Studies into primary and secondary infection processes by Plasmodiophora brassicae on canola

The early stages of infection of canola roots by the clubroot pathogen Plasmodiophora brassicae were investigated. Inoculation with 1 x 105 resting spores mL-1 resulted in primary (root hair) infection at 12 h after inoculation (hai). Secondary (cortical) infection began to be observed at 72 hai. When inoculated onto plants at a concentration of 1 x 104 mL-1, secondary zoospores produced primary infections similar to those obtained with resting spores at a concentration of 1 x 105 mL-1. Secondary zoospores caused secondary infections earlier than resting spores. When the plants were inoculated with 1 x 107 resting spores mL-1, 2 days after being challenged with 1 x 104 or 1 x 105 resting spores mL-1, secondary infections were observed on the very next day, which was earlier than the secondary infections resulting from inoculation with 1 x 107 resting spores mL-1 alone and more severe than those produced by inoculation with 1 x 104 or 1 x 105 resting spores mL-1 alone. Compared with the single inoculations, secondary infections on plants that had received both inoculations remained at higher levels throughout a 7-day time course. These data indicate that primary zoospores can directly cause secondary infection when the host is under primary infection, helping to understand the relationship and relative importance of the two infection stages of P. brassicae.