Wheat Leaf Blotch Research Articles

TRITICALE RESISTANCE TO THE MAIN PHYTOPATHOGENIC ORGANISMS OF NORTHWEST REGION OF THE RUSSIAN FEDERATION

The modern triticale varieties have high feed advantages, they have a good tolerance to difficult environments and they are resistant to majority of phy-topathogens. For the period of 2005-2006 phytosanitary monitoring for the triticale samples of different ecologo-geographic origin with spring (327 samples) and winter (37 samples) type of development from N.I. Vavilov All-Russian Scientific Research Institute of Plant Growing on the resistance to the basic wheat patho-gens of Northwest region of the Russian Federation (brown and yellow rust, wheat leaf blotch, powdery mildew) was carried out. In addition, the authors deter-mine the duration of interphase periods, the height of plants, the mass of 1000 grains. The pathogen of powdery mildew does not develop on samples of triticale. The maximal number of triticale samples, introduced from the European part of the former Soviet Union (Ukraine, Belarus) was characterized by absence of de-feat symptoms by brown rust and wheat leaf blotch agents. Pathogenesis of the yellow rust agent was not found on the triticale samples, received from Central Asia, Northern Europe, Australia, South America, Asia and Africa. As was revealed, the greatest degree the attribute of triticale origin influenced on wheat leaf blotch pathogenesis, in the least – on brown and yellow rusts pathogenesis. As was revealed, the criterion of a triticale origin influenced on wheat leaf blotch pathogenesis in the greatest degree, and in the least – on brown and yellow rusts. In the greater degree the plants development type affect the wheat leaf blotch pathogenesis on triticale samples of the Russian selection and in smaller - on triticale samples, received from European part of Former USSR, Northern Europe, East Europe, Central Europe. Keywords: triticale, cereal crop diseases, leaf rust, yellow rust, wheat leaf blotch, powdery mildew.

УСТОЙЧИВОСТЬ ТРИТИКАЛЕ К ОСНОВНЫМ ВОЗБУДИТЕЛЯМ БОЛЕЗНЕЙ, РАСПРОСТРАНЕННЫМ В СЕВЕРО-ЗАПАДНОМ РЕГИОНЕ РОССИЙСКОЙ ФЕДЕРАЦИИ

The modern triticale varieties have high feed advantages, they have a good tolerance to difficult environments and they are resistant to majority of phytopathogens. For the period of 2005-2006 phytosanitary monitoring for the triticale samples of different ecologo-geographic origin with spring (327 samples) and winter (37 samples) type of development from N.I. Vavilov All-Russian Scientific Research Institute of Plant Growing on the resistance to the basic wheat pathogens of Northwest region of the Russian Federation (brown and yellow rust, wheat leaf blotch, powdery mildew) was carried out. In addition, the authors determine the duration of interphase periods, the height of plants, the mass of 1000 grains. The pathogen of powdery mildew does not develop on samples of triticale. The maximal number of triticale samples, introduced from the European part of the former Soviet Union (Ukraine, Belarus) was characterized by absence of defeat symptoms by brown rust and wheat leaf blotch agents. Pathogenesis of the yellow rust agent was not found on the triticale samples, received from Central Asia, Northern Europe, Australia, South America, Asia and Africa. As was revealed, the greatest degree the attribute of triticale origin influenced on wheat leaf blotch pathogenesis, in the least – on brown and yellow rusts pathogenesis. As was revealed, the criterion of a triticale origin influenced on wheat leaf blotch pathogenesis in the greatest degree, and in the least – on brown and yellow rusts. In the greater degree the plants development type affect the wheat leaf blotch pathogenesis on triticale samples of the Russian selection and in smaller on triticale samples, received from European part of Former USSR, Northern Europe, East Europe, Central Europe.

Synthesis and fungicidal activity of tubulin polymerisation promoters. Part 3: Imidazoles

A novel class of experimental fungicides has been discovered, which consists of special tetrasubstituted imidazoles. They are highly active against important phytopathogens, such as Botrytis cinerea (grey mould), Uncinula necator (grape powdery mildew), Mycosphaerella graminicola (wheat leaf blotch) and Alternaria solani (potato and tomato early blight). Their fungicidal efficacy is due to their ability to promote fungal tubulin polymerization, which leads to a disruption of microtubule dynamics. These imidazoles are five-membered ring analogs of similar substituted triazolopyrimidines and pyridazines with the same mode of action. A concise four-step synthesis route has been used to prepare them from commercially available starting materials.

Spring air temperature accounts for the bimodal temporal distribution of Septoria tritici epidemics in the winter wheat stands of Luxembourg

Septoria tritici is the causal agent of leaf blotch in wheat and among the most damaging fungal cereal pathogens in the humid regions of central Europe. The percentage of the leaf area colonized by S. tritici was recorded weekly between April and July every season between 2004 and 2010. A total of 11 cultivars with moderate susceptibility [ratings of 4–6 on a 1 (resistant) to 9 (susceptible) scale] were included. The disease level was assessed on the upper three leaf layers at 2 locations between 2004 and 2006 and at 3 locations between 2007 and 2010. The period between sowing and the point of time, when 50% of the leaf area was necrotized due to colonization by S. tritici (T50) was estimated for each year, site, cultivar and leaf layer by non-linear regression. T50 values followed a bimodal distribution with one maximum at 245 days after sowing (DAS; early epidemics) and one maximum at 270 DAS (late epidemics). Early epidemics were preceded by almost constant daily average temperatures of 13.2 ± 0.8 °C between 181 and 210 DAS. Late epidemics were preceded by an approximately linear increase in temperature from 8.7 ± 0.9 to 12.1 ± 0.9 °C during the same period of time. Based on these differences, it seems possible to predict whether an early or a late epidemic can be expected at least 35 days before the epidemic outbreak. Temperature sums calculated with a base temperature of 6.6 °C starting at sowing and ending when T50 was reached were not significantly different between early and late epidemics (P = 0.73) and averaged 1721 ± 49°days. Fungicide applications, which resulted into a delay of the epidemic development similar to the difference between early and late epidemics, resulted in a yield increase between 11.7 and 12.6%.

Synthesis and fungicidal activity of tubulin polymerisation promoters. Part 2: Pyridazines

Special tetrasubstituted pyridazines are potent fungicides by promoting the tubulin polymerisation, hereby disrupting the microtubule dynamics in the fungus. They are monocyclic analogs of similar substituted triazolopyrimidines and pyridopyrazines with the same mode of action. The fungicidal activity of these pyridazines was evaluated against the plant pathogens Botrytis cinerea (grey mould), Mycosphaerella graminicola (wheat leaf blotch) and Alternaria solani (potato and tomato early blight). Structure–activity relationship studies revealed the importance of a methyl and a chlorine substituent next to both ring nitrogen atoms and two aryl or heteroaryl groups in the other two pyridazine positions.

Population Structure ofMycosphaerella graminicolaand Location of Genes for Resistance to the Pathogen: Recent Advances in Argentina

Leaf blotch of wheat (Septoria triticiRob. ex Desm., teleomorphMycosphaerella graminicola(Fückel) Schröt. in Cohn) causes significant losses in wheat. During the last decades studies about the genetic variability of the pathogen and location of the resistance have been intensive around the world. The knowledge about the genetic variation ofM. graminicolais very important because it could allow us to determine which genotypes predominate within a geographic area. It also can be used to evaluate the germplasm resistance of wheat cultivars with isolates with high genetic differences. In addition, the knowledge of the genes conditioning resistance in different genotypes allows getting precise combination in new germplasm. The incorporation of the known genes in new cultivars could contribute to broadening the resistance to the pathogen. A paper about genetic variability of the pathogen and location of the resistance, with special emphasis in the work carried out in Argentina, is presented.

Risk assessment studies on succinate dehydrogenase inhibitors, the new weapons in the battle to control Septoria leaf blotch in wheat

Chemical control of Septoria leaf blotch, caused by Mycosphaerella graminicola, is essential to ensure wheat yield and food security in most European countries. Mycosphaerella graminicola has developed resistance to several classes of fungicide and, with the efficacy of azoles gradually declining over time, new modes of action and/or improvements in host varietal resistance are urgently needed to ensure future sustainable disease control. Several new-generation carboxamide fungicides with broad-spectrum activity have recently been introduced into the cereal market. Carboxamides inhibit succinate dehydrogenase (Sdh) of the mitochondrial respiratory chain (complex II) but, because of their single-site specificity, these fungicides may be prone to resistance development. The objective of this study was to assess the risk of resistance development to different Sdh inhibitor (SDHI) fungicides in M. graminicola. UV mutagenesis was conducted to obtain a library of carboxin-resistant mutants. A range of SDHI resistance-conferring mutations was found in Sdh subunits B, C and D. Pathogenicity studies with a range of Sdh variants did not detect any fitness costs associated with these mutations. Most of the amino acid residues identified (e.g. B-S221P/T, B-H267F/L/N/Y, B-I269V and D-D129E/G/T) are directly involved in forming the cavity in which SDHI fungicides bind. Docking studies of SDHI fungicides in structural models of wild-type and mutated Sdh complexes also indicated which residues were important for the binding of different SDHI fungicides and showed a different binding for fluopyram. The predictive power of the model was also shown. Further diagnostic development, enabling the detection of resistant alleles at low frequencies, and cross-resistance studies will aid the implementation of anti-resistance strategies to prolong the cost-effectiveness and lifetime of SDHI fungicides.

Niementowski-type synthesis of pyrido[3,2-e][1,2,4]triazines: potent aza-analogs of pyrido[2,3-b]pyrazine fungicides

Abstract Novel trisubstituted pyrido[3,2- e ][1,2,4]triazines have been found to possess similar biological activity to the corresponding pyridopyrazine fungicides against important phytopathogens such as Mycosphaerella graminicola (wheat leaf blotch), Magnaporthe grisea (rice blast), and Rhizoctonia solani (rice sheath blight). They have been prepared for the first time from a monocyclic triazine by Niementowski-type ring condensation.

Emergence of Tan Spot Disease Caused by Toxigenic Pyrenophora tritici-repentis in Australia Is Not Associated with Increased Deployment of Toxin-Sensitive Cultivars

The wheat disease tan (or yellow leaf) spot, caused by Pyrenophora tritici-repentis, was first described in the period 1934 to 1941 in Canada, India, and the United States. It was first noted in Australia in 1953 and only became a serious disease in the 1970s. The emergence of this disease has recently been linked to the acquisition by P. tritici-repentis of the ToxA gene from the wheat leaf and glume blotch pathogen, Stagonospora nodorum. ToxA encodes a host-specific toxin that interacts with the product of the wheat gene Tsn1. Interaction of ToxA with the dominant allele of Tsn1 causes host necrosis. P. tritici-repentis races lacking ToxA give minor indistinct lesions on wheat lines, whereas wheat lines expressing the recessive tsn1 are significantly less susceptible to the disease. Although the emergence and spread of tan spot had been attributed to the adoption of minimum tillage practices, we wished to test the alternative idea that the planting of Tsn1 wheat lines may have contributed to the establishment of the pathogen in Australia. To do this, wheat cultivars released in Australia from 1911 to 1986 were tested for their sensitivity to ToxA. Prior to 1941, 16% of wheat cultivars were ToxA-insensitive and hence, all other factors being equal, would be more resistant to the disease. Surprisingly, only one of the cultivars released since 1940 was ToxA insensitive, and the area planted to ToxA-insensitive cultivars varied from 0 to a maximum of only 14% in New South Wales. Thus, the majority of the cultivars were ToxA-sensitive both before and during the period of emergence and spread of the disease. We therefore conclude that the spread of P. tritici-repentis in Australia cannot be causally linked to the deployment of ToxA-sensitive cultivars.

Salicylic acid induces resistance to Septoria nodorum berk. in wheat

The effect of salicylic acid (SA) on oxalate oxidase and peroxidase activities and hydrogen peroxide (H2O2) production in leaf cells has been studied in wheat of the susceptible cultivar Zhnitsa infected by the fungus Septoria nodorum, a pathogen of wheat leaf blotch. The results show that fungal hyphae spread into interstices between mesophyll cells and that infected tissues contain H2O2. Treatment with SA results in enhanced H2O2 production in mesophyll cells, which is due to activation of oxalate oxidase and peroxidase in the cell wall. It is proposed that the modulating effect of SA on oxide reductase activities is involved in the induction of protective response to fungal infection in wheat plants.

Trichoderma spp. as elicitors of wheat plant defense responses against Septoria tritici

Leaf blotch of wheat is a widespread and highly active disease that affects wheat production. In addition to the use of chemicals and proper cultivation methods, microbial antagonists are used to control plant pathogens. Trichoderma spp. stimulate a systemic induced response in plants. Therefore, the efficacy of Trichoderma spp. against wheat leaf blotch was evaluated under greenhouse conditions. The susceptible plants were sprayed with Septoria tritici conidiospores. In order to select an efficient method of pretreatment with Trichoderma spp., leaf spraying and seed coating with 14 isolates were tested in 2003 and 2004. The extent of leaf necrosis area and pycnidial coverage was estimated. Antagonism was assessed by the capacity of each Trichoderma spp. isolate to restrict the progress of leaf blotch, 21 days after inoculation. Of the two methods, seed coating was more efficacious against leaf blotch than leaf spraying. Amongst the 14 isolates tested, the isolate prepared from T. harzianum (Th5) produced the highest level of protection. None of the treatments caused changes in plant stem diameter or dry weight. Trichoderma spp. did not get into leaves while S. tritici was present, even in asymptomatic leaf extracts. In addition, the leaf apoplast antifungal proteolytic activity was measured in plants 7, 15, and 22 days after sowing. This antifungal action decreased in plants only inoculated with S. tritici, but increased in those grown from seeds coated with the T. harzianum (Th5) isolate. This increase conferred resistance to the susceptible wheat cultivar. The endogenous germin-like protease inhibitor coordinated the proteolytic action. These results suggest that T. harzianum stimulates a biochemical systemic induced response against leaf blotch.

Transcriptome profiling of the response of Mycosphaerella graminicola isolates to an azole fungicide using cDNA microarrays

SUMMARY Resistance to azole antifungals is a major problem in the control of diseases caused by fungal pathogens of both humans and plants. Potential for the development of azole resistance in the wheat leaf blotch pathogen Mycosphaerella graminicola, the causal agent of the most economically significant foliar disease of wheat in north-western Europe, is now of particular concern after the recent emergence of widespread resistance to quinone outside inhibitor fungicides. Using a cDNA microarray representing around 25% of the genome, we have profiled the transcriptional response of M. graminicola to epoxiconazole, currently the most widely used azole fungicide on cereal crops. By comparing the transcription profiles of two M. graminicola isolates with contrasting sensitivities to epoxiconazole we show qualitative and quantitative differences in differentially expressed genes, including those involved in ergosterol biosynthesis, mitochondrial respiration and transport mechanisms. This represents the first study investigating the response of a plant pathogenic fungus to a fungicide using cDNA microarray technology.

Mutations in the CYP51 gene correlated with changes in sensitivity to sterol 14α‐demethylation inhibitors in field isolates of Mycosphaerella graminicola

In France, as in many other European countries, Mycosphaerella graminicola (Fuckel) Schröter in Cohn (anamorph Septoria tritici), the causal agent of wheat leaf blotch, is controlled by foliar applications of fungicides. With the recent generalization of resistance to strobilurins (QoIs), reliable control is mainly dependent upon inhibitors of sterol 14 alpha-demethylation (DMIs). To date, strains with reduced sensitivity to DMIs are widespread, but disease control using members of this class of sterol biosynthesis inhibitors has not been compromised. In this study, sensitivity assays based on in vitro effects of fungicides towards germ-tube elongation allowed the characterization of seven DMI-resistant phenotypes. In four of them, cross-resistance was not observed between all tested DMIs; this characteristic concerned prochloraz, triflumizole, fluquinconazole and tebuconazole. Moreover, the highest resistant factors to most DMIs were found only in recent isolates; according to their response towards prochloraz, they were classified into two categories. Molecular studies showed that DMI resistance was associated with mutations in the CYP51 gene encoding the sterol 14 alpha-demethylase. Alterations at codons 459, 460 and 461 were related to low resistance levels, whereas, at position 381, a valine instead of an isoleucine, in combination with the previous changes, determined the highest resistance levels to all DMIs except prochloraz. Mutations in codons 316 and 317 were also found in some isolates exhibiting low resistance factors towards most DMIs.

A novel substitution I381V in the sterol 14alpha-demethylase (CYP51) of Mycosphaerella graminicola is differentially selected by azole fungicides.

SUMMARY The recent reduction in the efficacy of azole fungicides in controlling Septoria leaf blotch of wheat, caused by Mycosphaerella graminicola, has prompted concerns over possible development of resistance, particularly in light of the recent emergence of widespread resistance to quinone outside inhibitors (QoIs). We have recently implicated alterations in the target-encoding sterol 14alpha-demethylase protein (CYP51), and over-expression of genes encoding efflux pumps, in reducing sensitivity to the azole class of sterol demethylation inhibitors (DMIs) in M. graminicola. Here we report on the prevalence and selection of two CYP51 alterations, substitution I381V and deletion of codons 459 and 460 (DeltaY459/G460), in populations of M. graminicola. Neither alteration has previously been identified in human or plant pathogenic fungi resistant to azoles. The presence of DeltaY459/G460 showed a continuous distribution of EC(50) values across isolates with either I381 or V381, and had no measurable effect on azole sensitivity. Data linking fungicide sensitivity with the presence of I381V in M. graminicola show for the first time that a particular CYP51 alteration is differentially selected by different azoles in field populations of a plant pathogen. Substitution I381V although not an absolute requirement for reduced azole sensitivity, is selected by tebuconazole and difenoconazole treatment, suggesting an adaptive advantage in the presence of these two compounds. Prochloraz treatments appeared to select negatively for I381V, whereas other azole treatments did not or only weakly impacted on the prevalence of this substitution. These findings suggest treatments with different members of the azole class of fungicides could offer a resistance management strategy.

The effect of Trichoderma harzianum and T. koningii on the control of tan spot (Pyrenophora tritici-repentis) and leaf blotch (Mycosphaerella graminicola) of wheat under field conditions in Argentina

The effect of six isolates of Trichoderma harzianum and one isolate of T. koningii on the incidence and severity of tan spot (Pyrenophora tritici-repentis) and leaf blotch of wheat (Mycosphaerella graminicola) was evaluated under field conditions. Significant differences between wheat cultivars, inoculum types and growth stages were found. Three of the isolates tested (T2 for M. graminicola, T7 for P. tritici-repentis and T5 for both of them) showed the best performance in controlling leaf blotch and tan spot when coated onto seed or sprayed onto wheat leaves at different growth stages, with significant severity reduction up to 56%. At tillering, six of the isolates reduced the severity of P. tritici-repentis and M. graminicola compared to the control by up to 39% and 12–53%, respectively. In some experiments, the biocontrol preparation (T2 and T5) gave a level of disease control similar to that obtained with Tebuconazole (70 and 48%, respectively). The effect of Trichoderma against P. tritici-repentis was also observed at the heading stage, when six of the treatments reduced disease severity by 16–35%. This is the first report on the efficacy of Trichoderma spp. against wheat necrotrophic pathogens under field conditions in Argentina.

Analysis of expressed sequence tags from the wheat leaf blotch pathogen Mycosphaerella graminicola (anamorph Septoria tritici)

Mycosphaerella graminicola is a major fungal pathogen of wheat as the causal agent of Septoria leaf blotch disease. As a first step toward a greater understanding of the mechanism of host infection we have generated, sequenced, and analyzed three M. graminicola EST libraries from conditions predicted to resemble independent phases of the host infection process, including one library generated from the fungus during interaction with its host. A total of 5180 ESTs were sequenced and clustered into 886 contigs and 2039 singletons to give a set of 2925 unique sequences (unisequences). BLASTX analysis revealed 33% of the unknown M. graminicola unisequences to be orphans. Very limited inter-library overlap of expression was seen with the majority of unisequences (contigs and singletons) being library-specific. Analysis of EST redundancy between libraries demonstrated a significant difference in gene expression in the three conditions. Comparisons made against fully sequenced genomes revealed most M. graminicola sequences to be homologous to genes present in both pathogenic and non-pathogenic Ascomycete filamentous fungi. A range of sequences having significant homology to verified pathogenicity/ virulence genes ( HvPV-genes) of either plant or mammalian fungal and Oomycete pathogens were also identified (<1e−20). The generation of, and the diversity present within, this EST collection will facilitate future efforts aimed at a more detailed study of the transcriptome of the fungus during host infection.

Physiologic variation in Mycosphaerella graminicola from western Canada

Mycosphaerella graminicola (anamorph: Septoria tritici), the causal agent of septoria leaf blotch of wheat, has increased in prevalence in the prairie region of Canada in recent years. It is important to know if variation in virulence occurs in the local population as was reported in other countries. Seventy-four pathogen isolates were collected from hexaploid wheat fields in Manitoba and Saskatchewan and tested for virulence on a set of differential genotypes, including four hexaploid and two tetraploid wheat genotypes. Seedlings were inoculated with a conidial suspension and assessed 14 and 21 days later for the presence of necrosis, chlorosis, and pycnidia to identify virulence patterns. Reactions of the differential host genotypes to different isolates ranged from susceptible to near-immune. Adult-plant reactions were similar to seedling reactions for all genotypes used in this study. A difference in virulence patterns on one genotype suggested that physiological specialization occurs and that two virulence groups are present in the pathogen population from western Canada, based on the set of wheat genotypes used in this study

Effect of foliar‐applied potassium chloride on septoria leaf blotch of winter wheat

The effect of foliar‐applied potassium chloride on Septoria tritici, the anamorph of Mycosphaerella graminicola, was quantified and possible modes of action investigated during controlled‐environment and field experiments. A field experiment in harvest year 1997 showed c. 50% reduction in the area of leaf 2 of winter wheat plants affected by septoria leaf blotch after foliar application of potassium chloride, compared with untreated controls. Similarly, in harvest year 1998 potassium chloride reduced, by about one‐third, the area of the flag and penultimate leaf affected by S. tritici. However, a significant yield increase was not observed, although grains m−2 did show an increase of borderline significance. Applications of epoxiconazole reduced the area of leaf 4 affected by S. tritici compared with untreated controls, whereas applications of chlorothalonil, potassium chloride or polyethylene glycol proved ineffective against disease development. This may suggest that potassium chloride is relatively immobile and possesses contact activity similar to that of chlorothalonil. In 1998, similar reductions in leaf area affected were observed with the inert osmoticum polyethylene glycol in the field, suggesting that the control provided by potassium chloride may be achieved by adverse osmotic effects on the pathogen. Scanning electron microscopy of germinating conidia on wheat plants showed inhibition of conidial germination by both potassium chloride and polyethylene glycol at the same calculated osmotic potential on the leaf surface.

Genetic and Physical Mapping of a Gene Conditioning Sensitivity in Wheat to a Partially Purified Host-Selective Toxin Produced by Stagonospora nodorum

ABSTRACT A toxin, designated SnTox1, was partially purified from culture filtrates of isolate Sn2000 of Stagonospora nodorum, the causal agent of wheat leaf and glume blotch. The toxin showed selective action on several different wheat genotypes, indicating that it is a host-selective toxin (HST). The toxic activity was reduced when incubated at 50 degrees C and activity was eliminated when treated with proteinase K, suggesting that the HST is a protein. The synthetic hexaploid wheat W-7984 and hard red spring wheat Opata 85, the parents of the International Triticeae Mapping Initiative (ITMI) mapping population, were found to be sensitive and insensitive, respectively, to SnTox1. The ITMI mapping population was evaluated for toxin reaction and used to map the gene conditioning sensitivity. This gene, designated Snn1, mapped to the distal end of the short arm of chromosome 1B. The wheat cv. Chinese Spring (CS) and all CS nullisomic-tetrasomic lines were sensitive to the toxin, with the exception of N1BT1D. Insensitivity also was observed when the 1B chromosome of CS was substituted with the 1B chromosome of an insensitive accession of Triticum dicoccoides. In addition, a series of 1BS chromosome deletion lines were used to physically localize the sensitivity gene. Physical mapping indicated that Snn1 lies within a major gene-rich region on 1BS. This is the first report identifying a putative proteinaceous HST from S. nodorum and the chromosomal location of a host gene conferring sensitivity.

Molecular Variability of Mycosphaerella graminicola as Detected by RAPD Markers

AbstractA total of 90 isolates of Mycosphaerella graminicola, the cause of septoria tritici leaf blotch of wheat, were tested for DNA polymorphism using 15 decamer random primers. There was a high level of genetic variability among isolates. In 131 random amplified polymorphic DNA (RAPD) fragments, which were produced, 96% were polymorphic. Based on multilocus analysis, 40 different molecular phenotypes were detected. These molecular phenotypes were randomly distributed among sampling sites, suggesting that no clonal structure existed in the population. Cluster analysis showed that the maximum similarity value among isolates was approximately 81% and no identical isolates were detected, indicating that every isolate was a unique genotype. The high degree of DNA polymorphism, the large number of different molecular phenotypes, their random distribution and the results of the cluster analysis all suggested that sexual reproduction has a major role in the genetic structure of M. graminicola in western Canada. The presence of sexual reproduction provides the opportunity for development of new virulent genotypes in the population and suggests that the pathogen may adapt rapidly to any race‐specific sources of resistance. Therefore, when breeding for resistance to M. graminicola, emphasis should be placed on use of non‐race‐specific resistance.