Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/20/2014
Publication Date: 3/7/2014
Citation: Okubara, P.A., Schroeder, K.L., Abatzoglu, J.T., Paulitz, T.C. 2014. Agroecological factors correlated to Rhizoctonia spp. in dryland wheat production zones of Washington state, USA. Phytopathology. 104(7):683-691. Interpretive Summary: Certain fungal root pathogens cause Rhizoctonia root rot and damping off, and result in chronic yield losses for producers of wheat in the dryland regions of the Pacific Northwest, USA. These pathogens are thought to act as a complex, and can infect and reduce yields of legume and Brassica crops used in rotation with wheat. This three-year survey identified the regions in which occurred three different Rhizoctonia root rot-causing pathogens, and identified meteorological and soil chemical factors that were associated with their presence in the soil. Population densities of the most virulent pathogen of wheat were negatively correlated to precipitation, whereas those of the other two pathogens were positively correlated to low temperature. Sand and clay content also were differentially associated with the three pathogens. Management of Rhizoctonia root rot and damping off should account for the likelihood that each pathogen is impacted by a unique group of agroecological variables.
Technical Abstract: The necrotrophic soilborne fungal pathogens Rhizoctonia solani AG8 and R. oryzae are principal causal agents of Rhizoctonia root rot of wheat in dryland cropping systems of the Pacific Northwest (PNW). A three-year survey of 33 parcels at eleven growers’ sites and 22 plots at twelve Washington State University cereal variety testing locations was undertaken to understand how these pathogens were distributed. Pathogen population densities in soils, quantified using real-time PCR, were correlated to precipitation, high and low temperatures, and soil texture factors in a pathogen-specific manner. For example, population densities of R. solani AG8 were negatively correlated with precipitation and positively correlated with temperature maxima. However, R. oryzae was not correlated with precipitation and temperature maxima. Furthermore, both R. solani AG8 and R. oryzae were more abundant in soils with higher sand and lower clay content. Principal component analysis indicated that unique groups of meteorological and soil factors were associated with each pathogen. Tillage did not affect the populations of R. oryzae, but affected R. solani AG8 at P=0.06. The previously planted crop had a significant (P<0.05) effect on populations of R. solani AG8; lower populations occurred when the previous rotation was a broadleaf crop. Our findings showed that populations of R. solani AG8 were consistent with the general distribution of bare patch symptoms found in fields, based on other surveys and observations, but was present at many sites in which bare patch symptoms were not evident. Management of Rhizoctonia root rot and bare patch should account for the likelihood that each pathogen is impacted by a unique group of agroecological variables.