P lasmodiophora brassicae infection threshold—how many resting spores are required for generating clubroot galls on canola ( Brassica napus )

Clubroot, caused by Plasmodiophora brassicae , is an important disease of canola and other brassica crops. Recommending improved management strategies may be guided by an evolving understanding of host and pathogen biology. In order to better understand infection thresholds, clubroot-susceptible canola plants (cultivar Westar) were inoculated with resting spores of P. brassicae in a soil inoculation assay and a plant inoculation assay. In the soil inoculation assay, seven-day-old seedlings were transplanted into soil containing clubroot resting spores at a final concentration of each of the 10× serial dilutions from 1 to 1 × 10 6 spores/mL soil. In the plant inoculation assay, seven-day-old seedlings were inoculated with 1-µL aliquots of a resting spore suspension at a concentration of each of the 10× serial dilutions from 1 × 10 3 to 1 × 10 7 spores/mL, and after 48 h, the inoculated seedlings were transplanted into soil. After 42 days, clubroot development, incidence and severity were visually assessed. From the soil inoculation assay, clubroot was observed to establish in the soil containing as low as 1 spore/mL soil of resting spores. From the plant inoculation assay, very low clubroot incidence was observed when the plants were inoculated with ≤ 100 resting spores total. Our results indicated the urgent need for more sensitive methods of clubroot diagnosis and emphasized the difficulty of the creation of P. brassicae single-spore isolates.