Effects of temporary flooding rotations on Rhizoctonia solani AG-3 and on soil bacterial communities in western Washington

Amy Leigh Salamone

Temporary flooding rotations (TFR) are sometimes employed in the Skagit Valley of western Washington to enhance shorebird habitat and control soilborne potato pathogens, including Rhizoctonia solani. In growth chamber experiments, viability of R. solani AG-3 isolate #13-8 on naturally-infected potato tuber discs diminished by average 23, 40, and 9% over 38 weeks for constant and intermittent flooding, and dry soil treatments, respectively. Viability in inoculated soil-sand mix decreased by average 25% for constant flooding at 10°C, although 100% viability persisted for all treatments at 16°C. Mycoparasitism of R. solani was observed at 10° and 16°C for constant and intermittent flooding, and dry soil treatments only. Survival and pathogenicity of isolate #13-8 in bermed field microplots was assessed with four month Fall/Winter TFR treatments and subsequent potato cropping. Direct baiting indicated that soilborne R. solani increased 44% on average for constant flooding; qPCR primers designed specifically for isolate #13-8 ITS sequence indicated average 17% increase. Fall/Winter TFR had no impact on disease severity; hence it is unlikely that Fall/Winter TFR offers meaningful control of soilborne R. solani in the region. Pre-TFR and post-TFR soil nutrients and properties ie., P, NaHCO-P, K, SO4, Mn, Mg, Ca, Zn, Cu, B, EC, pH, and pH buffer index, responded similarly to Spring/Summer 2013/2014 and Fall/Winter 2013 field flooding events in barrel microplots. For Spring/Summer constant flooding, iron increased 46% average, but nitrate, organic matter, and cation exchange capacity decreased 85, 24, and 27%, respectively. Fall/Winter constant flooding resulted in 33, 4, and 5% increased sodium, organic matter, and cation exchange capacity, respectively. Soil bacterial communities were characterized pre- and post-TFR using Illumina MiSeq . Spring/Summer constant flooding resulted in average increase of Acidobacteria by 14% and Verrucomicrobia by 46%, while Fall/Winter constant flooding resulted in 20% average increase of Bacteroidetes. There was little effect, overall, on bacterial phyla and order composition when post-TFR was compared to pre-TFR communities implying that indigenous soil bacteria have resilient adaptive capabilities. This is the first report of enhanced susceptibility of R. solani to mycoparasitism under saturated soil conditions and the first molecular characterization of bacterial communities in an agricultural soil of western Washington.

Effects of temporary flooding rotations on Rhizoctonia solani AG-3 and on soil bacterial communities in western Washington

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