Spheroidal gall formation and seedborne infestation by Plasmodiophora brassicae as overlooked aspects of clubroot biology and epidemiology

Abstract

Plasmodiophora brassicae, the causal agent of clubroot, can infect many species of the Brassicaceae. Infection results in the formation of galls on the roots of susceptible plants. Most galls are spindle-shaped, but spheroid galls can also develop and are regarded as resistance structures. Histopathological characterization of spheroid galls revealed that they result from a proliferation of extrastelar tissues, and although pathogen spread is contained within these tissues, resting spores are still produced. A quantitative PCR protocol was also developed to estimate the numbers of resting spores present as external contaminants of seeds and tubers harvested from clubroot-infested fields. Quantifiable levels of infestation were found on 7 of 46 samples tested, suggesting that farmers should avoid the planting of common, untreated seeds from clubroot-infested fields. Collectively, the results presented in this thesis advance our understanding of P. brassicae biology and epidemiology, and provide the foundation for future studies.