Multivariate analysis of risk factors associated with Scots pine blister rust in Northern Sweden

Scots pine blister rust (SPBR), caused by the rust fungus Cronartium pini, has existed in Swedish pine forests for approximately 200 years. However, recent outbreaks have led to substantial damage in young Scots pine stands, particularly in northern Sweden. A better understanding of the current epidemics and the environmental factors influencing their occurrence is essential for sustainable forest management. In 2021 and 2022, field surveys were conducted in young forest stands across northern Sweden to assess SPBR incidence, alongside the collection of geographical, topographical, and vegetation data. Additionally, to investigate climate impact on SPBR, historical and projected climate data were obtained from public databases. The surveys revealed frequent presence and high incidence of SPBR in Norrbotten, with the highest observed disease incidence reaching 48.8 %. Using logistic regression with generalized linear mixed models and beta regression, we found that SPBR prevalence and higher incidence were significantly associated with higher elevation, moist soil conditions, and higher abundance of the telial host Melampyrum spp. Moreover, warmer temperatures combined with higher precipitation were also positively correlated with higher disease incidence. Based on future climate projections under various Representative Concentration Pathway (RCP) scenarios, we estimated the potential impact of climate change on SPBR dynamics. The results suggest that rising temperatures and precipitation levels may have a greater impact on SPBR risk in Norrbotten, the estimated absolute increase in disease incidence may reach 20 % by the end of the 21st century with RCP8.5. In other RCP scenarios, the climate impact on SPBR incidence increase is generally modest, typically below 5 %.