Plasmodiophora brassicae
 Written by Michelle A. Grabowski

Host Range and Distribution: Plasmodiophora brassicae can be found worldwide in all temperate zones. It infects over 300 species in 64 genera of crucifers and can be found in both cultivated and wild crucifers. Economically important hosts include cabbage, collards, kale, mustard, brussel sprouts, radish, turnip, rutabaga, cauliflower, broccoli, rape, and kohlrabi.
 

Isolation: Cysts are produced within the tissue of clubbed roots. Fresh or frozen club roots that have been infected for at least 5 weeks can be used for isolation of Plasmodiophora brassicae. These should be cleaned, chopped, mixed with water, and blended at high speed in a blender until a smooth pulpy consistency is achieved. Filter the solution through cheesecloth. Centrifuge the filtrate for 7 minutes, or until three distinct layers can be seen. The bottom most layer is debris, the middle white layer is mostly starch, and the top gray fluid contains cysts of P. brassicae. Pour off cysts and repeat centrifuge process to further purify the isolates.
 

Identification: The most characteristic symptom of P. brassicae within a host, is the clubbing of host roots. A wide variety of root pathogens, however, produce similar symptoms, including root knot nematode and gall inducing fungi and bacteria. Presence of P. brassicae within a host is verified through dissection of clubbed roots. Clusters of enlarged host cells containing plasmodium and amoebae can then be observed with proper staining.
 

Symptoms: Symptoms vary slightly from host to host. The first observable above ground symptom is day wilting. Otherwise healthy looking plants wilt on hot dry days, recovering once the sun sets or temperatures cool. As the disease progresses, leaves yellow and die. Diseased plants are obviously stunted compared to uninfected plants and will often be localized in low, wet areas of the field. When dug up, roots exhibit a variety of symptoms. New infections cause small knot like galls on roots, where as more developed infections display long spindle shaped clubs on primary and lateral roots. Some hosts, such as turnips and radishes, do not form clubs when infected. These hosts have black sunken lesions along the root surface.
 

Ecology and Life Cycle:Plasmodiophora brassicae is an obligate parasite. It survives in the soil only as dormant cysts. Cysts can survive for up to 6-8 years without the presence of a host, and will germinate in response to the presence of crucifer root exudates. Primary zoospores released from germinating cysts infect host root hairs by encysting on the root surface and entering through developing epidermal cells in the form of an amoeba like cell. Older roots can also be infected if wounding is present to provide an entrance to the pathogen.

In the root hairs, amoeboid cells of the pathogen join together to form a multinucleate plasmodium. This plasmodium divides and forms multiple secondary zoospores, which are released into the soil. Secondary zoospores infect healthy parts of the initial host or infect nearby plants. These zoospores also enter through the host root hairs, but the infecting amoeboid cells migrate into the cortical cells of the host.

Once in the cortex, the amoeboid pathogen infects one host cortical cell where it may multiply or join with other amoeboid cells to form a plasmodium. As the plasmodium develops, it releases plant hormones (IAA) which cause the host cells to enlarge up to 20 times of its normal size. As the plasmodium grows, it divides and infects neighboring cells causing them to enlarge. Clusters of these enlarged cells are responsible for the clubbing on the roots and are referred to as ‘Kankheitsherd’. These Kankheitsherd are diagnostic of P. brassicae and can be observed in cross sections of infected roots.

Not all amoeboid cells infect cortical cells. Some move into the vascular tissue and infect the cambial cells of the host. The soft undeveloped cell walls of the cambial cells allows P. brassicae to easily travel up and down the root, infecting cortical cells, vascular ray cells, and cambial cells as it goes. The infection and resultant swelling of the vascular ray cells is responsible for the characteristic wilt symptoms associated with P. brassicae. As the ray cells swell to abnormal sizes, sections of xylem are pushed aside, and the continuity of the water column is broken.

Plasmodium in all host cells eventually undergo meiosis and develop into resting cysts. These new cysts will be released into the soil as other soil microorganisms decompose the club root.
 
 

Karling, J.S., The Plasmodiophorales. Hafner Publishing Company, New York. 1968
 

                                                                       Acknowledgments

    Thanks to Dr. Marc Cubeta at the NCSU Vernon James Research Station, Plymouth, NC for providing pictures, diseased plant material, and information for the project.  And to Dr. D. Michael Benson for all of his advice and guidance in assembling the page.

Return to Pathogen Profile Page

Links to Other Great Plasmodiophora brassicae Pages !

P. brassicae and Host DNA

Resistance in Brassica rapa

P. brassicae as a slimemold

Other good sites to visit

North Carolina State University Department of Plant Pathology

American Phytopathological Society