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

As for radial occlusion, fungal growth in the sapwood was minimal (Fig. 4), and did not differ between provenances.
Hence, data are not given in detail for the different provenances. After 6 weeks, the average fungal penetration into the
sapwood ranged from 0.2-2.0 mm and from 0.2-6.0 mm in France and Sweden, respectively. After 21/24 weeks, the corresponding
ranges were 0.0-1.6 mm and 0.0-9.0 ram. In both countries, fungi generally regressed or remained stable from 6 to
21/24 weeks (Fig. 4). There were no differences between the 2 strains of fungi, but both fungi expanded further into the
sapwood in Sweden than in France.
DISCUSSION
Comparison Between Provenances
The observed differences between provenances in all variables studied (reaction zone length, sapwood occlusion, and
fungal growth - vertically as well as radially) do not demonstrate a systematic pattern justifying general conclusions about
provenance differences. In Sweden however, foreign provenances responded differently than native provenances to fungus
infection. As this was not the case in France, no latitudinal or altitudinal trends can be detected. On the other hand, systematic
differences in defense reactions may have been masked by the considerable inter-tree variation, possibly due to variations
in micro-ecological conditions from one provenance plot to another, or even to genetic differences between trees of the same
provenance. A larger number of replications would have been required. Even better would have been to work with clones
instead of provenances. Thus in this experiment, reaction zone size alone did not provide enough intbrmation to evaluate
possible genetic differences in host resistance to blue-stain fungi.
In Sweden, lodgepole pine also responded in the same manner as foreign Scots pine provenances, which suggests it
may be similarly susceptible to European bark beetles.
Comparison Between Treatments
As observed earlier, fungal inoculations produced larger reaction zones than the sterile zones (Wong and Berryman
1977, Solheim 1988, Ross et al. 1992). For the variables measured, the two fungal strains caused larger differences in France
than in Sweden, and this locality effect will be discussed below. The French strain of L. wingfieldii invariably caused larger
reaction zones than the Swedish strain in France, and similar reaction zones in Sweden. As the pattern was similar for fungat
growth, the French strain could be considered more aggressive than the Swedish one. The observed difference cannot be
attributed to some environmental or host factor, as inoculations of the two strains occurred parallel in the same trees. Thus,
some inherent difference between the fungal strains should cause this difference in aggressiveness. For sapwood, the French
strain occluded more tissue than the Swedish strain after 21 weeks in France, hence its pathogenicity also could be higher. In
all treatments however, sapwood occlusion was shallow, despite extensive reaction zone formation in the phloem (cf.
Parmeter et al. 1992). This can be explained by the low density of inoculation, allowing the tree to contain the infection.
Comparison Between Localities
All variables measured indicated clear differences between localities. Reactions zones following fungal inoculations
were larger in Sweden than in France, as was sapwood occlusion. That was also the case for fungal growth, although not so
clearly tbr the phloem. In contrast, sterile inoculations caused larger reaction zones in France than in Sweden for the phloem,
but not for the sapwood.
As these differences were also true inside comparable pairs of provenances (e.g., "Kal" vs "Vas", "Fin" vs "Olo",
Spe" vs "Inv", "Rud" vs "Tab" and "Klo" vs "Enz"), a provenance effect can be discarded. However, the finding that sterile
inoculations resulted in larger reaction zones in France than in Sweden whereas fungal inoculations yielded the opposite
pattern, is difficult to explain. This contradiction implies a higher sensitivity to mechanical wounding in France than in
Sweden, but also a more efficient containment of the fungi in the former than in the latter case. Possibly, differences in
prevailing temperature conditions may have favored fungal growth in Sweden, as L. wingfieldii is known to have a low
temperature preference (Lieutier and Yart 1989).
Although the experimental conditions were planned to be as similar as possible, trees may have differed in phenology
between the localities. Judging from the relative shoot lengths 6 weeks after inoculation, Lorio's growth-differentiation
174
:i