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Decline in the concentration of resting spores of Plasmodiophora brassicae following a susceptible crop.

Dalton, J.A., Gossen, B.D., Al-Daoud, F., Pageau, D., and McDonald, M.R. 2016. Decline in the concentration of resting spores of Plasmodiophora brassicae following a susceptible crop. Can. J. Plant Pathol. 37: 117.

Abstract

The concentration of resting spores of Plasmodiophora brassicae Woronin in soil increases rapidly when susceptible crops are grown in short rotation. Some of these spores survive for many years, but the rate of decline of resting spores in soil is not known. The objective of this research was to quantify the decline in resting spore concentration following a susceptible canola (Brassica napus L.) crop. A large-scale cropping rotation study that includes clubroot-susceptible canola has been conducted over many years in a field infested with P. brassicae at the AAFC research farm at Normandin, Québec. Bulked, replicated soil samples were collected from large plots of continuous canola and from plots with a break interval of 1, 2, 3, 5, and 6 years since the last susceptible canola crop. Resting spore populations in soil were quantified using a multiplex qPCR protocol that included a competitive internal positive control to minimize differential inhibition among the samples. The concentration of resting spores in soil declined over time in a quadratic relationship (y = 1E + 07e-0.759x . R2 = 0.65) as the length of break from the susceptible canola crop increased. Resting spore concentration declined by 96% after a 1-yr break, 99% after a 2-yr break from canola, but then declined slowly compared to continuous canola (1.3 x 108 spores g-1 soil). These results provide an explanation for a recent report that the seed yield of clubroot-resistant cultivars is reduced when grown in short rotations with a susceptible crop relative to a 2- to 3-yr break.

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