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

Temporal and spatial dispersal of Thekopsora areolata basidiospores, aeciospores, and urediniospores

Cherry spruce rust causes huge yield losses in Norway spruce seed production in Fennoscandia. The causal agent, Thekopsora areolata, has three types of spores that disperse during spring: basidiospores are produced on basidia that grow out from teliospores in overwintered bird cherry leaf litter to infect new pistillate spruce cones, aeciospores are released from old diseased spruce cones to infect bird cherry leaves, and urediniospores are produced from new bird cherry leaves for reinfection. No study has examined the dispersal of T. areolata spores, including the basidiospores that cause primary infection in spruce cones. In this study, teliospores of T. areolata were germinated in the laboratory and the morphology of basidiospores was described. T. areolata spores were sampled in Ultuna, Sweden and Joutsa, Finland with 21 spore traps at each site. Peaks in aeciospores were observed from 11 to 25 May and from 2 to 8 June at the Finnish site, and from 4 to 18 May at the Swedish site. Urediniospores were first observed 2-3 weeks after the peaks in aeciospores and they were mainly distributed within 10 m from the bird cherry trees. Peaks of 1-2 weeks in basidiospore detection coincided with multiple rain events. The basidiospore peak overlapped with the spruce pollen peak in Finland but not in Sweden. The quantities of basidiospores from different spore traps within 100 m from the spore source had no gradient. Information on spatial and temporal spore release is important for making decisions on disease management strategies

Genetic evidence for sexual reproduction and multiple infections of Norway spruce cones by the rust fungus Thekopsora areolata

Rust fungi are obligate parasites, of plants, with complex and in many cases poorly known life cycles which may include host alteration and up to five spore types with haploid, diploid, and dikaryotic nuclear stages. This study supports that Thekopasora areolata , the causal agent of cherry‐spruce rust in Norway spruce, is a macrocyclic heteroecious fungus with all five spore stages which uses two host plants Prunus padus and Picea abies to complete its life cycle. High genotypic diversity without population structure was found, which suggests predominantly sexual reproduction, random mating and a high gene flow within and between the populations in Fennoscandia. There was no evidence for an autoecious life cycle resulting from aeciospore infection of pistillate cones that would explain the previously reported rust epidemics without the alternate host. However, within cones and scales identical multilocus genotypes were repeatedly sampled which can be explained by vegetative growth of the fertilized mycelia or repeated mating of mycelium by spermatia of the same genotype. The high genotypic diversity within cones and haplotype inference show that each pistillate cone is infected by several basidiospores. This study provides genetic evidence for high gene flow, sexual reproduction, and multiple infections of Norway spruce cone by the rust fungus T. areolata which expands the general understanding of the biology of rust fungi.202

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

Relationship and genetic structure among autoecious and heteroecious populations of Cronartium pini in northern Fennoscandia

Epidemics of Scots pine blister rust, caused by Cronartium pini, have become an increasing problem in northern Finland and Sweden. The biology of the rust fungus is complex, with two different life cycle forms that cannot be morphologically distinguished, and it is unclear to what extent the two forms contribute to the epidemics. Genetic structure of fourteen populations of C. pini were investigated in Fennoscandia. Distinction between the two life cycle forms, a heteroecious and an autoecious one, was made by determining zygosity using microsatellite markers, and AFLP markers were developed to analyse population genetic relationships. The results showed that the two life cycle forms are clearly differentiated and occur in separate populations. Within the life cycle forms, geographic differentiation was evident, probably due to restricted gene flow as well as connection with different alternating hosts. The host-alternating form dominated in the epidemic regions in northern Fennoscandia. Implications for silvicultural practices are discussed

Alternate host screening of Thekopsora areolata in Scandinavia: a new record on Prunus grayana

The cherry spruce rust caused by Thekopsora areolata (Fr.) Magnus results in significant losses in spruce seed production in the forest industry. The pathogen is present in Asia and Europe but absent from North America where it has been considered as a potential threat and listed as a quarantine organism by the United States Department of Agriculture. A comprehensive list and in-depth information regarding the alternate hosts of this pathogen are important for conducting epidemiological studies and for optimal disease control. Prunus padus L. is the main alternate host reported for T. areolata. In this study, we investigated the susceptibility of domestic and exotic Prunus spp. and other potential alternate host-plant species native to Scandinavia to T. areolata infection through a field survey and aeciospore inoculation experiments in the greenhouse and laboratory. No new susceptible species were found. In Sweden, a new record of Prunus grayana Maxim. with low susceptibility to T. areolata was found. In addition, we updated the list of currently confirmed alternate hosts of T. areolata according to field observations and inoculation results. Prunus padus and Prunus serotina Ehrh., as well as their hybrids and subspecies of Prunus padus, are highly susceptible, while Prunus depressa Pursh, Prunus grayana, Prunus spinosa L., and Prunus tenella Batsch are considered slightly susceptible

Genetic evidence for sexual reproduction and multiple infections of Norway spruce cones by the rust fungus Thekopsora areolata

Rust fungi are obligate parasites, of plants, with complex and in many cases poorly known life cycles which may include host alteration and up to five spore types with haploid, diploid, and dikaryotic nuclear stages. This study supports thatThekopasora areolata, the causal agent of cherry-spruce rust in Norway spruce, is a macrocyclic heteroecious fungus with all five spore stages which uses two host plantsPrunus padusandPicea abiesto complete its life cycle. High genotypic diversity without population structure was found, which suggests predominantly sexual reproduction, random mating and a high gene flow within and between the populations in Fennoscandia. There was no evidence for an autoecious life cycle resulting from aeciospore infection of pistillate cones that would explain the previously reported rust epidemics without the alternate host. However, within cones and scales identical multilocus genotypes were repeatedly sampled which can be explained by vegetative growth of the fertilized mycelia or repeated mating of mycelium by spermatia of the same genotype. The high genotypic diversity within cones and haplotype inference show that each pistillate cone is infected by several basidiospores. This study provides genetic evidence for high gene flow, sexual reproduction, and multiple infections of Norway spruce cone by the rust fungusT. areolatawhich expands the general understanding of the biology of rust fungi

Relationship and genetic structure among autoecious and heteroecious populations of Cronartium pini in northern Fennoscandia

Epidemics of Scots pine blister rust, caused by Cronartium pini, have become an increasing problem in northern Finland and Sweden. The biology of the rust fungus is complex, with two different life cycle forms that cannot be morphologically distinguished, and it is unclear to what extent the two forms contribute to the epidemics. Genetic structure of fourteen populations of C. pini were investigated in Fennoscandia. Distinction between the two life cycle forms, a heteroecious and an autoecious one, was made by determining zygosity using microsatellite markers, and AFLP markers were developed to analyse population genetic relationships. The results showed that the two life cycle forms are clearly differentiated and occur in separate populations. Within the life cycle forms, geographic differentiation was evident, probably due to restricted gene flow as well as connection with different alternating hosts. The host-alternating form dominated in the epidemic regions in northern Fennoscandia. Implications for silvicultural practices are discussed. (C) 2021 The Authors. Published by Elsevier Ltd

Alternate host screening of Thekopsora areolata in Scandinavia: a new record on Prunus grayana

The cherry spruce rust caused by Thekopsora areolata (Fr.) Magnus results in significant losses in spruce seed production in the forest industry. The pathogen is present in Asia and Europe but absent from North America where it has been considered as a potential threat and listed as a quarantine organism by the United States Department of Agriculture. A comprehensive list and in-depth information regarding the alternate hosts of this pathogen are important for conducting epidemiological studies and for optimal disease control. Prunus padus L. is the main alternate host reported for T. areolata. In this study, we investigated the susceptibility of domestic and exotic Prunus spp. and other potential alternate host-plant species native to Scandinavia to T. areolata infection through a field survey and aeciospore inoculation experiments in the greenhouse and laboratory. No new susceptible species were found. In Sweden, a new record of Prunus grayana Maxim. with low susceptibility to T. areolata was found. In addition, we updated the list of currently confirmed alternate hosts of T. areolata according to field observations and inoculation results. Prunus padus and Prunus serotina Ehrh., as well as their hybrids and subspecies of Prunus padus, are highly susceptible, while Prunus depressa Pursh, Prunus grayana, Prunus spinosa L., and Prunus tenella Batsch are considered slightly susceptible.202

Geographic distribution of Cronartium flaccidum and Peridermium pini in Sweden and Finland

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