Abstract
Background. Sydowia polyspora is a widespread conifer-associated fungus with ecological roles that vary among host and disease contexts. However, how its genomic variation corresponds to geography and host symptom severity remains poorly understood. We examined whether genomic differentiation among S. polyspora isolates recovered from symptomatic Pinus taeda varied between geographic regions and among host symptom-severity classes. Methods. We analyzed whole-genome sequence data from 25 S. polyspora isolates. Genome-wide single-nucleotide polymorphisms were used to evaluate population structure through principal component analysis and quantify differentiation among geographic and symptom-severity groups using FST. Nucleotide diversity and Tajima’s D were estimated in 5-kb windows to characterize genomic variation within groups. Results. Isolates overlapped broadly in principal component analysis, but pairwise comparisons revealed geographically variable differentiation. Differentiation between high- and low/moderate-severity groups was minimal in northern Alabama, whereas the corresponding southern comparison showed broadly distributed genomic differentiation (mean FST = 0.37). Northern and southern isolates exhibited more modest differentiation (mean FST = 0.11). Nucleotide diversity and Tajima’s D varied across the genome but showed substantial overlap among geographic and symptom-severity groups. Genomic differentiation between symptom-severity groups was evident in southern but not northern Alabama, suggesting that associations between S. polyspora genetic background and host symptom severity may vary geographically.