Temporal Population Genetic Dynamics of Wheat Sharp Eyespot Pathogen Rhizoctonia cerealis in the Willamette Valley, Oregon, U.S.A.

Abstract

Wheat sharp eyespot, caused by the soilborne basidiomycete Rhizoctonia cerealis anastomosis group DI (AG-DI), has become an increasingly important stem disease worldwide and has resulted in yield losses in the Willamette Valley of Oregon since 2014. However, the temporal genetic dynamics of R. cerealis populations remain poorly understood. In this study, 440 pure isolates of R. cerealis AG-DI were isolated from the same experimental field in the springs of 2017, 2019, 2021, and 2023. Population genetic analyses based on six microsatellite markers detected no population differentiation among years. The two previously reported genetic pools neither merged nor produced new pools, indicating a stable population structure over time. Together with earlier reports of R. cerealis population genetics from China, these findings suggest that the R. cerealis population in the Willamette Valley was consistent with a founder effect. Although Hardy-Weinberg disequilibrium and linkage disequilibrium tests indirectly indicate the presence of sexual recombination, these signals may represent historical variation rather than substantial ongoing sexual reproduction within the region. The temporal genetic changes observed in the R. cerealis population may be linked to indirect selection pressure imposed during long-term breeding and screening of eyespot (strawbreaker foot rot)-resistant wheat cultivars in the same field. These findings provide insight into the evolutionary processes shaping R. cerealis populations and have implications for disease management and resistance breeding.