Seasonal variation in Norway spruce response to inoculation with bark beetle-associated bluestain fungi one year after a severe drought

In 2018, up to 4 million m3 Norway spruce was killed by the bark beetle Ips typographus in Sweden. The event unique for Sweden, terms of both affected volume and fact that it triggered severe drought stress, not ample availability relatively defenseless storm-felled trees. outbreak continued 2019 2020, each year with twice as many trees 2018. aim this study quantify seasonal variation potential lag-effects tree defense capacity after a stress. Inoculation beetle-associated bluestain fungus, repeated four times one-month-intervals between May August 2019, were carried out at three field sites provenances Swedish East European origin representing early late bud burst, respectively. All had experienced moderate stress site-specific correlated positively cumulative precipitation two months before inoculation. Sites two-month levels <100 mm larger necrotic lesions phloem following inoculation, an indication lower capacity. Lesion size did differ provenances, all able confine fungal infection successfully. There some differences lesion size, Skärsnäs Norberg having significantly June than May, site Lugnet large also May. Lesions generally smaller July June. cross-sectional area number traumatic resin ducts measured sapwood samples from one site, Lugnet, additional aspect defenses. produced August. This is line positive correlation duct area, indicating stronger inoculation spring induced response. more but provenances.