Phylogenetic placement of Spermospora avenae, causal agent of red leather leaf disease of oats

Abstract

Spermospora avenae causes the economically important red leather leaf disease of oats, which reduces grain yield and hay quality. It was first reported in the USA in 1936 and subsequently in Australia in 1978. Despite this, its phylogenetic placement is unknown, attributed merely to Ascomycota. Twenty-three S. avenae single spore isolates were obtained from affected crops in South Australia and western Victoria from 2008 to 2016. DNA was extracted from each and sequenced using Illumina technology. To identify its closest relatives, a draft genome was de novo assembled and contigs with the highest depth, hypothesised to be the rRNA gene region, were compared to NCBI using the BLASTN function. Contigs that had homologous sequence to the rRNA gene region were used to identify closely related species, which turned out to be Rhynchosporium species. Sequence data from the α-tubulin, β-tubulin, and ITS gene regions of Rhynchosporium species, identified as phylogenetically informative for this genus, were mapped to the S. avenae contigs. Phylogenetic analysis of the ITS region and multilocus concatenation demonstrated that S. avenae is nested within Rhynchosporium, closely related to R. orthosporum and R. lolii. When ITS sequences from other related genera sourced from GenBank were added to the analysis, it appears that Rhynchosporium is paraphyletic and should be split into two genera. Culturally, S. avenae prefers a semi-solid low nutrient medium (ie. ¼ strength PDA made with 1.25% agar) and cool temperature (optimum 15 °C). This corresponds well with the cold wet seasonal conditions required for disease development in the field.