| Three studies were conducted relating to greenhouse selection for resistance to Plasmodiophora brassicae in oriental subspecies of Brassica campestris.; In the first study, 265 cultivars of leafy, oriental brassicas were screened for resistance to 18 collections of Plasmodiophora brassicae, the causal agent of clubroot. The screens were conducted in the greenhouse at low and high level inoculum concentrations. Eleven cultivars of B. campestris L. ssp. pekinensis (Lour) Olsson, five cultivars of B. campestris L. ssp. chinensis (Rupr) Olsson and three cultivars of B. campestris L. ssp. parachinensis (Bailey) Olsson demonstrated repeated segregation at the lower level of inoculum (1,000 spores/ml). All 265 cultivars were susceptible at the higher concentration (1,000,000 spores/ml). Three cultivars were used in pedigree and recurrent selection schemes for increased resistance. After three cycles of selfing resistant individuals significantly increased resistance was obtained in S(,3) lines derived from each cultivar. The second study was conducted in order to help define changes in virulence spectra other researchers have noted when Plasmodiophora brassicae, as an obligate biotroph, is maintained for breeding and pathology studies on different susceptible hosts.; Sporangia from 18 collections of Plasmodiophora brassicae were formulated into an initial mixture used for cycling the pathogen through nine susceptible hosts representing five cruciferous species. P. brassicae populations retrieved from each of the hosts after 1-2, 5-6, 9-11 cycles of propagation were inoculated onto a differential host range drawn primarily from the European Clubroot Differential Set and compared with the original collection mixture. Significant reductions in the virulence spectrum were noted after only a few cycles of propagation in some species.; The third study was undertaken to identify better methods of storing P. brassicae resting spores, traditionally maintained in frozen root galls. Resting spores were stored inside foil packets with four soil media at three temperatures and retrieved for viability testing after three months, six months and one year. Best results were obtained over all soil types with cool storage (3-5(DEGREES)C). |
