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

Abstract

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.

Interaction of drought‐ and pathogen‐induced mortality in Norway spruce and Scots pine

“…Our study does not enable us to evaluate the impact of drought and pathogen infection on C transport in the phloem. However, lesions in the phloem were particularly large in spruce subjected to both drought and inoculation, as previously reported for other sapwood‐ and phloem‐infecting fungi that cause larger phloem lesions under drought conditions (Netherer et al, 2016; Schoeneweiss, 1975, 1983; Terhonen et al, 2019). Drought has also been suggested to cause phloem transport failure (Sevanto, 2014).…”

Acute Drought Is an Important Driver of Bark Beetle Infestation in Austrian Norway Spruce Stands

“…Such ecosystem modeling approaches have, however, rarely been linked to site conditions and tree/bark beetle interactions actually investigated at local level and, furthermore, have yet to be verified. To address this problem, seasonal variations in the stress status of mature Norway spruce and shifting capability to defend I. typographus attacks were studied in the course of an elaborate drought manipulation experiment performed in a forest stand located in the Eastern Austrian Rosalia Mountains (Netherer et al, 2015(Netherer et al, , 2016. The "Rosalia Roof experiment" for the first time provided strong empirical evidence of the negative impact of drought stress on Norway spruce defense against I. typographus; yet, distinct differences in attractiveness of stressed and control trees were not observed (Netherer et al, 2015;Matthews et al, 2018).…”

European Spruce Bark Beetle, Ips typographus (L.) Males Are Attracted to Bark Cores of Drought-Stressed Norway Spruce Trees with Impaired Defenses in Petri Dish Choice Experiments

“…To understand the host selection behavior of male I. typographus towards Norway spruce trees differing in drought-stress status and resistance, we conducted Petri dish arena choice experiments. Trees for our experimental trials were selected from the Rosalia Roof Study II, a rainfall exclusion study conducted in the Austrian Rosalia Mountains, forming the eastern foothills of the Alps [1,22,31]. The main project goals of the Rosalia study were to examine the influence of site water supply on Norway spruce biochemistry, physiology and chemical defense, the subsequent effects on tree attractiveness and susceptibility to attacks by the European spruce bark beetle, I. typographus, and susceptibility to its associated ophiostomatoid fungi.…”

“…The experimental inoculation of blue-stain fungi is an established method to test for tree resistance towards subcortical fungal and insect attacks [20,21]. Larger wound reaction zones indicate a slower containment of fungal growth due to the impaired defense reactions of stressed trees [22]. Moreover, in field experiments with young Norway spruce, increased terpene induction in response to fungal inoculation strongly inhibited subsequent spruce bark beetle colonization [23].…”