Infection of canola by secondary zoospores of Plasmodiophora brassicae produced on a nonhost

Clubroot, caused by Plasmodiophora brassicae, has two infection stages (primary and secondary). Although primary infection occurs in many plant species, secondary infection only continues to completion in susceptible hosts. As part of a larger study of clubroot pathogenesis, secondary zoospores collected from infected root hairs of canola and ryegrass were inoculated onto healthy roots of both plant species. The treatments consisted of all possible combinations of the two plant species and the two sources of inoculum. At 5 days after inoculation, levels of root hair infection were similar and in a range of 50–68% on roots in all of the treatments. Secondary infection was also observed from all of the treatments, with approximately 50% on canola and 40% on ryegrass. The proportion of secondary infection and the number of secondary plasmodia were higher in canola inoculated with zoospores from canola than in ryegrass inoculated with zoospores from ryegrass, with the other combinations intermediate. At 35 days after inoculation, typical clubs developed on 14% of the canola plants inoculated with secondary zoospores from canola, and tiny clubs developed on 16% of the canola plants inoculated with zoospores from ryegrass. Secondary infection occurred in about one-third of ryegrass plants but no clubs developed, regardless of inoculum source. These results indicate that resistance to secondary infection in ryegrass is induced during primary infection. This is the first report that secondary zoospores produced on a nonhost can infect a host and reconfirms that secondary infection can occur in a nonhost.