In 1991 Pacific yew (Taxus brevifolia Nutt.) was reported as a new host for
Phytophthora lateralis Tucker and Milbrath which is an aggressive root rot pathogen
thought previously to be specific to Port-Orford-cedar. This study was designed to
compare the pathogenicity of P. lateralis on the two hosts, and to characterize sites
where Pacific yew mortality occurs. The specific objectives were: 1) compare root
colonization and mortality of Pacific yew and Port-Orford-cedar seedlings and
rooted cuttings; 2) compare lesion length on inoculated Pacific yew and Port-Orford-cedar
branches and stems; 3) compare zoospore attraction to freshly cut Pacific yew
and Port-Orford-cedar rootlets; 4) compare amount of mortality of Pacific yew and
Port-Orford-cedar in infested drainages and determine extent of yew mortality; and
5) characterize sites where P. lateralis causes Pacific yew mortality.
Root colonization of P. lateralis was significantly greater in cedar than in yew.
Seedling mortality averaged 58% for cedar and 4% for yew. Lesion length on the
cedar seedling stems was twice the lesion length on yew stems, and cedar branches
had lesions four times longer than yew branches. Abundant zoospore aggregation
occurred on cedar rootlets along the zone of elongation and the region of maturation.
In comparison, far fewer zoospores encysted along the yew rootlets, and were
concentrated on the root hairs. The stream survey of three infested drainages in
southwest Oregon and northwest California revealed a total of 1199 dead Port-Orford-cedar (46% mortality), and 86 dead Pacific yew (10% mortality). At sites
where P. lateralis-induced mortality occurred, the interaction of slope and distance
from the stream was negatively correlated with tree death.
Based on results of this study, we conclude that Pacific yew is less susceptible
to P. lateralis than Port-Orford-cedar, but that mortality of Pacific yew in the field
is greater than previously reported. In addition, Pacific yew mortality was observed
most often on level to nearly-level sites close to the stream's edge where root
exposure to P. lateralis-infested water was frequent in scope and duration. / Graduation date: 1995
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/35176 |
| Date | 10 April 1995 |
| Creators | Murray, Marion S. |
| Contributors | Hansen, Everett M. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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