Abstract:
Phaeocryptopus gaeumannii is a pathogenic fungus on Douglas-fir needles which has become a
serious threat to timber production in the last decade along the Oregon coast This research was undertaken
to describe the general biology and pathology of F. gaeumannii on Douglas-fir needles and how
environmental variables affect fungal development. The biology of P. gaeumannii was described using
histopathology methods such as scanning electron microscopy, thin section light microscopy, and plastic
impressions of needle surfaces. Needle penetration occurred through stomata as ascospore germ tubes
differentiated into appressoria in the outer stomatal chamber. Penetration pegs grew between the guard
cells and into the needle mesophyll. Internal hyphae were observed only in intercellular spaces but were
often well attached to host cells. Epiphytic growth of P. gaeumannii on needles was extensive and
persisted throughout summer, fall, winter, and spring months. These hyphae continued to produce
appressoria and penetrate stomata over time. Hyphal incidence on needle surfaces and inside needles was
greater at high disease sites compared to low disease sites. No asexual stage was observed, however, new
strands of hyphae grew from pseudothecial initials as these emerged from stomata in fall and winter
months. These hyphae also persisted on needle surfaces.
Using temperature gradient plates, ascospores germinated and grew over a range of temperatures
from 14°C to 30°C. Optimum germination and growth occurred at 18°C and 22°C respectively. Dry
conditions for more than 24 hours significantly reduced both germination and growth of ascospores. Using
inoculated seedlings, successful infection of needles occurred after 24 hours of exposure to moisture in a
greenhouse mist chamber. As moisture periods increased, the overall colonization of needles by P.
gaeumannii hyphae also increased.
