Seasonal variability in Norway spruce defense capacity to a bark beetle-associated phytopathogenic fungus: effects of weather, soil moisture and tree phenology

Drought weakens tree defenses, predisposing Norway spruce (Picea abies) to spruce bark beetle (Ips typographus) attack. The extreme 2018 summer drought in Sweden triggered an unprecedented bark beetle outbreak. Our objective was to quantify how weather, soil moisture, and tree provenance influence Norway spruce defense capacity to a necrotrophic beetle-associated pathogen. Trees at three sites in Sweden were inoculated with the phytopathogenic fungus Leptographium europhioides on four occasions during each of the 2019 and 2021 growing seasons. At each site, we inoculated spruce provenances of Swedish or East European origin, with early and late spring bud burst, respectively. Tree defense capacity, expressed as the extent of necrotic lesion formation following fungal inoculation, was used as a proxy for resistance to bark beetle attack. Spruce defense capacity (i.e. lesion size) differed with water availability (both precipitation and soil moisture conditions) but not with the timing of spring bud burst. There were within-season differences, with trees having less efficient defenses (producing larger lesions) in the early season (June). On intermediate soil moisture sites, lesions were larger in 2019 than in 2021. In both years, there was a significant negative correlation between lesion size and water availability in the autumn of the previous year. Spruce defense capacity varied with local environmental conditions but not with provenance phenology. Variations between study years reflected the sensitivity of spruce defenses to climatic variability and the partial recovery of tree resistance 3 years after the 2018 drought.