View or print this publication - Northern Research Station - USDA ...

METHODS
The study was installed in a mixed conifer stand (elevation - 1460 m) in northeastern Oregon. On June 29, 1993,
seven Douglas-fir and seven western larch were selected from an area of less than 1 ha. The trees were chosen based on
similarities in size and lack of any visible injuries or disease. Three inoculation sites were established equidistant around the
circumference of each tree at dbh (1.4 m height). The three treatments that were randomly assigned to the inoculation sites
were: (1) sterile wound, (2) L. abietinum, and (3) O. pseudotsugae. At each inoculation site, a 1.2-cm diameter hole was
drilled to the sapwood surface with a sterile bit. For the sterile wound, a cotton plug was placed in the hole that was then
covered with duct tape. For the fungal inoculations, a small agar plug containing the appropriate fungus was placed in the
hole that was then similarly covered. The fungi used in the study had been isolated from Douglas-fir beetles collected in
British Columbia, Canada (Solheim unpubl.).
On August 25, 1993, the outer bark covering each inoculation site was removed and the length of the necrotic lesion
on the phloem surface was measured.
The mean dbh for trees of each species were compared by a t-test. Lesion length was subjected to analysis of
variance for a split-plot design with tree species representing whole plots and fungal inoculation representing subplots.
RESULTS
There was no significant difference (P < 0.19) in dbh between Douglas-fir (mean = 51.0 cm) and western larch (mean
= 48.8 cm). The tree species x treatment interaction was significant (F = 3.29; P < 0.05) for lesion length. For both tree
species, inoculation with either fungus resulted in a larger lesion than that produced by a sterile wound (Fig. 1). The length
of the lesions induced by a sterile wound or inoculation with O. pseudotsugae did not differ between tree species, but the
lesion induced by inoculation with L. abietinum was significantly shorter in western larch than in Douglas-fir (Fig. 1).
DISCUSSION
There were no obvious differences in the general appearance of lesions (i.e., intensity of resin accumulation, radial
growth, etc.) between tree species, so lesion length should provide a relatively accurate indication of fungal growth within the
phloem of the host trees. The smaller lesions produced by western larch in response to L. abietinum may reflect a more rapid
and effective defensive reaction to the fungus. These results suggest that L. abietinum is not as well adapted to utilize
western larch as a host compared with Douglas-fir. If L. abietinum is critical to the survival of Douglas-fir beetle brood in
live host trees, then this may help to explain the complete mortality of brood in western larch. In contrast, there was no
difference in the induced response to O. pseudotsugae between the two tree species. Our results must be interpreted cautiously
since we have only measured lesion length in phloem tissue. Lesion length is not always directly correlated to
pathogenicity (Solheim 1988). Fungal growth and tree response may be quite different in the xylem compared with the
phloem. However, the difference in response to fungal inoculations that we observed suggests that further research on the
host tree-fungus interactions in this system would be worthwhile. The significance of these results will only become apparent
when we learn more about the role that these fungi play in the population dynamics of the Douglas-fir beetle.
SUMMARY
There was no difference in lesion length at the phloem surface between Douglas-fir and western larch following
sterile wounds or inoculation with O. pseudotsugae. However, lesion length following inoculation with L. abietinurn was
significantly shorter in western larch compared with Douglas-fir. These results may help to explain the lack of Douglas-fir
beetle brood survival in western larch.
225