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Spatial and temporal heterogeneity in Rhizoctonia solani AG2-2IIIB inoculum density distribution in sugar beet fields

Michael Zellner: Bavarian State Research Centre for Agriculture

<div><em>Rhizoctonia solani</em> AG2-2IIIB, the soilborne fungus that causes Rhizoctonia Crown and Root Rot (RCRR), is responsible for considerable losses in sugar beet yield and quality. The RCRR incidence and severity as well as the location, shape and size of RCRR patches varies between seasons. Investigating the spatial and temporal distribution of the inoculum density (ID) would contribute to clarifying the underlying mechanisms. For a high spatial resolution analysis 49 composite soil samples were taken from a 1 ha grid on a sugar beet field in June 2017. For a large-scale spatial analysis a 15 ha site was probed with three composite samples each derived from an individual 1 ha grid in July 2017. For the temporal analysis soil samples were taken from artificially inoculated sugar beet research plots of a crop rotation field trial in June and September 2013, 2014 and 2015. Viable mycelium was isolated from soil samples by quinoa seed baiting. <em>R. solani</em> AG2-2IIIB was quantified by SYBR Green qPCR. The ID was unevenly distributed in the highly resolved 1 ha sampling area with zones of high, medium, low and zero ID. On the large scale two 1 ha sampling areas showed a high ID and one a low ID. The mean ID over all sugar beet research plots decreased threefold until June 2014 followed by a steady increase up to double the initial ID in September 2015. The data show that the ID varies greatly, both spatially and temporally which contributes to the dynamic nature of RCRR.</div>