Seasonal water stress and the resistance of Pinus yunnanensis to a bark-beetle-associated fungus

We examined the influence of seasonal water stress on the resistance of Pinus yunnanensis (Franch.) to inoculation with Leptographium yunnanense, a pathogenic fungus associated with the aggressive bark beetle, Tomicus n. sp. Experiments took place between October 1997 and November 1999 in two plots located at the top and at the foot of a hill in Shaogiu, China, a region characterized by dry winters and wet summers. Following isolated and mass fungal inoculations, we observed the reaction zone length, fungal growth in the phloem, and the occlusion, blue-staining and specific hydraulic conductivity of the sapwood. Measurements of soil and needle water contents and predawn needle water potentials confirmed that trees were subject to mild water stress during winter, especially at the drier hilltop site. Measures of tree resistance to fungal infection of phloem and sapwood were congruent and indicated that trees were most susceptible to inoculation during the wet summer, especially at the lower-elevation plot. Specific hydraulic conductivity decreased after inoculation in summer. The results indicate that mild seasonal water stress is not likely responsible for the recent extensive damage to young P. yunnanensis stands by Tomicus n. sp. in the vicinity of our study plots. Rather, the results suggest that mild water stress enhances tree resistance to fungal pathogens associated with Tomicus n. sp.