Biocontrol of clubroot disease: how successful are endophytic fungi and bacteria?

The commercial aspect of growing Brassica crops has always been influenced by the worldwide occurrence of the clubroot pathogen, Plasmodiophora brassicae. Clubroot symptoms reduce crop yield dramatically and the resilient protist is hard to eradicate from infested soils. Chemical treatments are not so efficient and their use is allowed only in a few areas of the world, none of them in the EU. The majority of clubroot control is mediated by using resistant crops, but not all species have good or durable resistance sources available, and these can be overcome by evolving or new Plasmodiophora pathotypes. Some commercially available biocontrol agents have been tested and found to reduce clubroot on crops such as rapeseed, cauliflower and Chinese cabbage to some extent. More biocontrol organisms have been isolated and described in recent decades but for many commercial application is still a long way off. In this review we summarize trends for bacterial and fungal endophytes for clubroot biocontrol as well as mechanisms behind the effects reported, such as antibiosis, defense induction or competition for space and nutrients. There are indeed plenty of studies on biocontrol of clubroot but not many have reached a point where the biocontrol agents are ready to be applied at field scale. The potential of endophytic microbes in pest management against clubroot disease is huge.