Distribution of viable resting spores of Plasmodiophora brassicae in infested fields.

Resting spores of Plasmodiophora brassicae Woronin, the causal agent of clubroot of Brassicaceae crops, can survive in soil for many years. The objectives of this study were to assess the vertical distribution of viable spores in newly-infested and heavily-infested fields in Alberta, Canada, and the horizontal distribution of viable spores in a hot spot in one infested field. Soil cores (1-3 cores per site, 2-3 sites per field) were collected at 7.5 cm increments from 0-45 cm depth from three fields. Also, samples (5 per site) to 15 cm depth were collected from 15 sites within a 0.5 ha section of a hot spot in one field. Spores were quantified using qPCR and propidium monoazide-assisted qPCR (PMA-PCR). PMA inhibits amplification of DNA of non-viable cells in qPCR. High variability in the vertical and horizontal distribution of spores was observed in each field. In general, spores were detected at greater depths in heavily-infested fields (10^3-10^5 spores g^-1 soil at 30-45 cm deep) as compared to the newly-infested field (10^5 spores g^-1 soil, only in 0-7.5 cm sample). Most of the spores found in the heavily-infested field were viable. The horizontal distribution of viable spores was highest in the middle of the hot spot (10^5-10^6 spores g^-1 soil) and declined rapidly towards the outer sampling sites (10^3-10^4 spores g^-1 soil). This supports previous reports on the high site to site variability of spore concentration within a field, and that resting spores move downward in the soil profile over time.