Host genotype, resistance status and seasonal temperature shape the Norway spruce phyllosphere mycobiome.

The influence of the phyllosphere microbiome on plant health and disease development is well established. However, the roles of host resistance phenotype, seasonal variations, and accumulated temperature in shaping the phyllosphere mycobiome of Norway spruce (Picea abies) remain poorly understood. In this study, we investigated (i) the phyllosphere mycobiome assembly of a subset of Heterobasidion partially resistant and susceptible Norway spruce genotypes, (ii) how fungal community composition in these genotypes changes across seasons and with temperature, and (iii) the extent to which the phyllosphere mycobiome shared taxa with the seed-associated fungal communities of the same genotypes. Needle samples were collected from Heterobasidion partially resistant and susceptible Norway spruce seedlings growing in an open field in May, July and September 2022. Fungal communities were characterized using internal transcribed spacer (ITS) amplicon sequencing. Alpha diversity metrics (richness, diversity and evenness) of the phyllosphere fungal community were significantly different between susceptible and partially resistant Norway spruce genotypes. Although the distributions showed substantial overlap between resistance classes, all three alpha-diversity metrics differed significantly between the groups (p < 0.05). Community composition was associated with both host resistance phenotype and temperature-related variables, with average and accumulated temperature explaining a greater proportion of variation than host resistance phenotype. Less than 1% of phyllosphere fungal taxa overlapped with the seed-associated fungal communities. Network analyses further revealed seasonal differences in fungal association patterns between partially resistant and susceptible seedlings.