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Vertical profile of Plasmodiophora brassicae resting spores in mineral and muck soils
T. J. CRANMER (1), B. D. Gossen (2), A. Deora (1), M. R. McDonald (1). (1) University of Guelph, Guelph, ON, Canada; (2) Agriculture and Agri-Food Canada, Saskatoon, SK, Canada

Canola (<i>Brassica napus</i> L.) is a major crop in Canada, with an economic value of over $19 billion per year. <i>Plasmodiophora brassicae</i> Woronin, the causative agent of clubroot, can cause substantial reduction in yield of canola and is spreading rapidly in western Canada. Resting spores of <i>P. brassicae</i> can survive in soil for many years, but information on their vertical distribution in the soil profile is lacking. Vertical soil cores from the soil surface to 53-cm depth were collected by hand from naturally infested mineral soil sites near Bassano Alberta and Milgrove Ontario, and a muck soil (70% organic matter) from the Holland Marsh in Ontario. A multiplex TaqMan qPCR assay, including an internal control based on <i>GFPuv1</i>, was used to quantify resting spore concentration in soils at selected points along the vertical profile. Spore concentrations ranged from 1 x 103 to 6 x 106 gram-1 of dry soil. Resting spores were present throughout the soil profile, with concentrations of 1.0 x 103 spores gram-1 or more at 45–53 cm below the surface in each soil type. Resting spores are generally produced near the soil surface, so this observation indicates that spores are likely carried down into the soil profile by movement of water. These results, showing that resting spores can be found well below the plow layer, have important implications for developing future clubroot management techniques.

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