Cabbage and Cauliflower (Brassica sp.)-Clubroot

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 cruciferous group, including wild mustard, but cabbage is among the most susceptible group members to clubroot. There are races within the clubroot pathogen population; certain strains of clubroot vary in the range of cruciferous species affected. The microorganism that causes clubroot occurs worldwide and also possibly infects plants in the rose, poppy, or grass families, including Creeping bent grass, orchardgrass, and perennial ryegrass. 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.

Clubroot survival in composting is dependent upon not only temperature but also moisture levels. If there are dry pockets (<40% moisture) in hot composting, clubroot spores have a high probability of surviving under those conditions. Incubation of compost at 60% moisture and 50°C for 7 days or 24 hours at 60°C have been shown to prevent clubroot survival.

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, and infections at this stage can cause dramatic yield impacts as surviving plants can be dramatically stunted. 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 with close cultural management.

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.

Cultural control Optimal management of clubroot includes planting seed or noninfected transplants in non-infested soil.

  • Plant in pathogen-free fields.
  • Infected transplants are the most prevalent source for the spread of pathogens into an uninfected area. Use only uninfected seedbeds and clean transplant media, trays, and equipment when producing transplants. Reused trays have been reported to be at high risk for pathogen contamination, especially if they have been in contact with the ground. If seedbeds have shown disease, soil fumigation is recommended if transplant production is continued. Do not mask the disease in transplant production with heavy lime applications to seedbeds or transplant-growing media. Disease will show once seedlings are transplanted to a soil of lower pH.
  • Long rotations (6 years or longer) help prevent a pathogen buildup and can reduce disease if susceptible weedy hosts are managed throughout the rotation.
  • If growing susceptible crops in suspect or infested fields, incorporate enough finely ground limestone the year before planting to raise the soil pH to at least 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. Regularly monitor the pH in subsequent years to determine the stability of the change.
  • If planting in a suspect or infected field, incorporate hydrated lime (1,500 lb/A) at least 6 weeks before planting, whether pH is neutral or alkaline, to get a bigger boost in disease control.
  • The form of nitrogen fertilizer can also influence disease. Using calcium nitrate may result in less disease compared to ammonium sulfate or urea.
  • Control wild mustards if they are a weed problem.
  • If clubroot occurs, hilling-up plants can encourage production of adventitious roots, which may result in a better yield.
  • Work in pathogen-free fields before moving into infested fields. Thoroughly clean soil from machinery and equipment.
  • Protect irrigation sources from infected fields, prevent water or soil flow from an infested field into an irrigation source. Spores are moved easily in water, including winter rains.
  • The cabbage variety 'Badger Shipper' offers resistance to some races of the disease. Clubroot-resistant cauliflower is also available.

Chemical control

  • 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. When combined with hydrated lime applications, good crop yields resulted in NY. 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 500F at 6.45 fl oz/100 gal water as a transplant drench or 2.6 pt/A soil incorporation. Product may cause plant stunting or delay and shorten harvest. Preharvest interval is 50 days for cauliflower and 7 days for cabbage. 12-hr reentry, except for high exposure activities (i.e. hand weeding) that have a 48-hr reentry.
  • Ranman 400SC at 12.9 to 25.75 fl oz/100 gal water as a transplant drench or at 20 fl oz/A for soil incorporation. Studies by Ohio State University showed reductions in vigor of mustard greens and radish; however, a nonsignificant yield improvement in broccoli was found. Preharvest interval is 0 days. 12-hr reentry

Biological control

  • 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 Donald, C., and Porter, I. 2009. Integrated control of clubroot. J Plant Growth Regul 28:289-303.

Faggian, R., and Strelko, S.E. 2009. Detection and measurement of Plasmodiophora brassicae. J Plant Growth Regul 28:282-288.

Fayolle, L., Noble, R., Coventry, E., Aime, S., and Alabouvette, C. 2006. Eradication of Plasmodiophora brassicae during composting of wastes. Plant Pathology 55:553-558.

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. Phytopathology 75:670-673.