Cause:
The disease is caused by Plasmodiophora brassicae, a funguslike microorganism that can survive in soil 18 years or more after an infected crop. It can spread by 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. The microorganism that causes clubroot occurs worldwide and also infects plants in the rose, poppy, and grass families. However, these plants rarely show typical symptoms of the disease.
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.
Note the large clubs on the left plant and smaller, clubbed roots on the right plant.
Note the clubbing of the roots.
Cultivar
Resistance: The cabbage variety 'Badger Shipper' offers resistance to some races of the disease.
Cultural
control:
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 let drainage water or soil from an infested field enter an irrigation source. Spores are easily moved in irrigation water.
Work in disease-free fields before moving people and machinery into infested fields. Thoroughly clean soil off machinery and equipment before moving from an infested field to a clean field.
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.
If clubroot occurs, hilling-up plants can encourage production of adventitious roots which may result in a better yield.
Chemical
control:
Preplant soil treatment with PCNB (Terraclor 75 WP or Terraclor F). PCNB does not completely control club root but reduces the number of clubs and secondary root rots so that the crop is nearly normal size. 12-hr reentry.
Broadcast: For transplant or direct-seeded fields, use 40 lb/A Terraclor 75 WP or 7.5 gal/A Terraclor F, depending
on soil type. Disk or rototill the PCNB into the top 4 inches of soil. The treatment is effective for two seasons if the soil is only rototilled and cultivated, not plowed.
Bands: For transplanted or direct-seeded fields. Although a savings in chemical may be made the first year by
applying it in bands before planting and cultivating it into the top 4 inches of soil, the second-year benefit from this
application of chemical is lost.
Use starter solutions at 1 cup/plant at planting: PCNB at 2 lb/100 gal water of the 75 WP or Terraclor F at 3
pints/100 gal water. Recommended only for commercial growers.
Calirus 150 at 0.75 to 1.5 gal in 100 gal water as a drench on 7- to 14-day intervals, start shortly before or after sowing. 4-hr reentry.
Omega at 6.45 fl oz/100 gallons as transplant drench or 2.6 pt/A as soil incorporation. Product may cause plant stunting or delay and shorten harvest. Preharvest interval is 50 days. 48-hr reentry.
References: Myers, DF. and Campbell, RN. 1985. Lime and the control of clubroot of crucifers: Effect of pH, calcium, magnesium, and their interactions. Phytopathology 75:670-673.
Content edited by:
Cynthia M. Ocamb on
January 1, 2010
