Belowground Defence Strategies Against Clubroot (Plasmodiophora brassicae)

The clubroot disease is one of the most devastating root-borne diseases of brassica crops. While breeding of resistant cultivars is still a method of choice, the control of clubroot by either biocontrol agents or even plant strengtheners could be improved. More environmentally friendly alternatives or additional means to make the resistance response of crop plants more durable are needed. Chemical control of clubroot is in many cases not successful; only liming has been used traditionally with good success. In some cases, the model plant Arabidopsis thaliana has been used; a plethora of work however has been done on oilseed rape/canola (in this chapter, the common name for Brassica napus will be chosen according to the name in the respective publications, mainly canola in Canada and oilseed rape in Europe) (Brassica napus). The clubroot pathogen is called Plasmodiophora brassicae and constitutes an obligate biotrophic protist that lives in close relationship with its host cell. The roots of the host plants are colonized, and the plant growth is altered upon infection. While shoots can be stunted and show wilt symptoms after longer infection periods, the root system is converted to a tumorous root tissue, called “clubroot” by alterations of plant hormones and metabolic pathways essential for pathogen nutrition. In this chapter, the major focus will, however, be on biocontrol of clubroot by either endophytic organisms or by plant strengtheners or plant growth regulators; and some mechanisms behind it, independent of which host plant was employed, will be discussed.