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Effect of inoculum concentration on growth of clubroot-resistant canola and Napa cabbage.

Dalton, J.A., Gossen, B.D., and McDonald, M.R. 2016. Effect of inoculum concentration on growth of clubroot-resistant canola and Napa cabbage. Can. J. Plant Pathol. 38: 128.

Abstract

Clubroot caused by Plasmodiophora brassicae Woronin reduces yield in canola (Brassica napus L.). Genetic resistance is essential for clubroot management. However, studies indicate that high spore loads may reduce growth and delay development in clubroot-resistant canola cultivars. The objective of this research was to compare the growth response of clubroot-resistant canola to P. brassicae resting spore load in soil. In 2014, clubroot-resistant and susceptible canola was planted at two adjacent sites at the Muck Crops Research Station (Holland Marsh, Ontario) with the same soil type and agronomic practices, but differing in spore loads of P. brassicae . Plant height was measured weekly from 4 – 8 weeks after planting. Clubroot incidence and severity were assessed using a standard 0 – 3 rating scale. Severe clubroot (100 DSI) developed in susceptible canola at both sites. Resistant cultivars had no clubroot symptoms. At 8 weeks after planting, at Site 1 (7 × 10^5 spores g^−1soil), plant height of resistant cultivars was 124% (±4.5) greater than the susceptible control. The proportion of resistant cultivars at flowering or pod development stage was 30% (±3.2) greater than the susceptible control. At 8 weeks after planting, at Site 2 (7 × 10^6 spores g^−1 soil), plant height of resistant canola was 46% (± 3.9) greater than the susceptible control. The proportion of resistant cultivars at flowering or pod development stage was 20% (±2.8) greater than the susceptible control. These field results support the observations from controlled environment trials that high concentrations of resting spores of P. brassicae cause reduced growth and delayed development of clubroot-resistant canola.

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