Club Root

Species


Clubroot disease is caused by a soil inhabiting fungus called Plasmodiophora brassicae.It is a plant parasite of the group Phytomnyxea. A relatively simple fungus, P. brassicae forms no mycelia (numerous threadlike branches) and is an obligate parasite, meaning that it can develop and multiply only in living host cells. Although the fungus attacks many species of the Brassicaceae, i.e. mustard family, the intensity of the disease on each crop greatly varies. Numerous forms or races have been identified, making plant resistance very difficult.

Related Species

Clubroot may be confused with a wide variety of root pathogens which produce similar symptoms. These include root knot nematode, gall inducing fungi and bacteria.

Host Site


  • Outbreaks of clubroot disease have been recorded worldwide and in all temperate zones. It attacks only plants in the Brassicaceae family but can be found in both cultivated and wild crucifers.
  • Primary hosts include: cabbage, collards, kale, mustard, brussel sprouts, radish, turnip, rutabaga, cauliflower, broccoli, rape, and kohlrabi.
  • The clubroot fungus infects susceptible host plants through root hairs. Once in the tissue, it stimulates abnormal growth forming a swollen club. This prevents water and nutrient absorption resulting in stunted plants, yellow leaves and/or wilting.
  • Infection is favored by excess soil moisture and low pH, although it can occur over a wide range of conditions. Once a plant is infected, an abundance of resistant fungal spores are produced in the "clubbed" tissues. As these tissues decay, spores are released into the soil where they can remain infectious for at least ten years. Contaminated soil moved by wind or water can infect nearby gardens causing outbreaks of disease in areas where susceptible crops are planted for the first time.

Identification


Symptoms
  • A key symptom of clubroot disease is the abnormal swelling of roots (thickest part at the center) and occasionally on stems below ground. These swellings range from small, round, enlarged areas to large spindle-shaped galls. All or part of the root system may be affected. In cases of mild infection no above ground symptoms may be evident. In severe cases, nutrient absorption and water uptake are disrupted and above ground symptoms are those commonly associated with nutrient deficiencies and drought stress. Plants may show varying degrees of yellowing (chlorosis). Infected plants may also wilt during the hot part of the day. Flagging, wilting of uppermost leaves is common on young infected plants. Premature death, stunting and reduced size and quality of heads are also symptoms associated with club root. (Partridge, 2008)
  • In radishes, clubroot causes distorted swellings on the base of the bulb and along the tap root.(Miller, Ohio)

Life Cycle


  • P. brassicae can survive in soil as dormant cysts or resting spores for up to 10 years, even without the presence of a host. Cysts can spread to different sections of the garden by infested soil, contaminated water supplies, tools or machinery, infected transplants, or even animals.
  • Germination requires moist, acidic soil and can occur over a wide temperature range of 54°-81 °F (12°-27°C). Disease development is favored by high soil moisture and soil temperatures between 64°-77° F (18°-25° C). There are a few cases of clubroot in soil with pH balance of 4.5-8.1.
  • Within the infected roots, the pathogen develops rapidly, releasing hormones that cause an increase in the number and size of cells, enlarging the roots 20 times the normal size, which results in "clubbing."
  • As the pathogen develops, new or secondary spores are produced. These are capable of infecting the same plant or adjacent plants, thus repeating the cycle.
  • Eventually, resting spores are formed within the diseased plant tissue, and these are released into the soil when the plant roots disintegrate.

Monitoring


Monitor plants for wilting yellowing leaves, or signs of decay and stunted growth. When infection occurs at an early stage of growth, young plants are stunted and may die, whereas plants infected in a later stage fail to produce normal size fruit. When diseased plants are pulled from the soil, the roots are usually swollen and distorted. If you suspect a plant is infected with clubroot, dig it up and look for a bulbous mass of golf-ball-size galls instead of a healthy, fibrous root system.

Action Threshold


Act immediately at the first sign of clubroot. This disease is tenacious and aggressive.

Cultural and Physical Controls


  • Prevention is key. Rotate crops. Growing crucifers on the same soil no more than every third or fourth year (some sources suggest every 7 years) may be effective for stopping the development of spores, as long as your garden is not already heavily infested. Make sure susceptible weeds and plants, such as birdsrape mustard, field mustard, wild mustard are not present.
  • If you have a clubroot problem, plant brassicas in a different area of your garden each year for 5 to 10 years before reintroducing them to that original plot. But be aware that this is one tenacious pest: club root spores persist for many years by feeding off decaying organic matter while they lie in wait for the next broccoli or cabbage seedling to appear.
  • With transplanted crops, the use of pathogen-free seedbeds and uninfected plants is essential to prevent introduction of the disease.
  • Always clean and disinfect all tools especially before moving them from infested to non-infested parts of your garden.
  • After identifying an infected plant, carefully dig around the infected area and gently remove the entire root system from the soil to prevent the clubs from breaking up and potentially releasing thousands of spores. DO NOT COMPOST the infected plants.
  • Sowing a cover crop of winter rye in early spring has shown to substantially reduce viable clubroot spores in the soil. Make sure to till it into your soil 2 to 3 weeks after germination
  • Soil solarization (cooking the fungus out) can work because clubroot develops best in cold, wet environments. Simply spread clear, construction-grade plastic over fungus-infested soil and leave it in place for four to six weeks. The sun's radiant energy kills the fungus. Thinner sheets of plastic (1 to 2 mils) heat the soil more quickly and absorbs more solar rays than thicker sheets. Soil solarization works best during warmer months, generally between May and September.
  • To avoid introducing club root to your soil from an outside source, start your own plants from seed in a sterile potting mix.
Resistant Varieties

Several clubroot resistant cabbage varieties do exist: Badger Shipper from Wisconsin, developed by a chance cross of cabbage with kale, as well as additional cabbage breeding lines from Oregon. However, plant resistance has not been very useful in clubroot control because of rapid development of new types of the fungus.

Biological Controls


Studies indicate that Trichoderma, a type of fungus founds in most soils, and Streptomyces spp., a bacteria, may be useful biological controls for clubroot.

Chemical Controls


  • Since clubroot is favored by a low pH, liming soil to pH 7.2 or above may be helpful. Raising soil pH too high, however, may interfere with the growth of succeeding crops other than crucifers. Calcitic lime is usually preferable to dolomitic lime, except for soils low in magnesium, where dolomitic lime is more effective.
  • Although the way in which lime reduces infection by P. brassicae is not understood, it is thought that control is enhanced when the calcium concentration in the soil is very high. Because the use of nitrogen fertilizers will reduce soil pH (nullifying the beneficial effects of liming) and because some crops in rotation with crucifers are sensitive to heavy lime applications, it is important to check the soil pH regularly.

References


Grabowski, Michelle A. Plasmodiophora Brassicae. Fact Sheet. University of North Carolina, Department of Plant Pathology. Web. 16 May 2010. <http://www.cals.ncsu.edu/course/pp728/Plasmodiophora/Plasmodiophora.html>.

Miller, Sally A., Randall C. Rowe, and Richard M. Riedel. Fact Sheet; Clubroot of Crucifers. Fact Sheet. The Ohio State University Extension; Plant Pathology. Web. 16 May 2010. <http://ohioline.osu.edu/hyg-fact/3000/3118.html>.

Partridge, J. E. Club Root of Crucifers. Fact Sheet. University of Nebraska-Lincoln; Department of Plant Pathology, 2008. Web. 16 May 2010. <http://nu-distance.unl.edu/homer/disease/hort/crucifer/CrClbRoot.html>.

Sloane, Lauren. "Club Root." Organic Gardening. Rodale INC., 2009. Web. 16 May 2010. <http://www.organicgardening.com/feature/0,7518,s1-2-9-1189,00.html>.

Zitter, T. A. Vegetable MD Online: Vegetable Crops: Clubroot of Crucifers. Fact Sheet. Cornell University, Oct. 1985. Web. 16 May 2010. <http://vegetablemdonline.ppath.cornell.edu/factsheets/Crucifers_Clubroot.htm>.

CHEAH, L.H., VEERAKONE, S. and KENT, G. Biological Control of Clubroot On Cauliflower With Trichoderma and Steptomyces Spp.Crop & Food Research, Private Bag 11 600, Palmerston North http://www.nzpps.org/journal/53/nzpp_530180.pdf

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