|Abstract or Summary
- 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,
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.