Relationships between yield, rotation length, and abundance of Olpidium brassicae and Pyrenochaeta sp. in the rhizosphere of oilseed rape

Oilseed rape yields in the UK have been found to decline with more frequent cropping in a rotation. Previously, two soil-borne organisms (Olpidium brassicae (Chytridiomycota) and Pyrenochaeta sp. (Ascomycota)) were identified as having high relative abundance in rhizosphere fungal communities associated with oilseed rape crops where yield decline had been recorded. In order to better understand these organisms' association with the oilseed rape crop, the current study was designed to investigate the fungal rhizosphere microbiome of oilseed rape grown in a wide range of rotational frequencies. Samples collected from a long-term rotation trial site at three time points through the growing season were used to determine fungal community composition, and quantification of O. brassicae and Pyrenochatea sp. Analyses showed the combined root and rhizosphere fungal communities were similar across all oilseed rape rotations, largely due to the high relative abundance of O. brassicae, irrespective of cropping frequency. Olpidium brassicae abundance peaked in March (mid-season) in all rotations, before declining in abundance by June (pre-harvest). In contrast, Pyrenochaeta sp. increased in abundance throughout the season, with significantly higher levels reached in June than earlier in the season. Pyrenochaeta sp. had a greater abundance early in the season (January) in continuously grown and alternate oilseed rape (grown one year in two) than in rotations with longer gaps between oilseed rape crops This study concludes that O. brassicae cannot be solely associated with yield decline of OSR observed in short rotation cropping due to its prevalence in the extended rotations examined (up to 6-year gap).