Defence reactions of mature Norway spruce (Picea abies) before and after inoculation of the blue-stain fungus Endoconidiophora polonica in a drought stress experiment

We performed an inoculation experiment using the blue-stain fungus Endoconidiophora polonica at the Rosalia Roof study site, which was set up to study drought effects on Norway spruce susceptibility to attacks by the European spruce bark beetle Ips typographus. Tree resistance parameters such as resin flow rate and extent of hypersensitive wound reaction in the phloem were investigated prior to and after fungal infection. Sample trees with different drought treatments (trees fully covered or semi-covered by roofs, control trees) were inoculated with E. polonica in July 2014. In order to check for areas of phloem necrosis, the outer bark around the inoculation holes was scratched off 6 weeks later. We recorded the amount of resin exudation within 12 hours overnight in August and September 2013 and 2014. Although wound reaction zones did not differ in size between tree collectives of the various treatments, fully covered trees tended to exhibit larger necrotic areas compared to control trees. The least water supplied trees showed lowest resin flow rates prior to fungal inoculation, but were the only group to show significantly enhanced resin flow five weeks after the evaluation of defence reactions. Our results agree with earlier findings that wounding and few fungal inoculations can increase tree resistance in the medium term given not too severe water stress. Further investigations will clarify how water stress affects the availability of non-structural carbohydrates for secondary metabolism, when depletion of resources eventually occurs, and at which point trees are most susceptible to bark beetle attack.