Page 305 - PC2019 Program & Proceedings

PLANT CANADA 2019

P183. Resting spores of Plasmodiophora brassicae continue to develop after death of their host.
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2
Al-Daoud, F. ; Gossen, B. ; M.R. McDonald 1
1 University of Guelph
Agriculture and Agri-Food Canada
2
Plasmodiophora brassicae Woronin causes clubroot on canola (Brassica napus L.) and other brassica
crops. The pathogen infects host roots, where it stimulates hypertrophy and hyperplasia. Following
infection, young plasmodia develop into vegetative plasmodia, then immature resting spores, and finally
mature resting spores are released into the soil as the clubbed root deteriorates at plant maturity. Field
trials were conducted with clubroot susceptible canola, followed by bioassays, to determine if a living
host was required for spore maturation. Plants were treated 5 wk after seeding. Glyphosate was sprayed to
o
kill the roots, or clubs were harvested and kept at 5, 10 or 22 C or frozen for 3 days and then stored at
these temperatures. Control plants were left to grow for 9 wk. Resting spores from 9-wk-old clubs were
used as inoculum on canola in bioassays in a controlled environment. Clubroot symptoms were assessed
after 6 weeks. Spore maturation continued in all treatments except when young clubs were continuously
frozen or stored at 5 ºC. Freezing clubs before storing them at 5 or 10 ºC resulted in more clubroot in
bioassays, compared to clubs that were harvested (not frozen) and stored. This demonstrated that a living
host was not required for spore maturation. Indeed, host death may speed spore maturation. Clubroot
infected plants must be controlled early to avoid adding inoculum to the soil.

Bruce Gossen (bruce.gossen@canada.ca)

P184. Identifying clubroot resistance in canola and Brassica vegetable cultivars for Ontario, 2018
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2
1
Drury, S. ; B. Gossen ; M.R. McDonald
1 University of Guelph; Agriculture and Agri-Food Canada
2
Clubroot, caused by the obligate parasite Plasmodiophora brassicae Woronin, has been present for many
years on Brassica vegetables in Ontario, but was only recently identified on canola (Brassica napus L.).
Once P. brassicae is present in a field, eradication is difficult, but resistant cultivars can provide effective
management. Two field trials to assess clubroot reaction were conducted at the Muck Crops Research
Station, King, Ontario. Pathotype 2 is now predominant at this site, having replaced pathotype 6 that
mainly attacks Brassica vegetables. Trial 1 consisted of 3 putative clubroot-resistant canola cultivars, 2
cultivars expected to be susceptible and 3 susceptible control cultivars. Trial 2 assessed cultivars of
cabbage, cauliflower, broccoli, napa cabbage, rutabaga, and a susceptible Shanghai pak choi control. The
canola cultivars expected to be resistant were highly resistant (disease severity index, DSI = 1%) and the
cultivars not marketed as resistant were susceptible (DSI = 98%). The broccoli cultivars were all
susceptible, the rutabaga cultivars showed intermediate resistance, and there were both resistant and
susceptible cultivars of cabbage and cauliflower. Clubroot reduced fresh shoot weight in susceptible
canola by 57% and in susceptible vegetables by 42–65% relative to resistant cultivars. Inoculation studies
of canola under controlled conditions showed a similar pattern of response. This research will help
Ontario growers to select cultivars if P. brassicae pathotype 2 is present in their fields.

Sarah Drury (drury@uoguelph.ca)

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