Plasmodiophora brassicae

SPORANGIAL AND SPOROGENIC PLASMODIA produced within spindlelike, spherical, knobby, or club-shaped swellings on roots and rootlets. Infected plants have pale green to yellowish leaves, may show flagging and wilting in the middle of hot, sunny days. Severely infected young plants usually die.

SPOROGENIC PLASMODIA (sp) function as haploid resting spores, are globose to subglobose, colourless, 1.6-4.3 µm diam. Upon germination, they produce zoospores with two flagella of unequal length. They penetrate host root hairs and then transform into a plasmodium. After a few days, each plasmodium clavates into multinucleate portions surrounded by separate membranes. Subsequently, each portion develops into a sporangial plasmodium. Single sporangial plasmodium contains four to eight secondary zoospores. After their extricate from the host root, they fuse in pairs and produce dikaryotic zygotes able to cause new infections and form new plasmodia with many dikaryotic nuclei. Following karyogamy, the nuclei undergo first fusion (karyogamy) and later meiosis. Finally, the plasmodium turns into resting spores (sporogenic plasmodia).

PLANT HOST AND DISTRIBUTION. Plasmodiophora brassicae affects many plant species of the family Brassicaceae.

The organism is widely distributed all over the world.

NOTES. Plasmodiophora brassicae causes the clubroot disease of brassicaceous plants. It is an obligate, intracellular parasite. Since it lives within the host cells, it is an endoparasite. It cannot live as a saprotroph. However, its sporogenic plasmodia (resting spores) persist for years in the soil. Plasmodiophora brassicae prefers acid soils.

REFERENCES

Alexopoulos C. J., Mims C. W., Blackwell M. 1996. Introductory mycology, 4th ed. John Wiley & Sons, INC. New York, Chichester, Brisbane, Toronto, Singapore.

Agrios G. N. 1988. Plant pathology, 3rd edition, Academic Press, INC. San Diego, New York, Berkeley, Boston, London, Sydney, Tokyo, Toronto.