Assessments of Population Structure, Diversity, and Phylogeography of the Swiss Needle Cast Fungus (Phaeocryptopus gaeumannii) in the U.S. Pacific Northwest

Abstract

Swiss needle cast (SNC) is a foliar disease of Douglas-fir (Pseudotsuga menziesii) caused by Phaeocryptopus gaeumannii (Rohde) Petrak. This fungus is endemic to western North America, where it has historically had little impact in native forests. However, increasing disease severity in western Oregon since the 1990s has prompted renewed interest in P. gaeumannii and SNC. For this study, we analyze multilocus microsatellite genotypes from 482 single-spore isolates from 68 trees across 14 sites in the western Coast Range of Oregon and southwestern Washington. This study assesses genotypic variation and genetic structure at several levels of population hierarchy. Despite the observation that most of the genetic variation occurred within subpopulations, our analyses detected significant differentiation at all hierarchical levels. Clustering among the 482 isolates based on genetic distance clearly supports the existence of two previously described cryptic lineages of P. gaeumannii in the western United States. The two lineages occur in varying proportions along latitudinal and longitudinal gradients in western Oregon and Washington, suggesting a relationship between climate and phylogeography. Sites near Tillamook, Oregon, where SNC is most severe, consist of sympatric subpopulations in which the two lineages comprise roughly equal proportions.

Environmental variables associated withNothophaeocryptopus gaeumanniipopulation structure and Swiss needle cast severity in Western Oregon and Washington

“…The protocol for isolating N. gaeumannii from infected Douglas-fir needles is described in detail in Bennett and Stone (2016). Briefly, needles with pseudothecia of N. gaeumannii were attached to the lids of Petri dishes with double-sided adhesive tape, placed over water agar, and incubated for 48-72 hr.…”

“…The DNeasy Plant Kit (Qiagen) was used to extract genomic DNA from vegetative mycelium. For each isolate, ten SSR loci (Winton, Stone, & Hansen, 2007) were amplified in three multiplexed PCR reactions (Bennett & Stone, 2016). For each isolate, ten SSR loci (Winton, Stone, & Hansen, 2007) were amplified in three multiplexed PCR reactions (Bennett & Stone, 2016).…”

“…The manufacturer's protocol was modified with the addition of a maceration procedure (Bennett & Stone, 2016). The amplification was performed with the thermal cycling protocol described in Bennett and Stone (2016 Corporation) and were also visually examined for each locus and isolate to confirm accuracy. The Qiagen Type-It Microsatellite PCR kit (Qiagen, Hilden, Germany) and its associated protocols were used, but with reaction volumes of 12.5 µl.…”

“…Lineage 2 has a more restricted distribution (Bennett & Stone, 2016;Winton et al, 2006) and is most abundant along the western Coast Ranges in Oregon and Washington within a few kilometers of the coast. The abundance of Lineage 2 decreases further inland, where it is often supplanted entirely by Lineage 1 (Bennett & Stone, 2016;Winton et al, 2006). The factors influencing the spatial distributions of these lineages are not currently understood.…”

“…The abundance of Lineage 2 decreases further inland, where it is often supplanted entirely by Lineage 1 (Bennett & Stone, 2016;Winton et al, 2006). Previous studies have identified strong genetic differentiation between these two lineages (Bennett & Stone, 2016;Winton et al, 2006). Previous studies have identified strong genetic differentiation between these two lineages (Bennett & Stone, 2016;Winton et al, 2006).…”