Phylogenetics, population genetics, molecular epidemiology, and pathogenicity of the Douglas-fir Swiss needle cast pathogen Phaeocryptopus gaeumannii

A hierarchical series of studies, based mainly on molecular data, was conducted to elucidate the life history of the Douglas-fir Swiss needle cast pathogen Phaeocryptopus gaeumannii at macro- and micro-evolutionary scales. This information was then utilized to design and evaluate molecular diagnostic tools for use in studies on the epidemiology of a Swiss needle cast outbreak near Tillamook, Oregon. Phylogenetic analyses of partial nuclear ribosomal gene sequences indicated that P. gaeumannii, currently classified in the Venturiaceae, is closely related to neither Phaeocryptopus nudus, type of the genus, nor Venturia inaequalis, type of the Venturiaceae. Instead, it is closely related to members of the "sooty molds" (Capnodiales), particularly the common and morphologically similar Douglas-fir epiphyte Rasutoria pseudotsugae (Euantennariaceae). Single-strand conformation polymorphisms, revealing DNA sequence variation in five loci, were used to investigate population biology of P. gaeumannii from a worldwide collection of isolates. In western Oregon, P. gaeumannii population structure suggests a predominantly selfing reproductive mode within two reproductively isolated sympatric lineages. One lineage was widely distributed both locally and abroad. The second lineage was restricted to western Oregon and suggested a correlation with symptoms of Swiss needle cast. A novel application of real-time PCR allowed species-specific detection and quantification of P. gaeumannii and proved a good measure of its biomass in Douglas-fir needles. Compared to other techniques (ergosterol and a DNA probe), real-time PCR correlated best with visual estimates of needle colonization and additionally proved useful early in the first year of the colonization process before visible development of fruiting structures. While all four methods provided evidence that sites expressing a range of disease severity differed in the degree of fungal colonization, only real-time PCR consistently separated both moderately and severely diseased sites from relatively healthy sites. Seedling inoculation experiments, fulfilling Koch's postulates, demonstrated that P. gaeumannii is the causal agent of Swiss needle cast, as observed in the Tillamook epidemic. Furthermore, the incorporation of virulence tests provided independent, non-molecular evidence that Oregon's pathogen population is not homogeneous. One strain, isolated from a severely diseased site, caused significantly greater symptom severity than strains derived from less damaged sites

Determining the Novel Pathogen Neodothiora populina as the Causal Agent of the Aspen Running Canker Disease in Alaska

Neodothiora populina Crous, G.C. Adams & Winton was determined to be a new pathogen of trembling aspen (Populus tremuloides) growing in Alaska, based on completion of Koch’s Postulates in replicated forest and growth chamber inoculation trials. The pathogen is responsible for severe damage and widespread rapid mortality of sapling to mature aspen (≥ 80 years) in the boreal forests of interior Alaska, due to large diffuse annual (1–2 years) cankers. Isolation of the pathogen was challenging, and identification based on cultural characters was difficult. Fruiting bodies were not found on wild diseased trees, but erumpent pycnidia were found in bark overlying cankers on several stems inoculated with pure cultures

Widespread Mortality of Trembling Aspen (Populus tremuloides) Throughout Interior Alaskan Boreal Forests Resulting from a Novel Canker Disease

Over the past several decades, growth declines and mortality of trembling aspen throughout western Canada and the United States have been linked to drought, often interacting with outbreaks of insects and fungal pathogens, resulting in a “sudden aspen decline” throughout much of aspen’s range. In 2015, we noticed an aggressive fungal canker causing widespread mortality of aspen throughout interior Alaska and initiated a study to quantify potential drivers for the incidence, virulence, and distribution of the disease. Stand-level infection rates among 88 study sites distributed across 6 Alaska ecoregions ranged from \u3c 1 to 69%, with the proportion of trees with canker that were dead averaging 70% across all sites. The disease is most prevalent north of the Alaska Range within the Tanana Kuskokwim ecoregion. Modeling canker probability as a function of ecoregion, stand structure, landscape position, and climate revealed that smaller-diameter trees in older stands with greater aspen basal area have the highest canker incidence and mortality, while younger trees in younger stands appear virtually immune to the disease. Sites with higher summer vapor pressure deficits had significantly higher levels of canker infection and mortality. We believe the combined effects of this novel fungal canker pathogen, drought, and the persistent aspen leaf miner outbreak are triggering feedbacks between carbon starvation and hydraulic failure that are ultimately driving widespread mortality. Warmer early-season temperatures and prolonged late summer drought are leading to larger and more severe wildfires throughout interior Alaska that are favoring a shift from black spruce to forests dominated by Alaska paper birch and aspen. Widespread aspen mortality fostered by this rapidly spreading pathogen has significant implications for successional dynamics, ecosystem function, and feedbacks to disturbance regimes, particularly on sites too dry for Alaska paper birch