Survey and characterization of oomycete species associated with root rot in UK red raspberry ( <i>Rubus idaeus</i> )

Abstract Phytophthora rubi has been reported as the primary causal agent of raspberry root rot (RRR) in the UK, resulting in severe economic losses. Recent observations suggest the RRR complex may be more diverse than previously thought. This study investigated the Peronosporales species present in UK raspberry plants and factors affecting their diversity and abundance. Eighty‐six symptomatic root and 86 cane samples were collected from seven farms in the UK in 2020 and 2021. Five additional samples were collected from blackberry roots. Symptomatic tissue was plated onto selective media, and resultant isolates were identified by ribosomal internal transcribed spacer (ITS) barcoding. Twelve isolates of five species of Phytophthora ( P . citrophthora , P . pini , P . erythroseptica , P . cryptogea and P . pseudocryptogea ) and seven isolates of two Phytopythium species ( Pp . vexans and Pp . litorale ) were collected. Isolate pathogenicity was assessed on detached leaves, with Pp . litorale and Pp . vexans exhibiting significantly higher pathogenicity on raspberry leaves than an isolate of P . rubi . High‐throughput sequencing of the ITS region of symptomatic root samples revealed farm location had a significant effect on Peronosporales community composition ( p = 0.001), and both farm location and plant variety significantly affected the abundance of taxa ( p &lt; 0.002 and p &lt; 2 × 10 −16 , respectively). This study presents Pp . vexans and Pp . litorale as new potential pathogens for further study. Furthermore, we recommend the inclusion of other Peronosporales species into resistance screens in raspberry breeding programmes to produce genotypes more resilient to community composition shifts in raspberry root pathospheres in the UK.