First Report of Sydowia polyspora Causing Current Season Needle Necrosis on Abies grandis in Sweden

Current season needle necrosis (CSNN) is a conifer foliar disease and a particular problem for Christmas tree plantations, as it causes necrosis and premature foliage shedding, resulting in significant economic and aesthetic devaluation. In September 2022, CSNN symptoms were detected on 10-year-old Abies grandis trees at a plantation located in Southern Sweden. Field surveys were employed to assess the extent of damage, and the causal agent of damage was identified via isolation and tissue culturing, microscopy, molecular diagnostics and pathogenicity testing. DNA sequencing and phylogenetic analysis of obtained cultures identified Sydowia polyspora as the potential causal agent of the observed CSNN symptoms. The pathogenicity of S. polyspora was confirmed through seedling inoculation and successful re-isolation from induced necroses. Field surveys indicated a high incidence (98%) of CSNN, with severity levels typically affecting up to 50% of the crown. To our knowledge, this is the first documented occurrence of CSNN on A. grandis in Sweden. Extreme weather conditions, that is, drought and high precipitation, may exacerbate CSNN development, increasing tree susceptibility to S. polyspora infections. These findings underscore the increasing threat of weak or latent pathogens like S. polyspora to trees under changing climatic conditions, emphasising the need for proactive management strategies, including shielding and resistance breeding.