Effects of inoculum potential on screening for resistance to Plasmodiophora brassicae in greenhouse trials

Several factors, including growth medium, inoculum density, and inoculum storage affected the reaction of resistant and susceptible Brassicas to Plasmodiophora brassicae in the greenhouse. A high level of disease was achieved using Peat-litte mix R and a commercial greenhouse mix. There was litte difference in disease incidence when spore suspensions were pipeted into planting holes or when seedlings were dipped into spore suspensions. Seedlings transplanted from sand or Petri dishes gave higher levels of disease than direct seeding. Two-year frozen storage of clubs reduced the inoculum potential to a level unable to define resistance. Inoculum levels of 103-7 spores per ml from fresh clubs, or 105-7 spores per ml from clubs frozen for 2 or 4 years, produced 90% club incidence of susceptible cauliflower and Chinese cabbage, A concentration of only 106-8 spores per ml from fresh clubs was required for maximum disease expression in a cauliflower line partially resistant to clubroot