Cause The disease is caused by Plasmodiophora brassicae, a fungus-like microorganism that can survive in soil 18 years or more after an infected crop. It can be spread through any means that moves soil: wind and water, footwear and equipment, and in infected transplants. Soils that are cool, wet (70% to 80% water-holding capacity) and acidic favor the pathogen.
Clubroot probably affects all species of the Crucifer family, including wild mustard. There are races of the pathogen that infect certain host species. The microorganism that causes clubroot occurs worldwide and possibly infects plants in the rose, poppy, and grass families. However, these plants rarely show typical symptoms of the disease. Viable spores of Plasmodiophora brassicae were detected on canola, pea and wheat seeds as well as on potato tubers harvested from clubroot-infested fields in Alberta, Canada.
Symptoms The distinctive symptom is abnormally large roots-fine roots, secondary roots, the taproot, or even on the underground stem. Roots develop clubs (swellings) that can be 5 or 6 inches wide. The largest clubs usually are just below the soil surface on the larger roots. Affected seedlings will not show any root swellings until about 3 weeks after infection. When susceptible plants are attacked in the seedling stage, they can die. When plants are attacked at a later stage, the disease rarely kills, but roots that are severely distorted have a reduced capacity to absorb minerals and water from soil. Plants wilt in hot weather but partly recover at night. Consequently, top growth may be stunted, yellowish, and likely to prematurely bolt or to wilt in hot weather. But even with extensive root clubbing, top growth may be nearly normal.
Sampling Because the pathogen cannot be cultured in the laboratory, traditional testing of soil for P. brassicae has been based on plant bioassays (soil baiting). The OSU Plant Clinic currently offers a molecular test for a common clubroot race in Oregon that can be used on plant tissue; research by OSU shows that current soil testing is unreliable due to chemical inhibitors in western Oregon soils.
- Early infection of seedlings can result in severe symptoms; so it is important to use only uninfected seedbeds and clean transplant media, trays, and equipment.
- Do not lime seedbeds or transplant-growing media heavily. It may mask the disease, which could flare up once seedlings are transplanted to a soil of lower pH.
- Never allow drainage water or soil from an infested field to enter an irrigation source. Spores are moved easily in irrigation water.
- Work in pathogen-free fields before moving people and machinery into infested fields. Thoroughly clean soil from machinery and equipment before moving from an infested field to a clean one.
- If growing susceptible crops in suspect or infested fields, incorporate enough finely ground limestone the year before planting to raise the soil pH above 7. Use lime applications that increase soil pH as well as level of soil calcium. Thoroughly mix lime into the soil to maximize potential disease control. Lime inhibits disease development, but will not prevent a disease outbreak if the spore load in the soil is sufficiently high. Different soil types vary considerably in their response to efforts to alter the pH with lime. Therefore, measure the initial soil pH, follow Soil Moisture Potential (SMP) test recommendations, and monitor the changes after application. Periodically monitor the pH in subsequent years to determine the stability of the change.
- If planting in a suspect or infected field, incorporating hydrated lime (1,500 lb/A) at least 6 weeks before planting, whether pH is neutral or alkaline, gives additional disease control.
- Long rotations (6 years or longer) help prevent a pathogen buildup and reduce disease.
- Control wild mustards if they are a weed problem.
- The form of nitrogen fertilizer can also influence disease. Using calcium nitrate may result in less disease compared to ammonium sulfate or urea.
- The variety, Ore. CR-1, is resistant to some strains of the fungus.
- If clubroot occurs, hilling-up plants can encourage production of adventitious roots, which may result in a better yield.
- Preplant soil treatment with Blocker 4F. PCNB does not control clubroot completely but reduces the number of clubs and secondary root rots so that the crop is nearly normal size. 12-hr reentry.
- For transplanted or direct-seeded fields, use 5.62 gal/A (55 fl oz/1000 ft row).
- Use 3 pints/100 gal for transplant solutions. Recommended only for commercial growers.
- Omega F at 6.45 fl oz/100 gallons as transplant drench or 2.6 pints/A as soil incorporation with a minimum band width of 9 inches. Product may cause plant stunting or delay and shorten harvest. Preharvest interval is 50 days. 12-hr reentry, except for high exposure activities (i.e. hand weeding) that have a 48-hr reentry.
- Prestop at 1.4 to 14 oz/10 gal water (0.1% to 1.0% suspension) as a soil drench or incorporation into growing medium. Apply at seeding and repeat 7 to 14-days later for best suppression of clubroot. 0-hr reentry. O
References Lahlali, R., and Peng, G. 2014. Suppression of clubroot by Clonostachys rosea via antibiosis and induced host resistance. Plant Pathology 63:447-455.
Myers, D.F., and Campbell, R.N. 1985. Lime and the control of clubroot of crucifers: Effect of pH, calcium, magnesium, and their interactions. Phytopatholgy 75:670-673.