Root hair infection by Plasmodiophora brassicae in clubrootresistant and susceptible genotypes of Brassica oleracea,B. rapa and B. napus

The pathogenesis of clubroot, a disease of cruciferous crops caused by the fungusPlasmodiophora brassicae, starts with infection of the root hairs. This process was studied in 13 accessions ofBrassica oleracea, B. napus and B. rapa with varying levels of plant resitance toP. brassicae. Seedlings were grown in a mineral solution, inoculated with resting spores ofP. brassicae, and the number of plasmodia developing in root hairs was recorded. When compared with the standard susceptible cultivar Septa, both higher and lower resistance to root hair infection was found in the accessions of the differentBrassica species. No complete resistance to root hair infection was found. Over the accessions studied, there was no correlation between the plant resistance estimated from greenhouse tests and the resistance to root hair infection of seedlings. The resistance of all accessions must at least partly be caused by other mechanisms which operate after the root hair plasmodia are formed.