Martin Schebeck

Climate change poses severe pressures to European conifer forests. Using non‐native tree species, such as Douglas fir ( Pseudotsuga menziesii ), is one proposed strategy to circumvent adverse effects for forest management. However, novel forest health risks can impair the cultivation of non‐native trees. In 2022, we observed large Douglas fir trees (approximately 40–50 years old, diameter at breast height (dbh) 21–41 cm) that had recently died in spring or summer 2022 in three forest stands in Eastern Austria. Intensive resin flow, blue‐staining of the sapwood and the absence of bark‐ and wood‐boring insects indicated a fungal infection. Isolations from blue‐stained sapwood of the dead trees consistently yielded cultures of the opportunistic pathogen Diplodia sapinea . In a greenhouse wound inoculation experiment, seven D. sapinea isolates obtained from Douglas fir caused phloem necrosis, blue‐staining of sapwood and mortality and thus displayed pathogenicity towards seedlings of both Ps. menziesii and its common host, Scots pine ( Pinus sylvestris ). Diplodia sapinea produced significantly longer areas of blue‐stain as well as higher and faster mortality in Ps. menziesii compared to P. sylvestris . We conclude that D. sapinea substantially contributed to the death of seven of the 13 examined large Douglas fir trees. While this fungus has been described as a pathogen of young Douglas fir trees before, this is the first report that it can potentially kill large individuals of this conifer species under drought conditions. Thus, our results indicate that D. sapinea could represent a severe threat to the cultivation of Ps. menziesii in European forestry.

The North American oak lace bug (OLB), Corythucha arcuata , is an invasive species in Europe and a serious threat to oak‐dominated forests. Survival at low temperatures is one major factor determining the spread of invasive insects. Thus, we studied key traits, that is, cold‐tolerance strategy, supercooling points (SCP) and chilling‐related mortality, of overwintering adults to assess their potential to withstand harsh winters. Samples for SCP measurements were collected once a month from November 2020 until March 2021 at three different locations in Hungary; specimens for chilling experiments were sampled in November 2020, January 2021 and March 2021. SCPs of overwintering adults ranged from −29.68 to −7.49 °C, with only moderate variation among months; C. arcuata is a freeze‐avoidant species. Mortality rates of adults exposed to two sub‐zero temperatures above the SCP (−3 and −5 °C) for 1, 2 and 3 weeks ranged between 0% and 69.1%, suggesting that OLB has a moderate risk to die from chilling injuries. Exposure time and sampling date affected mortality, with lowest survival rates after 3 weeks, collected in March 2021.