Andrea Ganthaler (Department of Botany, University of Innsbruck , Sternwartestrasse 15, 6020 Innsbruck ,) Andrea Ganthaler Carlos Trujillo-Moya (Department of Forest Growth, Silviculture & Genetics, Austrian Research Centre for Forests BFW , Seckendorff-Gudent-Weg 8, 1131 Vienna ,) Carlos Trujillo-Moya Stefanie Burger (VetCore Facility for Research, University of Veterinary Medicine , Veterinärplatz 1, 1210 Vienna ,) Stefanie Burger Juraj Hlavaty (Unit of Morphology, Center for Pathobiology, University of Veterinary Medicine , Veterinärplatz 1, 1210 Vienna ,) Juraj Hlavaty Stefan Kummer (VetCore Facility for Research, University of Veterinary Medicine , Veterinärplatz 1, 1210 Vienna ,) Stefan Kummer Waltraud Tschulenk (Unit of Morphology, Center for Pathobiology, University of Veterinary Medicine , Veterinärplatz 1, 1210 Vienna ,) Waltraud Tschulenk Ingrid Walter (VetCore Facility for Research, University of Veterinary Medicine , Veterinärplatz 1, 1210 Vienna ,) Ingrid Walter Oxford University Press 2025 Infection of Norway spruce (Picea abies L.) by the rust Chrysomyxa rhododendri is a major problem in European subalpine forests, causing severe defoliation and reduced growth. However, as with most pathogens from high-elevation environments, little is known about the host–pathogen interaction, the associated plant cellular damage and responses, and their differential expression in susceptible and resistant host trees. Here we report on the development of the biotrophic pathogen in the host tissues, from infection by basidiospores to release of aeciospores, by analysing needles at different time points after infection by histology and transmission electron microscopy. Ultrastructural changes in the host cells, ranging from cell reorganization and degradation to the accumulation of secondary compounds, were localized and characterized in both susceptible and a resistant genotype. Chrysomyxa rhododendri formed a dense mycelium in the intercellular spaces of the needle mesophyll of susceptible trees, followed by the formation of subepidermal spermogonia and aecia. Symptomatic needle yellowing corresponded to the spatial expansion of the mycelium and was caused by degradation, but not collapse, of the mesophyll cells with chloroplasts. In needles of the enhanced resistant genotype, only few fungal hyphae appeared, but distinct modifications of the cell walls and an accumulation of electron dense material in the intercellular space appeared. In addition, large tannin droplets were observed around fungal structures, indicating an increased accumulation of polyphenols. The findings are consistent with observations on other heterocyclic rusts and with known physiological and molecular responses of infected trees, including a reduced photosynthetic activity, changes in the needle phenolic profile and a local hypersensitive response. Highly resistant trees may be able to limit fungal growth and associated damage by rapidly enhancing structural and chemical barriers in the needle mesophyll. PDFDocument eng 2026-03-10T09:47:29.827092Z 2025 6054777 b application/pdf http://creativecommons.org/licenses/by/4.0/