Epidemiological and histological components of crown rust resistance in oat genotypes

Crown rust, caused by Puccinia coronata f. sp. avenae, can cause significant damage in all regions where oats (Avena sativa L.) are cultivated. The primary means of controlling crown rust has been through genetic resistance, although in most cases resistance has been quickly overcome by the pathogen. More durable partial or non-specific resistance may possess different mechanisms from those underlying genes with specific effects. We studied the epidemiological and histological components of crown rust resistance with potential use in plant protection. Among the components evaluated, pustule density showed the clearest effect on resistance, while the latent period was not an important component. Cell death associated with the accumulation of autofluorescent and phenolic compounds was common in the resistant genotypes, but temporally distinct for the genotypes studied. Genotype Pc68/5*Starter, which has race-specific resistance, showed rapid cell death that prevented the development of pathogen colonies. Conversely, with cultivar URS 21 and genotypes 04B7113-1 and 04B7119-2, cell death and associated accumulation of autofluorescent and phenolic compounds was delayed until pathogen colonies were already established. Pathogen colonies developed normally in susceptible plants genotypes, and had usually produced sporogenic tissue by 5 days after inoculation. The data suggest that the resistance mechanisms, especially hypersensitivity and phenolic compound production, active in resistant plants are similar but may be differently expressed over time. The temporal variation in the expression of hypersensitivity and phenolic compound production reflects the level of field resistance in these genotypes.     

Charakterisierung der Virulenz einer Braunrost-Feldpopulation (Puccinia triticina f. sp. tritici) und Nachweis effektiver Braun-rost-resistenz-gene in Weizen (Triticum aestivum)

Puccinia triticina f. sp. tritici, the causal agent of leaf rust causes yield losses in wheat up to 60%. In order to avoid such losses, leaf rust resistance (Lr-) genes have been incorporated into wheat cultivars. The Lr- genes confer mostly vertical resistance, i.e. they are race specific. Therefore, knowledge of still effective resistance genes is required for efficient breeding of resistant cultivars. To get information on these virulences, a leaf rust population was monitored in field experiments in 2010. For this purpose naturally infection at three different timepoints of wheat development was monitored on Thatcher near isogenic lines (NILs) carrying 37 and accessions carrying 6 additional Lr-genes. Thatcher-NILs carrying Lr2a, Lr9, Lr19, Lr22a, Lr23, Lr24, Lr35, Lr38 and Lr49 showed a significantly lower infection with Puccinia triticina than the susceptible cultivar Thatcher. Thatcher-NILs carrying Lr13, Lr16, Lr37 and Lr46 showed no significant differences in comparison to Thatcher. In order to get information on the effectiveness of resistance genes, P. triticina isolates were collected from the NILs analysed in field trials and a leaf segment test was conducted followed by microscopic analyses. In the field and in the leaf segment test Lr9, Lr19, Lr24 and Lr38 and to some extent Lr3a turned out to be the most effective genes. By microscopic analyses, the infection process as well defense reactions activated before macroscopic symptoms are visible were monitored. By counting haustorial mother cells, it could be demonstrated which Lr-genes provide resistance, which were overcame and whether P. triticina isolates exist already at a low frequency, which may overcome a certain Lr-gene in the future. Thus microscopy offers a timesaving and effective method to detect susceptible or resistant plants and the upcoming of virulent races prior to typical symptom expression.     

Virulence of Puccinia triticina in Turkey and leaf rust resistance in Turkish wheat cultivars

Leaf rust caused by Puccinia triticina is a common disease on wheat in the coastal regions of Turkey. Collections of P. triticina from infected wheat leaves were obtained from the main wheat production zones of Turkey in 2009 and 2010. A total of 104 single uredinial isolates were tested for virulence on 20 lines of Thatcher wheat that differ for single leaf rust resistance genes. Forty-four different virulence phenotypes were identified over both years. Four phenotypes were found in both years. Phenotype FHPTQ found in 2009, with virulence to genes Lr2c, Lr3, Lr16, Lr26, Lr3ka, Lr17a, Lr30, LrB, Lr10, Lr14a, Lr18, Lr3bg, and Lr14b, was the most common phenotype at 15.4 % of the total isolates. Forty-three winter and spring wheat cultivars from Turkey were tested as seedlings with 13 different P. triticina virulence phenotypes from Canada, the US and Turkey. The infection types on the cultivars were compared with infection types on the Thatcher near isogenic lines to postulate the presence of seedling leaf rust resistance genes in the cultivars. Resistance genes Lr1, Lr3a, Lr10, Lr14a, Lr17a, Lr20, Lr23, and Lr26 were postulated to be present in the Turkish wheat cultivars. DNA of the wheat cultivars was tested with PCR markers to determine the presence of the adult plant resistance genes Lr34 and Lr37. Marker data indicated the presence of Lr34 in 20 cultivars and Lr37 in three cultivars. Field plot evaluations of the wheat cultivars indicated that no single Lr gene conditioned highly effective leaf rust resistance. Resistant cultivars varied for combinations of seedling and adult plant resistance genes.     

Is virulence phenotype evolution driven exclusively by Lr gene deployment in French Puccinia triticina populations?

Puccinia triticina is a highly damaging wheat pathogen. The efficacy of leaf rust control by genetic resistance is mitigated by the adaptive capacity of the pathogen, expressed as changes in its virulence combinations (pathotypes). An extensive P. triticina population survey has been carried out in France over the last 30 years, describing the evolutionary dynamics of this pathogen in response to cultivar deployment. We analysed the data set for the 2006–2016 period to determine the relationship between the Lr genes in the cultivars and virulence in the pathotypes. Rust populations were dominated by a small number of pathotypes, with variations in most of the virulence frequencies related to the corresponding Lr gene frequencies in the cultivated landscape. Furthermore, the emergence and spread of a new virulence matched the introduction and use of the corresponding Lr gene (Lr28), confirming that the deployment of qualitative resistance genes is an essential driver of evolution in P. triticina populations. However, principal component analysis (PCA) revealed that certain pathotype–cultivar associations cannot be explained solely by the distribution of Lr genes in the landscape. This conclusion is supported by the predominance of a few pathotypes on some cultivars, with the persistence of several other compatible pathotypes at low frequencies. Specific interactions are not, therefore, sufficient to explain the distribution of virulence in rust populations. The hypothesis that quantitative interactions between P. triticina populations and bread wheat cultivars—based on differences in aggressiveness—is also a driver of changes in pathotype frequencies deserves further investigation.     

Evaluation of the susceptibility of modern,wild, ancestral,and mutational wheat lines to Septoria tritici blotch disease

Globally, bread wheat production is threatened by fungal diseases, including the devastating disease Septoria tritici blotch (STB). Given the global importance of STB, and the difficulty in identifying novel sources of resistance to this disease, we screened a variety of wheat genotypes, including wild, ancestral, and mutagenized lines, for their STB response. This delineated a panel of wild wheat relatives and Watkins collection lines with exceptional resistance to a range of Zymoseptoria tritici isolates, some of which are highly virulent on modern, elite wheat varieties. Additionally, we characterized the STB susceptibility of 500 lines of the wheat cultivar Cadenza TILLING population and developed backcross derivatives of two TILLING lines that show dominant partial resistance to STB. These backcross lines are partially resistant to multiple isolates of Z. tritici, and, with the wild and ancestral lines identified, provide a useful reservoir of STB-resistant germplasm for use in wheat breeding programmes.     

Diversity in Puccinia triticina detected on wheat from 2008 to 2010 and the impact of new races on South African wheat germplasm

Samples of wheat and triticale infected with leaf rust were collected from 2008 to 2010 in South Africa to identify Puccinia triticina races. Races were identified based on their virulence profile on standard differential lines. Eight races were identified from 362 isolates. The dominant races were 3SA133 (syn. PDRS) in 2008 (78 %) and 2009 (34 %), and 3SA145 (47 %) in 2010. Race 3SA145 (CCPS) identified in 2009 was a new race in South Africa with virulence for the adult plant resistance gene Lr37. Another new race, 3SA146 (MCDS), was identified in 2010. Race 3SA146 is also virulent for Lr37 but unlike 3SA145, it is virulent for Lr1 and Lr23 and avirulent for Lr3ka and Lr30. Microsatellite analysis showed that 3SA145 and 3SA146 shared 70 % genetic similarity with each other, but only 30 % similarity with other races in South Africa, suggesting that both represent foreign introductions. In seedling tests of 98 South African winter and spring cultivars and advanced breeding lines, 27 % were susceptible to 3SA145 and 3SA146 but resistant to 3SA133. In greenhouse studies of 59 spring wheat adult plants, 19 % of breeding lines and 46 % of cultivars were susceptible to 3SA145, whereas 29 % of the lines and 53 % of cultivars were susceptible to 3SA146. The cssfr6 gene-specific DNA marker confirmed the presence of Lr34 gene for leaf rust resistance in a homozygous condition in 28 wheat entries. Five entries were heterogeneous for Lr34. Several entries which were susceptible as seedlings to the new races carried Lr34. These lines are expected to show lower levels of leaf rust as adult plants. Results of these studies indicate a continued vulnerability of South African wheat cultivars to new races and emphasise the importance of regular rust monitoring and the need to incorporate genes for durable resistance.     

Pathogenic and genetic diversity of Puccinia triticina from triticale in Poland between 2012 and 2015

Variation among Puccinia triticina isolates collected from triticale in Poland between 2012 and 2015 was studied based on virulence and simple sequence repeat (SSR) markers. Two hundred and forty-two single-uredinial isolates from four geographically separated locations were tested for virulence against 33 near-isogenic Thatcher lines containing known Lr resistance genes, and their molecular genotypes were characterized with 34 SSR markers. Structure and relationships of the regional and annual populations of P. triticina were analysed using an assignment-based approach for both the virulence and SSR data. The molecular marker analysis was based on two different models of SSR evolution: the stepwise mutation model with a variable mutation rate (SMMv) and the infinite alleles model (IAM). A highly significant deviation from Hardy–Weinberg equilibrium among SSR genotypes, a high proportion of heterozygotes, and a moderate association of relationships between virulence phenotypes and SSR genotypes were consistent with the occurrence of clonal lineages of related races within the population. While the results suggest that genetic drift and mutation affect variation within the pathogen population, it seems that migration has the most significant role in shaping the population structure of P. triticina occurring on triticale in Poland.     

Qualitative and quantitative variation in pathogenicity of races of coffee leaf rust (Hemileia vastatrix) detected in the State of São Paulo,Brazil

Between 1977 and 1981, seven qualitatively distinct new races of coffee leaf rust (Hemileia vastatrix) were detected in breeding plots in the State of São Paulo, Brazil. Four races carry complex virulence against the resistance genes S_H1, S_H2, S_H4 and S_H5 ofCoffea arabica. Three races match unidentified resistance genes ofC. canephora. Two of these were isolated from cv. Kouillou and one from the hybrid population Icatu. Pending further identification, these races were indicated by the number of their type cultures (Is. 2, 10 and 11). Is. 2 and Is. 10 showed extra virulence to some coffee genotypes and decreased virulence to other coffee genotypes, suggesting stabilizing selection. Three rust isolates were detected which differed quantitatively from the common rust race II. Is. 1 was moderately virulent to the coffee differential for S_H3 in the laboratory but avirulent in the greenhouse, indicating a host x pathogen x environment interaction. Is. 3 and 12 showed levels of virulence intermediate between race II and Is. 2 and 10, respectively. The results show that incomplete resistance of coffee toH. vastatrix, at various levels, can be race-specific. The nature of race formation of coffee leaf rust in Brazil and breeding strategies for obtaining durable resistance are discussed.     

Components of quantitative resistance to leaf rust in wheat cultivars: diversity,variability and specificity

The aim of this study was to investigate the potential diversity and pathogen‐specificity of sources of quantitative resistance to leaf rust caused by Puccinia triticina in French wheat germplasm. From a set of 86 genotypes displaying a range of quantitative resistance levels during field epidemics, eight wheat genotypes were selected and challenged in a greenhouse with three isolates of the pathogen, belonging to different pathotypes. Five components of resistance were assessed: infection efficiency, for which an original methodology was developed, latent period, lesion size, spore production per lesion, and spore production per unit of sporulating tissue. High diversity and variability for all these components were expressed in the host × pathotype combinations investigated; isolate‐specificity was found for all the components. The host genotypes displayed various resistance profiles, based on both the components affected and the isolate‐specificity of the interaction. Their usefulness as sources of quantitative resistance was assessed: line LD7 probably combines diversified mechanisms of resistance, being highly resistant for all the components, but displaying isolate‐specificity for all the components; cv. Apache did not show isolate specificity for any of the components, which could be related to the durability of its quantitative resistance in the field over more than 11 years.     

Genetic differentiation of the wheat leaf rust fungus Puccinia triticina in Europe

The objective of this study was to determine whether genetically differentiated groups of Puccinia triticina are present in Europe. In total, 133 isolates of P. triticina collected from western Europe, central Europe and Turkey were tested for virulence on 20 lines of wheat with single leaf rust resistance genes, and for molecular genotypes with 23 simple sequence repeat (SSR) markers. After removal of isolates with identical virulence and SSR genotype within countries, 121 isolates were retained for further analysis. Isolates were grouped based on SSR genotypes using a Bayesian approach and a genetic distance method. Both methods optimally placed the isolates into eight European (EU) groups of P. triticina SSR genotypes. Seven of the groups had virulence characteristics of isolates collected from common hexaploid wheat, and one of the groups had virulence characteristics of isolates from tetraploid durum wheat. There was a significant correlation between the SSR genotypes and virulence phenotypes of the isolates. All EU groups had observed values of heterozygosity greater than expected and significant fixation values, which indicated the clonal reproduction of urediniospores in the overall population. Linkage disequilibria for SSR genotypes were high across the entire population and within countries. The overall values of R_ST and F_ST were lower when isolates were grouped by country, which indicated the migration of isolates within Europe. The European population of P. triticina had higher levels of genetic differentiation compared to other continental populations.     

Genetic Control of Resistance to Tan Necrosis Induced by Pyrenophora tritici-repentis, Races 1 and 2, in Spring and Winter Wheat Genotypes

ABSTRACT The symptoms of tan spot of wheat, caused by Pyrenophora triticirepentis, include a tan necrosis component and an extensive chlorosis component. Since tan spot has become the major component of the leafspotting disease complex of wheat in western Canada, the need for resistant cultivars has increased. This study was conducted to determine whether the resistance to tan spot found in a diverse set of spring and winter wheat genotypes was due to resistance genes not previously reported. The genetic control of resistance to necrosis induced by P. triticirepentis race 1 and race 2 was determined, under controlled environmental conditions, for spring wheat genotypes Erik and 86ISMN 2137 and winter wheat genotypes Hadden, Red Chief, and 6B-365. Plants were inoculated at the two-leaf stage and disease reaction was assessed based on lesion type. Tests of the F(1) and F(2) generations, and of F(2:3) and F(2:8) families, indicated that one recessive gene controlled resistance to the necrosis component of tan spot caused by both race 1 and race 2 in each cross studied. Lack of segregation in crosses between the resistant cultivars indicated that the resistance gene was the same in all of the cultivars.     

Sources of resistance to Pseudomonas syringae pv. phaseolicola races in Phaseolus vulgaris

One thousand and forty-eight Phaseolus bean accessions were evaluated for resistance to six races of Pseudomonas syringae pv. phaseolicola . The accessions originated from regions of the Americas and Africa where the disease is important and included wild type accessions and some known resistance sources. Resistance, graded on a five-point scale, was of two types: qualitative, which was shown to be race-specific, and quantitative. Race specific resistance genes (R-genes) were detected in 49.4% of accessions with the following gene frequencies: R1 (10.3%), R2 (0.3%), R3 (25.0%), R4 (35.0%) and R5 (0.2%).
Evidence for quantitative variation in resistance, in the absence of specific R-genes, was shown by the distribution of infection scores, 76% of accessions showing maximum susceptibility (grades 4–5), 23% showing intermediate resistance (grades 2–4), and 1% showing high levels of quantitative resistance (grades 1–2). The last 1% of accessions showed interactions which were not race-specific and it is suggested that they may possess race non-specific resistance. It is possible that several of the accessions in this category carry the recessive gene derived from PI 150414. Other accessions were of unknown parentage and may represent new sources of quantitative, potentially race non-specific, resistance. It is suggested that the combination of race specific and race non-specific resistance could provide an effective strategy for establishing durable resistance.     

Regional and temporal differentiation of virulence phenotypes of Puccinia triticina from common wheat in Russia during the period 2001–2018

Leaf rust, caused by the fungus Puccinia triticina, is one of the most damaging rust diseases of wheat in Russia. Populations of P. triticina were monitored in seven regions of Russia from 2001 to 2018, with a total of 5,191 single urediniospore isolates from bread wheat (Triticum aestivum) being analysed. Populations have changed significantly in all regions since 2012, after 2 years of drought (2010–2011). Regional collections of P. triticina were also significantly different between the two periods 2001–2009 and 2012–2018, with changes along two geographic gradients from West Siberia to the north-west and south-west (North Caucasia) of the European part of Russia. All tested isolates were avirulent to resistance gene Lr9 in 2001–2009 but, since 2010, virulence to Lr9 has occurred and annually increased in the Asian part of Russia (Ural and West Siberia) due to deployment of cultivars with the Lr9 gene. Virulence to Lr2a and Lr15 was considerably lower in Dagestan (6%–33%) and all European regions (35%–67%) than in Asian regions (84%–96%). During 2001–2009, virulence on Lr1 was also lower in Dagestan (33%) and the European regions (50%–77%) than in Asia (91%–96%); however, by 2012–2018, nearly all isolates were virulent on Lr1. Remarkable changes were observed in frequencies of P. triticina races defined by their virulence/avirulence to Lr1 and Lr2a genes. We postulate the P. triticina population in Dagestan is specific to that area and pathogen populations in European and Asian parts of Russia are distinct.     

Comparative microscopic and molecular analysis of Thatcher near‐isogenic lines with wheat leaf rust resistance genes Lr2a,Lr3, LrB or Lr9 upon challenge with different Puccinia triticina races

Thatcher near‐isogenic lines (NILs) of wheat carrying resistance gene Lr2a, Lr3, LrB or Lr9 were inoculated with Puccinia triticina races of virulence phenotype BBBD, MBDS, SBDG and FBDJ. Puccinia triticina infection structures were analysed under the fluorescence microscope over a course of 14 days after inoculation (dai). The relative proportion of P. triticina and wheat genomic DNA in infected leaves was estimated with a semiquantitative multiplex PCR analysis using P. triticina‐ and wheat‐specific primers. The occurrence of a hypersensitive response (HR), cellular lignification and callose deposition in inoculated plants was investigated microscopically. In interactions producing highly resistant infection type (IT) ‘0;’, a maximum of two haustorial mother cells per infection site were produced, and there was no increase in the proportion of P.  triticina genomic DNA in infected leaves, indicating the absence of P. triticina growth. In comparison, sizes of P. triticina colonies increased gradually in interactions producing moderately resistant IT ‘1’ and ‘2’, with the highest proportion of P. triticina genomic DNA found in leaves sampled at 14 dai. In interactions producing susceptible IT ‘3–4’, the highest proportion of P. triticina genomic DNA was found in leaves sampled at 10 dai (45·5–51·5%). HR and cellular lignification were induced in interactions producing IT ‘0;’ and ‘1’ at 1 dai but they were not observed in interactions producing IT ‘2’ until 2 dai. No HR or cellular lignification were induced in interactions producing susceptible IT ‘3–4’. Furthermore, a strong deposition of callose was induced in Lr9 + BBBD and Lr9 + FBDJ (IT ‘0;’), whereas this defence response was not induced in resistant or susceptible interactions involving Lr2a, Lr3 or LrB, indicating that Lr9 mediated resistance was different from that conditioned by Lr2a, Lr3 or LrB.     

First report of oospore formation in Plasmopara viticola,the causal agent of grapevine downy mildew,in highland regions of southern Brazil

This work evaluates the formation of oospores of Plasmopara viticola, the causal agent of grape downy mildew (DM), in highland regions in southern Brazil. Leaves of susceptible and resistant grape genotypes naturally infected with the pathogen were collected in the autumn of 2017, 2018, and 2019 from vineyards located in the highlands of Santa Catarina state. Leaf tissues were evaluated by light microscopy and scanning electron microscopy. Oospores of P. viticola were identified in both susceptible and resistant host genotypes. They were concentrated in the central regions of the DM lesions, close to the leaf veins, and exhibited a rounded shape, yellowish colour, thick wall, and a diameter ranging from 16.28 to 49.15 µm. The formation of oospores is strong evidence that sexual reproduction is occurring in P. viticola in the climatic conditions of the highlands of southern Brazil. Sexual reproduction contributes to the maximization of genetic diversity via meiosis. Populations with high genetic variability are more likely to break resistance mechanisms conferred by resistance genes and to develop resistance to fungicides applied for disease control. To our knowledge, this is the first scientific study to prove the formation of P. viticola oospores in Brazil. The results presented provide a solid basis for further studies on sexual recombination in P. viticola. Genetic improvement programmes for grapevines, disease management methods, and disease prediction models need to consider the sexual reproduction of this pathogen, otherwise their effectiveness may be compromised.     

Mechanisms of date palm resistance to Bayoud disease: Current state of knowledge and research prospects

The Bayoud disease, caused by Fusarium oxysporum f. sp. albedinis (Foa), represents a major limiting factor of date palm culture in Morocco and constitutes a serious threat to the date palm plantations in Algeria and all other countries. Efficient disease prevention requires the development of resistant cultivars. In Morocco, among the cultivars listed, only six appear to be resistant to Bayoud disease, but they produce poor quality fruit. Thus, the Moroccan program of date palm genetic improvement is based on directed crossing between resistant cultivars and susceptible cultivars with good date quality traits to select resistant genotypes producing high quality fruits. In addition to the separation of the resistance to Bayoud disease and quality of the fruits characters, this breeding program is really complex due to the sex separation in the date palm, the duration of juvenile phase which is very long, and the lifespan of the date palm which requires a durable polygenic resistance. Then, the selected genotypes must be of female sex, of good date quality, and possess effective defense mechanisms against the pathogen. Moreover, the selection of the date palm resistance must necessarily take into account the mechanisms of pathogen aggressiveness. In this review, we will present and discuss studies developed on the Bayoud disease of the date palm, particularly on the disease control, the biochemical and molecular markers of resistance, the program of date palm genetic improvement of resistance, the Foa pathogenicity factors, and host defense mechanisms. It will also highlight the recent studies that showed that differential behaviour of the resistant and susceptible cultivars was not related to a difference of induction of the defense mechanisms, but to the suppression of their elicitation in the susceptible cultivars.     

Low diversity and fast evolution in the population of Puccinia triticina causing durum wheat leaf rust in France from 1999 to 2009, as revealed by an adapted differential set

No internationally agreed differential set is available for characterization of virulences in populations of Puccinia triticina causing wheat leaf rust on durum wheat. In a first step, 73 potentially differential host genotypes were tested with 96 durum leaf rust isolates collected in France. A differential set, adapted to the local epidemiological context and useful for comparison with international studies was selected, including French commercial cultivars, Thatcher lines with Lr genes, and international cultivars. In the second step, a sample of 310 isolates collected in France from 1999 to 2009 was characterized on this set. Diversity was very low, as only five pathotypes were distinguished. Genotyping of a subset of 76 isolates according to 20 SSR markers confirmed this low diversity, with 73 isolates belonging to a single dominant genotype. Population was strongly shaped by cultivars, and the findings explain the successive breakdown of resistance sources deployed in French durum wheat cultivars. The gene Lr14a, suggested to be an efficient source of resistance in several European and American countries, was overcome by pathotypes frequent in France since 2000. Postulation of resistance genes in the commercial cultivars led to a proposed simplified version of the differential set. This study, providing new information about leaf rust resistance genes present in the French durum wheat germplasm, highlights the need to diversify sources of resistance to P. triticina in this germplasm. The results are also discussed in terms of relatedness and intercontinental migration of P. triticina on durum wheat.