Low disease incidence and cone bagging in Picea abies are associated with low genotypic diversity in Thekopsora areolata

Thekopsora areolata infects pistillate cones of Picea spp. with monokaryotic basidiospores in the spring. Receptive monokaryotic hyphae in the cones are fertilized by monokaryotic spermatia in the summer, and dikaryotic aecia are produced in cones in late summer. Infected cones produce no fertile seeds, meaning the disease causes large reductions in seed production. To understand the seasonal variation of T. areolata genotypic diversity, 548 aecia from 55 infected cones were sampled from multiple seed orchards in 2015, 2019 and 2020. Cone bagging experiments were performed during two seasons to investigate the sexual reproduction of T. areolata. In addition to the published simple-sequence repeat (SSR) markers, we developed 10 new polymorphic SSR markers to improve the resolution of population genetic analysis. Aecia were genotyped with 18 SSR markers in total. In 2015, when disease incidence was high in the seed orchards, the T. areolata populations had high genotypic diversity (H = 4.69). In 2019 and 2020, when disease incidence was low, the T. areolata populations had lower genotypic diversity (H = 3.88 and 3.85) and several cones were dominated by a single multilocus genotype. The genotypic diversity of T. areolata in a recently established seed orchard was exceptionally low (H = 2.01). Seven bagged cones that were infected produced either aecial primordia or aecia with lower diversity than exposed cones. The results indicate that cross-fertilization is important for sexual reproduction and aecia formation of T. areolata, and genotypic diversity of T. areolata increased with higher disease prevalence