Quantifying resistance to Plasmodiophora brassicae in Brassica hosts

Plasmodiophora brassicae causes clubroot of crucifers. A quantitative PCR (qPCR)‐based protocol was developed to measure P. brassicae DNA in the roots of susceptible, intermediately susceptible, intermediately resistant and resistant Brassica hosts, and the non‐host wheat, at 5, 10, 15, 20 and 42 days post‐inoculation (dpi). The final reaction of each plant genotype was recorded as an index of disease at 42 dpi. Plasmodiophora brassicae DNA showed an increase in susceptible and moderately resistant hosts from 5 to 42 dpi, in contrast to a decrease in a highly resistant host and the non‐host wheat over the same period. Index of disease was significantly positively correlated with the amount of P. brassicae DNA in the roots at 5, 15, 20 and 42 dpi in one experiment, and at 10, 15, 20 and 42 dpi in a repeated experiment. Significant positive correlations also existed between the amounts of P. brassicae DNA in the roots at 42 dpi and those at 5, 10, 15 and 20 dpi in one experiment, and those at 10, 15 and 20 dpi in a repeated experiment. The results generated by the qPCR assay were validated by microscopic examination of roots inoculated with P. brassicae. The qPCR‐based protocol developed in this study allows for the accurate quantification of P. brassicae DNA in host root tissues as early as 5 dpi, and may serve as a useful tool to evaluate pathogen proliferation and development in the roots.