crop.