The 2011 ASHS Annual Conference

Clubroot of Brassica crops, caused by protist, Plasmodiophora brassicae Woronin,  causes clubbing of roots, which leads to wilting, stunting and reduction in yield.  The pathogen produces large numbers of resting spores which persist in soil for many years.  Recently, cabbage cultivars with clubroot resistance have been developed by Syngenta Seeds.  Four cultivars, Kilherb, Kilaton, Kilaxi, and Tekila were evaluated for clubroot resistance in relation to susceptible cabbages, Bronco (3 sites) or Atlantis (mineral soil, 2009).  Allegro 500F (40% fluazinam ) was applied to an additional treatment of susceptible cabbage (50 ml product in 100 L water, 100 ml solution per plant) immediately following transplanting. Trials were conducted on muck soil ( pH 6.3, 72% organic matter)  in 2009 and 2010 and on two  mineral soil sites, a Bierren sandy loam  (pH 6.5) in 2009 and  another sandy loam in 2010.  Sites were naturally infested with P. brassicae pathotype 6. Cabbage was harvested in September and October, as each  cultivar reached marketable size.  Heads were cut, weighed, and  a detailed quality assessment was done for cabbage from the mineral soil site in 2010.   Roots of all plants in each replicate plot were dug and assessed for clubroot incidence, and for severity using a 0-3 scale, where 0 was no visible clubbing and 3 was over two thirds of the root system with clubs. A disease severity index was calculated to transform the ratings to a 0-100 scale. Clubroot incidence and severity was very high at three of the four sites ranging from 87.5-100% on susceptible cv. Bronco. Clubroot incidence was low at the mineral soil site in 2009; incidence on susceptible Atlantis was 17.5%.   All of the resistant cultivars were highly resistant to clubroot (0- 3.8% incidence) in all trials.  Applying Allegro to Bronco reduced clubroot incidence and severity at two of three sites, compared to untreated Bronco, but did not reduce clubroot to the low level exhibited by the resistant cultivars. Cabbages Kilherb, Kilaton, Kilaxi, and Tekila demonstrated a consistently high level of resistance on different soil types and under different levels of disease pressure.