Rust Fungus Research Articles

Phylogenetics of the rust fungi (Pucciniales) of South Africa, with notes on their life histories and possible origins

ABSTRACT South Africa has an indigenous rust (Pucciniales) funga of approximately 460 species. This funga was sampled with species from as many genera as possible. The nuclear ribosomal large subunit (28S) region was amplified from samples representing 110 indigenous species, as well as the small subunit (18S) region and the cytochrome c oxidase subunit 3 (CO3) in some cases, and these were used in phylogenetic analyses. One new species is described, 12 new combinations made, six names reinstated, and two life history connections made. The life histories of this funga were summarized; it is dominated by species with contracted life histories. The majority of species are autoecious, with a small proportion being heteroecious. Of the autoecious species, many will likely be homothallic with no spermagonia. A shortened life history with homothallism allows for a single basidiospore infection to initiate a local population buildup under the prevailing unpredictable climatic conditions. Suggestions are made as to the possible origin of this funga based on the development of the modern South African flora. It is postulated that the rusts of South Africa are of relatively recent origin, consisting of three groups. Firstly, there is an African tropical element with members of the Mikronegerineae (Hemileia), the Sphaerophragmiaceae (Puccorchidium, Sphaerophragmium), and certain Uredinineae (Stomatisora). Their immediate ancestors likely occurred in the tropical forests of Africa during the Paleogene. Secondly, there is a pantropical element including the Raveneliaceae (e.g., Diorchidium, Maravalia, Ravenelia sensu lato, Uropyxis). This likely diversified during the Neogene, when the mimosoids became the dominant trees of the developing savannas. Thirdly, the Pucciniaceae invaded Africa as this continent pushed northward closing the Tethys Sea. They diversified with the development of the savannas as these become the dominant habitat in most of Africa, and are by far the largest component of the South African rust funga.

The effect of diversity on disease reverses from dilution to amplification in a 22-year biodiversity × N × CO2 experiment

Plant disease often increases with N, decreases with CO2, and increases as biodiversity is lost (i.e., the dilution effect). Additionally, all these factors can indirectly alter disease by changing host biomass and hence density-dependent disease transmission. Yet over long periods of time as communities undergo compositional changes, these biomass-mediated pathways might fade, intensify, or even reverse in direction. Using a field experiment that has manipulated N, CO2, and species richness for over 20 years, we compared severity of a specialist rust fungus (Puccinia andropogonis) on its grass host (Andropogon gerardii) shortly after the experiment began (1999) and twenty years later (2019). Between these two sampling periods, two decades apart, we found that disease severity consistently increased with N and decreased with CO2. However, the relationship between diversity and disease reversed from a dilution effect in 1999 (more severe disease in monocultures) to an amplification effect in 2019 (more severe disease in mixtures). The best explanation for this reversal centered on host density (i.e., aboveground biomass), which was initially highest in monoculture, but became highest in mixtures two decades later. Thus, the diversity-disease pattern reversed, but disease consistently increased with host biomass. These results highlight the consistency of N and CO2 as drivers of plant disease in the Anthropocene and emphasize the critical role of host biomass—despite potentially variable effects of diversity—for relationships between biodiversity and disease.

Unveiling Genetic Variation in Garlic Genotypes in Response to Rust Disease Using RAPD Markers

Garlic (<em>Allium sativum</em>), cultivated worldwide for its medicinal and nutritional value, faces challenges due to diseases caused by various pathogens. In this study, eleven garlic genotypes from Iran and one from China were selected and sown under natural infection rendered by the rust fungus (<em>Puccinia alli</em>) over two consecutive years. Subsequently, disease distribution and severity, percentage of infection and susceptibility of different garlic genotypes to rust disease were investigated. The results showed that genotype Solan was the most susceptible, with disease severity of 30.81%. In comparison, genotypes Aliabad and Bahar were resistant against rust disease with the lowest infection percentages of 13% and 16.37%, respectively. Further, genetic diversity was assessed using random-amplified polymorphic DNA (RAPD) markers. Out of 10 primers used, 95 scorable bands were generated, of which 66 (69.48%) were found to be polymorphic. A dendrogram was constructed based on RAPD polymorphism using the UPGMA method, and the genotypes were separated into six distinct clusters based on Jaccard's coefficient of similarity. Additionally, it was observed that there is no genetic differentiation among the genotypes based on their geographical origin. This study highlights the significant diversity in resistance and susceptibility among garlic genotypes, which can be harnessed in plant breeding programs.

First report of Puccinia xanthii causing rust disease on Xanthium orientale in Korea.

Puccinia xanthii Schw. is a microcyclic rust fungus, first found on Xanthium strumarium Lour in North Carolina, the United States. This rust fungus is native to the continental United States, Hawaii, Mexico, and the West Indies (Arthur 1934). It has become notoriously invasive and is now distributed in the Europe (Bulgaria, France, Hungary, Italy, Romania, Spain, and the former Yugoslavia), India, Indonesia, Australia, and South Africa (Parmelee 1969; Alcorn 1976; Wahyuno 2012). In East Asia, the fungus has been reported in Japan (Hiratsuka et al. 1992) and China (Zhao et al. 2014) but not in Korea. It has been reported mainly on the invasive weeds Xanthium and Ambrosia species. In addition, it rarely occurs on sunflowers (Helianthus spp.) in Australia (Alcorn 1976), South Africa (Pretorius et al. 2000), and North America (Gulya and Charlet 2002). In Korea, rust disease symptoms caused by a Puccinia fungus were first found on X. orientale L. at the roadside of Okcheon-gun, Chungcheongbuk-do (36 27'95.428"N 127 66'26.378"E) in October 2021 and were repeatedly observed in the same site in 2022. The similar symptom was additionally found on X. orientale in Yesan-gun, Oct. 2022. The symptoms were brown spots on round chlorotic haloes on the adaxial leaf surface and dark brown pustules on the abaxial leaf surface. Telia were brown to dark brown, round, mostly grouped, 0.28-0.61 mm in diameter, and mainly formed on the abaxial leaf surface but sometimes on the adaxial leaf surface. Teliospores were two-celled, pedicellate, and measured 37.6-110 × 12.4-21.5 μm in size; the wall was yellowish or almost colorless, smooth, 1.2-2.6 μm thick at the sides, and up to 7.4 μm thick at the apex. The morphological characteristics of the teliospores were identical to those of P. xanthii described by Arthur (1934) and Parmelee (1969). Based on phylogenetic analyses (e-Xtra 2) of the internal transcribed spacer (ITS) and partial large subunit (LSU) rDNA extracted from the teliospores, they were identified as P. xanthii. BLAST analysis showed that the sequences had high homologies (over 99.82%) with the reference strains of P. xanthii (EF635903 and KX999896). The representative specimens were preserved at the Animal and Plant Quarantine Agency Herbarium (PQK211005 for Okcheon-gun isolate and PQK220913 for Yesan-gun) and the sequences were deposited in GenBank (OR958716 and OR958692). A pathogenicity test was performed by dropping a suspension of germinating teliospores and basidiospores onto the adaxial leaf surfaces of apparently healthy X. orientale plants in Oct. 2022, using the isolate PQK220913 (OR958692). The three inoculated plants were placed together with three controls treated with only distilled water, in the dark at saturated humidity for 24 hours in an isolated greenhouse. After two weeks, typical rust symptoms were observed in the three infected plants, whereas no symptoms appeared in the control plants (e-Xtra 1). The causal fungus was identified as P. xanthii based on host relationships, successful experimental inoculation, morphological characteristics, and sequence similarity of partial DNA fragments. To our knowledge, this is the first report of P. xanthii on X. orientale in Korea. P. xanthii was additionally confirmed on X. orientale in Gumi-si, Boeun-gun, Seongju-gun, Naju-si, and Gunsan-si in 2023, indicating its wide distribution in Korea. It is expected that P. xanthii could be a candidate as a biological agent for controlling the invasive weed, X. orientale.

First Discovery of Stereostratum corticioides Causing Rust on the Culm of the Bamboo Pseudosasa japonica

A fungus strain <i>Stereostratum corticioides</i> PKVL1, belonging to the family <i>Pucciniaceae</i> that causes rust in plants, was discovered on the sheath of the bamboo <i>Pseudosasa japonica</i> leading to the death of the infected bamboo in the following year. Microscopic observation of the yellow fungal mass revealed teliospores with an oval, one-septate (two-celled) structure. The average length and width of teliospores were 31.83±3.57 μm and 20.74±1.72 μm, respectively. The large-subunit ribosomal RNA gene was amplified using the LR0R and LR7 primers, showing that the strain PKVL1 had a similarity of 99.34% to previously reported <i>S. corticioides</i>. In particular, the two <i>Stereostratum</i> strains form a separate cluster among the fungi in the family <i>Pucciniaceae</i>. This is the first report in the Republic of Korea of fungal rust occurring on the culm of bamboo rather than on the leaves.

Austropuccinia licaniae, first congeneric with the myrtle rust pathogen A. psidii

ABSTRACT In 1895 and 2001, rust fungi affecting Licania trees (Chrysobalanchaceae) in Brazil were described as Uredo licaniae by Hennings in the state of Goiás and as Phakopsora tomentosae by Ferreira et al. in the state of Amazonas, respectively. Recently, a Licania rust fungus collected close to the Amazonian type location sharing symptoms with the former two species was subjected to morphological examinations and molecular phylogenetic analyses using 28S nuc rDNA (ITS2-28S) and cytochrome c oxidase subunit III (CO3) gene sequences. Since the original type specimen of Ph. tomentosae is considered lost, we carefully reviewed the type description and questioned the identity of the telium, which justified the description of the fungus as a Phakopsora species. Furthermore, the additional revision of the type material described by Hennings revealed that Ph. tomentosae is a synonym of U. licaniae. Based on the morphological examinations, disease symptoms, and shared hosts, we concluded that the newly collected material is conspecific with U. licaniae. However, the phylogenetic analyses rejected allocation in Phakopsora and instead assigned the Licania rust fungus in a sister relationship with Austropuccinia psidii (Sphaerophragmiaceae), the causal agent of the globally invasive myrtle rust pathogen. We therefore favored a recombination of U. licaniae (syn. Ph. tomentosae) into Austropuccinia and proposed the new name Austropuccina licaniae for the second species now identified for this genus. The fungus shares conspicuous symptoms with A. psidii, causing often severe infections of growing leaves and shoots that lead to leaf necrosis, leaf shedding, and eventually to the dieback of entire shoots. In view of the very similar symptoms of its aggressively invasive sister species, we briefly discuss the current state of knowledge about A. licaniae and the potential risks, and the opportunity of its identification.

First Report of the Rust Fungus Melampsora ferrinii on Salix babylonica in China and a New Spermogonial and Aecial Host, Corydalis bungeana.

The occurrence of rust fungi on Corydalis bungeana Turcz. and Salix babylonica L. were found in same area of Hebei Province, China from 2022 to 2023. The life cycle connection of these rust fungi was suspected because Peng et al. (2022) reported the life cycle of Melampsora ferrinii Toome & Aime by inoculations, producing spermogonia and aecia on Corydalis species, and uredinia on S. babylonica. The morphology of the uredinial and telial stages on S. babylonica collected in the field was identical with the description of M. ferrinii by Toome and Aime (2015), and its identity was confirmed by phylogenetic analyses using the method of Ji et al. (2020) (LSU-PP087777, ITS-PP091274; Similarity with M. ferrinii: LSU-100%, ITS-99.85%). To confirm the life cycle of this rust fungus, inoculations were conducted on C. bungeana with basidiospores obtained from the teliospores on fallen leaves of Salix babylonica. The fallen leaves producing basidiospores were cut into small pieces (ca. 5 mm2) and placed on healthy leaves of C. bungeana. The inoculated plants were kept in a moist plastic box in darkness at 15-20℃ for 2 days and then transferred to the floor near windows at about 15-20℃ for observations. Ten days after inoculations small yellow spots of spermogonia appeared on the upper surface of the leaves of C. bungeana. About 7 days later, pale yellow aecia with aeciospores were produced mainly on the under surface of the leaves and petioles. The morphology of rust fungus on C. bungeana collected from the fields and obtained by inoculations was identical with the description by Peng et al. (2022). Phylogenetic analyses also showed that a specimen on C. bungeana collected from the field (LSU-OR607838, ITS-OR612063) were included into the same clade of M. ferrinii (Similarity: LSU-100 %, ITS-99.85). Based on morphology, inoculations and DNA sequence analyses, the rust fungi on C. bungeana and S. babylonica are identified as different stages of life cycle of M. ferrinii. This rust fungus has been reported to produce spermogonia and aecia on C. acuminata Franch., C. edulis Maxim. and C. racemosa (Thunb.) Pers. in China (Peng et al. 2022), and uredinia and telia on S. babylonica in USA, Argentina and Iran (Toome and Aime 2015, Abbasi et al. 2024), and on Salix sp. in Chile (Zapata 2016). Therefore, C. bungeana is a new host for M. ferrinii, and its field occurrence on S. babylonica is reported for the first time in China although Peng et al. (2022) reported successful results in its inoculations to S. babylonica in China. This report contributes to the control of rust diseases caused by this species. Specimens used in this experiment were deposited in the Fungal Herbarium of the Jilin Agricultural University, Changchun, China (HMJAU) and sequences newly analyzed were deposited in GenBank.

Genome biology and evolution of mating-type loci in four cereal rust fungi.

Permanent heterozygous loci, such as sex- or mating-compatibility regions, often display suppression of recombination and signals of genomic degeneration. In Basidiomycota, two distinct loci confer mating compatibility. These loci encode homeodomain (HD) transcription factors and pheromone receptor (Pra)-ligand allele pairs. To date, an analysis of genome level mating-type (MAT) loci is lacking for obligate biotrophic basidiomycetes in the Pucciniales, an order containing serious agricultural plant pathogens. Here, we focus on four species of Puccinia that infect oat and wheat, including P. coronata f. sp. avenae, P. graminis f. sp. tritici, P. triticina and P. striiformis f. sp. tritici. MAT loci are located on two separate chromosomes supporting previous hypotheses of a tetrapolar mating compatibility system in the Pucciniales. The HD genes are multiallelic in all four species while the PR locus appears biallelic, except for P. graminis f. sp. tritici, which potentially has multiple alleles. HD loci are largely conserved in their macrosynteny, both within and between species, without strong signals of recombination suppression. Regions proximal to the PR locus, however, displayed signs of recombination suppression and genomic degeneration in the three species with a biallelic PR locus. Our observations support a link between recombination suppression, genomic degeneration, and allele diversity of MAT loci that is consistent with recent mathematical modelling and simulations. Finally, we confirm that MAT genes are expressed during the asexual infection cycle, and we propose that this may support regulating nuclear maintenance and pairing during infection and spore formation. Our study provides insights into the evolution of MAT loci of key pathogenic Puccinia species. Understanding mating compatibility can help predict possible combinations of nuclear pairs, generated by sexual reproduction or somatic recombination, and the potential evolution of new virulent isolates of these important plant pathogens.

Scaling use of the rust fungus Puccinia punctiformis for biological control of Canada thistle (Cirsium arvense (L.) Scop.): First report on a U.S. statewide effort

Canada thistle (Cirsium arvense (L.) Scop., CT) is one of the worst weeds threatening temperate regions of the world. A host-specific rust fungus, Puccinia punctiformis (F. Strauss) Rohl., is known to cause systemic disease of CT, ultimately killing individuals and reducing stand densities. In 2013, it was demonstrated that fall inoculation of rosettes with coarsely ground leaves bearing P. punctiformis telia can successfully initiate epiphytotics. In the same year, a cooperative project between the Colorado Department of Agriculture and United States Department of Agriculture was initiated, in which CT patches across the state of Colorado (USA) were inoculated and tracked over subsequent years for changes in stem density. Here, we report our findings from 8 years (2014–2021) of monitoring effort. At most sites (N = 87), CT stem densities declined, from a mean (±SE) of 87.9 (±6.5) stems to 44.7 (±4.2). These declines however were spatially-autocorrelated, and likely attributable to local growing conditions, as mean annual daily maximum temperature and standard deviation of elevation, as well as climatic conditions around the times of both treatment and monitoring, were found to be important predictors of CT decline. Further, we observed that the amount of inoculum deployed, timing since last release, and method in which it was spread locally at a site were also associated with the magnitude of CT stem decline. These results are indicative of the value of P. punctiformis as a CT biological control agent.

Comparative Analysis of the Avirulence Effectors Produced by the Fungal Stem Rust Pathogen of Wheat.

Crops are constantly exposed to pathogenic microbes. Rust fungi are examples of these harmful microorganisms, which have a major economic impact on wheat production. To protect themselves from pathogens like rust fungi, plants employ a multilayered immune system that includes immunoreceptors encoded by resistance genes. Significant efforts have led to the isolation of numerous resistance genes against rust fungi in cereals, especially in wheat. However, the evolution of virulence of rust fungi hinders the durability of resistance genes as a strategy for crop protection. Rust fungi, like other biotrophic pathogens, secrete an arsenal of effectors to facilitate infection, and these are the molecules that plant immunoreceptors target for pathogen recognition and mounting defense responses. When recognized, these effector proteins are referred to as avirulence (Avr) effectors. Despite the many predicted effectors in wheat rust fungi, only five Avr genes have been identified, all from wheat stem rust. Knowledge of the Avr genes and their variation in the fungal population will inform deployment of the most appropriate wheat disease-resistance genes for breeding and farming. The review provides an overview of methodologies as well as the validation techniques that have been used to characterize Avr effectors from wheat stem rust. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

First report of Gymnotelium blasdaleanum causing Saskatoon serviceberry rust in Canada.

Saskatoon serviceberry (Amelanchier alnifolia) is a tall shrub native to the western regions of North America. In British Columbia, serviceberries are a resource for Indigenous communities, used in cooking and for medicinal purposes (Turner et al. 1990). The Saskatoon serviceberry is popular for ornamental and food purposes, with increasing demand for commercial production in British Columbia. (FLNR, 2003). In June 2023 leaf and fruit samples displaying signs of a rust fungus were collected from three ornamental Saskatoon serviceberry shrubs on the University of British Columbia's campus in Vancouver, BC (49°15'35.0"N 123°15'05.6"W). Morphological characteristics were taken from six leaves and berries. Infected leaf tissue was misshapen due to the development of aecia on the abaxial side of the leaves. Approximately 80% of the berries were enveloped in aecia, while infected leaf tissue constituted only around 5% across all three shrubs. Aecia were aecidioid (aecidium type) 0.25 to 0.35mm in diameter, hypophylous and, fructicolous. Peridial cells had a rhomboid shape measuring 21-29 × 18-23 μm (average dimension 25 × 20 μm, n = 30). Aeciospores were globoid with yellowish walls, verrucose and the dimensions ranged from 16-24 × 12-15 μm, (average dimensions 19 ×15 μm, n = 30) (Fig. x). These morphological features fit well with description of Gymnotelium blasdaleanum (Dietel & Holw.) Arthur (= Gymnosporangium libocedri (Henn.) F. Kern) (Arthur 1934). Judging from U.S. National Fungal Databases (https://fungi.ars.usda.gov/) G. blasdaleanum has never been reported in Canada, thus this discovery represents the first report of G. blasdaleanum in the country, specifically in the province of British Columbia. G. blasdaleanum is a hetero-demicyclic rust fungus with a broad aecial host range which includes several Rosaceous genera such as Amelanchier, Crataegus, Cydonia, Malus, Pyrus, and Sorbus (Farr & Rossman 2019). The telial host range is limited to California incense cedar (Calocedrus decurrens) and was not observed on nearby trees, some located within one kilometer of the serviceberries. To confirm morphological identification, DNA was extracted from infected leaves and fruits from all three plants using a modified protocol (Russell et al., 2010). The D1/D2 region was amplified using primers LR6 and 2Rust1R (Beenken, et al., 2012). A BLASTn search of all three sequences revealed 99.52 to 99.62% identity to available sequence in GenBank from Gymnotelium blasdaleanum (AF522168). Newly generated sequences were submitted to the GenBank under accession numbers OR567878, OR568568, and OR654105. Specimens from three Saskatoon serviceberry plants were provided to the UBC herbarium (Beaty Biodiversity Museum) and deposited in their fungarium (F35820). Infected fruits undergo deformation and premature dropping, posing a significant threat to ornamental and fruit production varieties of serviceberries (EPPO, 2006). The spread and establishment of the G. blasdaleanum is highly dependent on the presence of C. decurrens because of its limited range. However, California incense cedar is planted outside its natural range in suitable environments for G. blasdaleanum to thrive such as southern BC allowing the rust pathogen to spread to Canada. To prevent potential spread into other agricultural industries, particularly the 256-acre pear cultivation, continued planting of C. decurrens in British Columbia should be closely monitored. (BC MFLNRO, 2003).

Genome-Enabled Analysis of Population Dynamics and Virulence-Associated Loci in the Oat Crown Rust Fungus Puccinia coronata f. sp. avenae.

Puccinia coronata f. sp. avenae (Pca) is an important fungal pathogen causing crown rust that impacts oat production worldwide. Genetic resistance for crop protection against Pca is often overcome by the rapid virulence evolution of the pathogen. This study investigated the factors shaping adaptive evolution of Pca using pathogen populations from distinct geographic regions within the United States and South Africa. Phenotypic and genome-wide sequencing data of these diverse Pca collections, including 217 isolates, uncovered phylogenetic relationships and established distinct genetic composition between populations from northern and southern regions from the United States and South Africa. The population dynamics of Pca involve a bidirectional movement of inoculum between northern and southern regions of the United States and contributions from clonality and sexuality. The population from South Africa is solely clonal. A genome-wide association study (GWAS) employing a haplotype-resolved Pca reference genome was used to define 11 virulence-associated loci corresponding to 25 oat differential lines. These regions were screened to determine candidate Avr effector genes. Overall, the GWAS results allowed us to identify the underlying genetic factors controlling pathogen recognition in an oat differential set used in the United States to assign pathogen races (pathotypes). Key GWAS findings support complex genetic interactions in several oat lines, suggesting allelism among resistance genes or redundancy of genes included in the differential set, multiple resistance genes recognizing genetically linked Avr effector genes, or potentially epistatic relationships. A careful evaluation of the composition of the oat differential set accompanied by the development or implementation of molecular markers is recommended. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Rust HUBB: DNA barcode-based identification of Pucciniales

Rust fungi (Pucciniales, Basidiomycota) are a species-rich (ca. 8000 species), globally distributed order of obligate plant pathogens. Rust species are host-specific, and as a group they cause disease on many of our most economically and/or ecologically significant plants. As such, the ability to accurately and rapidly identify these fungi is of particular interest to mycologists, botanists, agricultural scientists, farmers, quarantine officials, and associated stakeholders. However, the complexities of the rust life cycle, which may include production of up to five different spore types and alternation between two unrelated host species, have made standard identifications, especially of less-documented spore states or alternate hosts, extremely difficult. The Arthur Fungarium (PUR) at Purdue University is home to one of the most comprehensive collections of rust fungi in the world. Using material vouchered in PUR supplemented with fresh collections we generated DNA barcodes of the 28S ribosomal repeat from > 3700 rust fungal specimens. Barcoded material spans 120 genera and > 1100 species, most represented by several replicate sequences. Barcodes and associated metadata are hosted in a publicly accessible, BLAST searchable database called Rust HUBB (Herbarium-based Universal Barcode Blast) and will be continuously updated.

Understanding the influence of applying plant extracts and microorganism culture filtrates against barley leaf rust disease

Rust fungi are devastating plant pathogens, and several Puccinia species have exerting substantial financial impacts on global barley cultivation. Fungicides are used on a large scale as an effective method for combating phytopathogenic fungi. The negative environmental impacts of fungicides are steadily escalating on a daily basis. Consequently, researchers are currently exploring alternative approaches to mitigate the use of fungicides, such as the utilization of plant extracts. This method has proven effective due to its incorporation of natural antifungal substances. Among the nine natural elicitors that were tested, the application of plant extracts on barley seedlings resulted in an increase in the incubation and latent periods of Puccinia hordei. These periods are integral components of partial and induced resistance, effectively mitigating the incidence of barley leaf rust disease by over 70% on mature plants. Similarly, the biochemical analyses demonstrated a notable augmentation in all the tested treatments’ overall phenolics and oxidative enzyme activities (peroxidase and polyphenol oxidase). Random amplified polymorphic DNA (SCoT) test serves as a viable approach for assessing the impact of plant extracts and microorganisms on barley plants. The results obtained from this study indicate that the detection of DNA polymorphism through SCoT analysis holds a significant potential powerful tool to evaluate genetic changes compared with untreated plants although some of them tested displayed high similarities at the morphological reaction.

First record of the rust fungus Uromyces gageae on Yellow Star-of-Bethlehem (Gagea lutea) in the Netherlands.

Yellow Star-of-Bethlehem (Gagea lutea) is a rare and threatened bulbous plant in the Netherlands, with its largest stronghold in the northern province of Drenthe. In 2022, numerous plants within a population of G. lutea were found to be infected by a rust fungus, which was identified as Uromyces gageae based on morphological characteristics. Further examination of collected teliospores revealed differences from Uromyces acutatus, a closely related rust species known to infect Ornithogalum and Gagea species. Rust symptoms on G. lutea plants were observed within the same population in April 2023, suggesting that teliospores surviving winter conditions serve as a viable source for recurrent infection. DNA of U. gageae and U. acutatus extracted from teliospores was used to obtain partial ribosomal DNA (rDNA) gene fragments by PCR. Amplicon sequencing revealed nucleotide variation between both rust species and verified the identity of the rust fungus on G. lutea as U. gageae. This confirmation substantiates the first documentation of U. gageae in the Netherlands. This study raises new avenues for research on the distribution and host range of U. gageae, as well as additional studies on the population dynamics of this potentially rare, wild plant-rust interaction.

Relationship between fungal bioaerosols and biotic stress on crops: a case study on wheat rust fungi

Relationship between fungal bioaerosols and biotic stress on crops: a case study on wheat rust fungi

In vitro and in planta investigation of succinate dehydrogenase inhibitor resistance conferred by target-site amino acid substitutions in Puccinia horiana, chrysanthemum white rust.

Resistance to succinate dehydrogenase inhibitor (SDHI) fungicides has been reported in some rust fungi within Pucciniales. However, measuring the resistance factors conferred by a specific substitution at the target site is difficult for most species because of the difficulty in performing in vitro experiments and the complexity of the binuclear state in these obligate parasites. We focused on Puccinia horiana because it easily forms homozygous basidiospores that are sensitive to SDHIs during in vitro germination, whereas the uredospores of other rust fungi are less sensitive. We identified two substitutions, SdhC-I88F and SdhD-C125Y, that drive SDHI resistance in Pu. horiana. Using basidiospore germination inhibition tests, we measured the resistance factors for six SDHI fungicides in Pu. horiana isolates harboring SdhC-I88F substitutions, wherein orthologous substitutions were most frequently observed in SDHI-resistant Pucciniales, such as soybean rust (Phakopsora pachyrhizi). The resistance factors were high for penthiopyrad and benzovindiflupyr (>150), moderate for oxycarboxin and inpyrfluxam (10-30), and low for mepronil and fluxapyroxad (3-10). The most potent SDHI against SdhC-I88F-harboring isolates was inpyrfluxam, with a half-maximal effective concentration (EC50 ) of 0.0082 mg L-1 owing to its high intrinsic activity. SdhD-C125Y played a minor, but significant role in increasing the resistance factors (one- to tenfold increases), depending on the individual SDHIs. This study is the first to use basidiospore germination inhibitory tests to quantify the resistance factors for SDHI-resistant Pucciniales. Owing to its homozygous binucleate nature and the high availability of basidiospores, Pu. horiana is useful for investigating SDHI resistance in Pucciniales. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Characterization of a Foxtail Mosaic Virus Vector for Rust Fungus Avirulence Gene Expression in Wheat

The wheat fungal pathogens Puccinia graminis f. sp. tritici ( Pgt), P. striiformis f. sp. tritici, and P. triticina, causing stem, stripe, and leaf rust, respectively, pose a threat to global wheat production. Genetic resistance in the form of resistance ( R) genes provides the best protection, but rust fungal populations change frequently by mutating avirulence (AVR) effectors matching specific R genes, thereby defeating resistance. Hence, characterization of AVR effectors is needed to understand the evolution of pathogen populations, provide insights for extending the effectiveness of R genes, and yield tools for the identification and isolation of R genes. Functional characterization of Avr genes in rust fungi is challenging in these biotrophic pathogens that lack a reliable and efficient transformation system. Studies indicate that the recently engineered foxtail mosaic virus (FoMV) shows promise as an expression system in cereals. In this study, we utilized two confirmed AVR effectors from Pgt, AVRSr35 and AVRSr50, to assess the applicability of FoMV for investigating rust fungus Avr genes. We showed that vector FoMV PV101 carrying PgtAvrSr35 induced a hypersensitive response (HR) in wheat having the corresponding Sr35 resistance gene. However, when carrying PgtAvrSr50, no HR or even mild viral symptoms were seen in a wheat line having Sr50, as this particular wheat line was not susceptible to FoMV. Several wheat cultivars did not support FoMV replication. The results here show that FoMV PV101 is effective as an expression vector for studying rust fungi AVR effectors, but its applicability relies on the susceptibility of wheat cultivars to FoMV. [Formula: see text] Copyright © 2024 His Majesty the King in Right of Canada, as represented by the Minister of Agriculture and Agri-Food Canada. This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Aecial and Telial Host Specificity of Puccinia coronata var. coronata, a Eurasian Crown Rust Fungus of Two Highly Invasive Wetland Species in North America.

The Eurasian crown rust fungus Puccinia coronata var. coronata (Pcc) was recently reported in North America and is widespread across the Midwest and Northeast United States. Pcc is a close relative of major pathogens of oats, barley, and turfgrasses. It infects two highly invasive wetland plants, glossy buckthorn (Frangula alnus) and reed canarygrass (Phalaris arundinacea), and could be useful as an augmentative biological control agent. We conducted large greenhouse trials to assess the host specificity of Pcc and determine any threat to cultivated cereals, turfgrasses, or native North American species. A total of 1,830 accessions of cereal crop species and 783 accessions of 110 other gramineous species were evaluated. Young plants were first inoculated with a composite uredinial inoculum derived from aecia. Accessions showing sporulation were further tested with pure urediniospore isolates. Sixteen potential aecial hosts in the families Rhamnaceae and Elaeagnaceae were tested for susceptibility through inoculation with germinating teliospores. Thirteen grass species within five genera in the tribe Poeae (Apera, Calamagrostis, Lamarckia, Phalaris, and Puccinellia) and four species in Rhamnaceae (Frangula alnus, F. californica, F. caroliniana, and Rhamnus lanceolata) were found to be susceptible to Pcc, with some species native to North America. All assessed crop species and turfgrasses were resistant. Limited sporulation, however, was observed on some resistant species within Poeae and four other tribes: Brachypodieae, Bromeae, Meliceae, and Triticeae. Among these species are oats, barley, and Brachypodium distachyon, suggesting the possible use of Pcc in studies of nonhost resistance.

Rust resistance of wheat and triticale samples in different plant growing stages

   Aim. To study resistance to leaf, yellow and stem rusts of winter and spring wheat and winter triticale samples from the collection of the N. I. Vavilov All‐Russian Institute of Plant Genetic Resources (VIR) and breeding centres of southern Russia in different phases of ontogenesis and to identify new sources with group resistance.   Material and Methods. The resistance of 158 wheat and triticale samples from the VIR collection and southern breeding centres to populations of leaf, stem and yellow rusts in different plant growth stages was analysed. In the juvenile phase the samples studied were inoculated with populations of rust fungi in climate chambers of the Federal Research Centre of Biological Plant Protection and the type of plant reaction to lesions was noted. In field conditions the samples were evaluated according to the CIMMYT scale on an artificial infectious background.   Results. In the adult plant stage under field conditions 79 wheat and triticale samples with group resistance to leaf, yellow and stem rusts were identified, and in the juvenile stage 26 samples. 22 samples showed resistance to three rusts in both phases of ontogenesis. Of these, 5 samples belong to the VIR collection, 3 winter triticale varieties (Dozor, Knyaz, Svat) and the Kavalerka wheat variety selection of the P.P. Lukyanenko National Grain Centre, 9 lines from the Donskoy Agrarian Research Centre and the Federal Rostov Agrarian Research Centre, and 4 winter triticale varieties (Ataman Platov, Zimogor, Kapral, Legion) of selections of the Federal Rostov Agrarian Research Centre.   Conclusion. The samples of wheat and winter triticale with group resistance which have been identified are promising for use in breeding for immunity to rust diseases.