Differences in population structure and zygosity between heteroecious and autoecious forms of Cronartium pini suggest selfing in the autoecious form

Cronartium pini causes Scots pine blister rust. This rust fungus has two different forms without differentiation in morphology and internal transcribed spacer: the heteroecious form has a macrocyclic life cycle and infects pine and an alternate host; the autoecious form only infects pine. Epidemics caused by these two forms impose severe risk on the pine forest in Sweden, therefore knowledge of their distribution and diversity is needed for strategic disease management. We designed microsatellite markers with improved resolution based on the C. pini genome, developed a multiplex amplification system, and analyzed the C. pini population diversity and structure in Sweden using 396 isolates. The heteroecious and autoecious populations showed clear differences in diversity, linkage disequilibrium, and structure. The heteroecious isolates had unique multilocus genotypes. Autoecious isolates shared the same genotypes more frequently, especially three autoecious multilocus genotypes that were commonly found over a in northern Sweden. The genetic distances among autoecious isolates are closer than those among the heteroecious isolates. The results confirmed that heteroecious C. pini populations were sexual and autoecious C. pini populations were clonal. We further discussed the hypothesis that autoecious C. pini originated from self-fertilization, and frequent self-fertilization and infrequent mutation generate homozygous but diverse genotypes

The inheritance of resistance to Scots pine blister rust in Pinus sylvestris

Scots pine blister rust is a rust fungal pathogen that has become more noticeable in recent years and has increased in recorded incidence in Northern Scandinavia. This has prompted an initiation of resistance breeding to the fungus in the Northern Swedish pine breeding program. To accomplish this, it is necessary to evaluate the breeding potential for increased resistance and putative impact on other breeding objective traits that may have genetic correlations to resistance. To assess the potential of the trait to be included in the breeding program we examined half sibling families in four trials of which two had high incidence of Scots pine blister rust in a range of 25.4-42.1 %. We assessed vitality and height in one year and rust lesion frequency at two later time points. We found that rust resistance had a narrow sense heritability of 0.36-0.41, while vitality reached 0.17 and height 0.25. We found a high genetic correlation between sites in rust resistance and no genetic correlation between rust resistance and either vitality or height. This means that breeding for increased resistance in Scots pine can be done effectively without risking a negative impact on established breeding objective traits and that resistance is stable across environments

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

African fan palm (Borassus aethiopum) and oil palm (Elaeis guineensis) are alternate hosts of coconut lethal yellowing phytoplasma in Mozambique

In this study, potential alternate hosts of the phytoplasma causing coconut lethal yellowing disease (CLYD) in Mozambique were investigated based on 16S rRNA and secA genes. The results reveal that the naturalized palm species, Elaeis guineensis and Borassus aethiopum are alternate hosts of CLYD phytoplasma in Mozambique. Based on the iPhyClassifier online software, the phytoplasma detected in B. aethiopum belongs to the 16Sr group XXII-A, which include ‘Candidatus Phytoplasma palmicola’ and ‘Candidatus Phytoplasma cocosnigeriae’. This is the first report associating ‘Candidatus Phytoplasma palmicola’ with wild naturalized palm species in the world. Key words: Alternate hosts, Borassus aethiopum, „Candidatus Phytoplasma palmicola‟, Elaeis guineensis, Mozambique, palm lethal phytoplasma phylogeny

DMI fungicide resistance in Zymoseptoria tritici is unlinked to geographical origin and genetic background: a case study in Europe

BACKGROUNDThe hemibiotrophic fungus Zymoseptoria tritici causing Septoria tritici blotch (STB), is a devastating foliar pathogen of wheat worldwide. A common group of fungicides used to control STB are the demethylation inhibitors (DMIs). DMI fungicides restrict fungal growth by inhibiting the sterol 14-alpha-demethylase, a protein encoded by CYP51 gene and essential for maintaining fungal cell permeability. However, the adaptation of Z. tritici populations in response to intensive and prolonged DMI usage has resulted in a gradual shift towards reduced sensitivity to this group of fungicides. In this study, 311 isolates were collected pre-treatment from nine wheat-growing regions in Europe in 2019. These isolates were analysed by high-throughput amplicon-based sequencing of nine housekeeping genes and the CYP51 gene.RESULTSAnalyses based on housekeeping genes and the CYP51 gene revealed a lack of population structure in Z. tritici samples irrespective of geographical origin. Minimum spanning network (MSN) analysis showed clustering of multilocus genotypes (MLGs) based on CYP51 haplotypes, indicating an effect of selection due to DMI fungicide use. The majority of the haplotypes identified in this study have been reported previously. The diversity and frequencies of mutations varied across regions.CONCLUSIONUsing a high-throughput amplicon-sequencing approach, we found several mutations in the CYP51 gene combined in different haplotypes that are likely to cause fungicide resistance. These mutations occurred irrespective of genetic background or geographical origin. Overall, these results contribute to the development of effective and sustainable risk monitoring for DMI fungicide resistance. (c) 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry

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