Fertile Soils Increase Susceptibility of Pinus sylvestris to Cronartium pini

Abstract

Cronartium pini, Scots pine blister rust, is a rust fungus that has caused severe damage in both old and young Scots pine (P. sylvestris) forest stands in northern Fennoscandia. To understand the many drivers of these outbreaks we examined putative factors and their explanatory power of C. pini incidence based on data collected in Lapland, northern Finland. Diseased stands were selected from the 10th Finnish National Forest Inventory (NFI) and control plots were randomly selected from the 11th NFI. A damage assessment was conducted in 2014, 4 years later in separate 10–15 sample plots within each stand. Stand and tree characteristics, including incidence of C. pini, were recorded. Site type (fertility) and age of trees significantly explained the incidence of C. pini in sample plots. Incidence of C. pini infection increased significantly in fertile sites, while the risk was non-significant in more arid sites. Risk of infection by C. pini was significantly reduced when the proportion of deciduous trees increased. There was an increased infection risk (up to over 50%) proportional to how well the trees grew for the last 5 years if Melampyrum spp. were present. Our results support previous observations that fertile sites are more susceptible to infection by C. pini. For the first time, we also demonstrate that an increase in the number of deciduous trees reduces the risk of C. pini infections. These results offer tools to improve forest management practices in reducing the risk of damage caused by C. pini in northern pine forests.