Short introduction to the disease

Clubroot disease has been known on cultivated brassicas in Europe at least as far back as the 13th century and quite possibly much earlier to Roman times and has been ascribed to many causes. The early 19th century Scottish view that it resulted from ‘‘unsatisfactory soil conditions or unbalanced fertiliser practices’’ was, in the absence of microbiological knowledge, an entirely reasonable assertion. Recent considerations of the ecology of the organism make an association of epidemics of P. brassicae with unbalanced host nutrition a quite tenable view. In 1873, the Russian biologist M. S. Woronin commenced studies of the disease in St. Petersburg and five years later identified Plasmodipophora brassicae Woron. as the causal agent of clubroot disease. More than a century of subsequent study has unraveled much concerning the biology and host-pathogen relationships of P. brassicae and the Brassicaceae.

Much is still unknown, not least the true taxonomic status of the microbe, its full life cycle, interactions with hosts, and means of controlling a disease that is of considerable and increasing economic significance.

All members of the family Brassicaceae are thought to be potential hosts for Plasmodiophora brassicae and within them it is able to complete both the root hair and cortical stages of its life cycle. Cultivated crops appear to be especially susceptible. Also cruciferous weeds, for example mustard, Sinapis, and decorative ornamentals including stocks (Matthiola) and wallflower (Cheiranthus) can be infected. The scientific model and rock garden plant Arabidopsis is also susceptible. Infection of seedlings may lead to plant death but invasion in later growth stages rarely kills. Infected plants typically exhibit reversible, foliar wilting when under slight soil moisture stress. As the disease progresses the leaves become reddened, chlorotic, necrotic, and abscise. Plants become stunted and flowering is accelerated, with the formation of poor-quality curds or spears in cauliflower and calabrese (green broccoli) and small unthrifty hearts in Chinese cabbage. Heavy-framed plants such as Brussels sprouts become physically unstable and crops may lodge badly. Seed number and oil quality are depressed in oil seed and turnip rape crops, whereas the roots of swede and turnip exhibit contusions and excrescences on their surfaces. Crop consistency is impaired as a consequence of erratic maturity. Yield, quality, palatability, and storability are reduced and land capital value is diminished. The root malformations produced are the cause for the diseased upper parts.

Clubroot is now found throughout the world wherever Brassica crops are grown and is assessed as the major source of disease-induced loss. The severity of infestation and symptom expression increases with the intensity and frequency of crop production.

(from Dixon, 2009, Journal of Plant Growth Regulation)

Meanwhile the genomes of several P. brassicae isolates have been sequenced in 2015 and 2016 providing the researchers with a wealth of information on this obligate biotrophic protist.