Spatial and temporal dynamics of seedborne, airborne, and foliar fungal communities in Swedish forest nurseries

Abstract

Fungal infections in forest nurseries can significantly affect the economic sustainability of tree seedling production. Increased knowledge of local fungal communities, especially prevalent pathogens, could improve preventative and targeted control strategies, ultimately reducing the usage of fungicides. This thesis aimed to acquire new knowledge and advance a general understanding of seedborne, airborne, and foliar fungal communities in Swedish forest nurseries. The studies were conducted in five forest nurseries. Fungal communities were assessed from Picea abies, Pinus sylvestris, and Larix sp. seeds differentiated between the surface and the tissue (paper I), non-symptomatic P. sylvestris needles (paper II), and deposited spores (paper IV) using high-throughput sequencing of the ITS2 rDNA region. Results showed that seedborne fungal communities were distinguished between the seed surface and the seed tissue, where fungi from the seed surface showed a strong host-affinity and regional dependence. This result suggests that the movement of seeds between different geographic regions, including frequently traded seeds, could be a potential source of the spread and introduction of fungal species. The foliar and airborne fungal communities comprised high species richness and showed clear temporal shifts over the seedling growing seasons. The experiments included microbial treatments to control fungal pathogens and stimulate seedling growth on P. sylvestris seedlings. However, the use of microbial treatments was not found to impact the seedling growth or the disease incidence, which was generally low, and had no impact on the foliar fungal communities. All the fungal communities showed a high prevalence of nursery fungal pathogens, e.g., Cladosporium sp., Botrytis cinerea, Phoma herbarum, and Sydowia polyspora. In this thesis, the disease incidence of Diplodia sapinea in P. sylvestris seedlings was confirmed (paper III). Additionally, low abundances of D. sapinea were detected from the seeds and the deposited airborne spores. The findings in this thesis may contribute to developing disease management strategies in forest nurseries owing to a better understanding of fungal communities, thereby highlighting the importance of managing fungal pathogens to maintain healthy tree seedling production.