A Large-Scale Survey of Races of Leptosphaeria maculans Occurring on Oilseed Rape in France

Nine avirulence genes (AvrLm1–AvrLm9) were identified in Leptosphaeria maculans, the causal agent of stem canker of oilseed rape (OSR), combinations of which could theoretically generate up to 512 different races of the fungus. L. maculans displays a high evolutionary potential to adapt to novel resistance genes as illustrated by the Rlm1 breakdown in France, where virulent populations became prevalent within three growing seasons. An improved knowledge of the race structure of the fungal population is therefore needed to ensure a better use of available major resistance genes. The objective of this study was to characterise the L. maculans population structure in France using a large-scale, rationalised sample of isolates. Experimental fields, planted with “trap plants” harbouring no major resistance gene, were sown at 20 locations. Single-pycnidium isolates were collected from leaf lesions that developed in early autumn and 1797 isolates were genotyped at Avr loci. The frequency of AvrLm6 and AvrLm7 was higher than 99%, whereas avrLm2 and avrLm9 alleles were fixed in the population. AvrLm1, AvrLm4, AvrLm5 and AvrLm8 were polymorphic. AvrLm3 isolates were detected at a very low frequency (less than 1%). Only 11 races were identified in France, with one race prevalent, namely Av5-6-7-(8) (i.e. virulent on Rlm1, Rlm2, Rlm3, Rlm4 and Rlm9), representing around 65% of the population. Disparities between the locations sampled were evident at all scales analysed. Some virulent races, such as those harbouring avrLm5, were present before the introduction of the corresponding resistance gene in the commercial OSR crop.     

Fitness Cost Associated with Loss of the AvrLm4 Avirulence Function in Leptosphaeria maculans (Phoma Stem Canker of Oilseed Rape)

Near-isogenic isolates of Leptosphaeria maculans differing at the AvrLm4 avirulence locus (AvrLm4 or avrLm4) were produced in vitro. Methods for inoculation of leaves of oilseed rape with ascospores or conidia were compared. The ‘ascospore shower’ inoculation was the most efficient method for use when inoculum is limited (e.g. ascospores produced in vitro). It was used in controlled environments to compare fitness of AvrLm4 and avrLm4 isolates at 5, 10, 15, 20 or 25 °C on leaves of oilseed rape cultivars Eurol and Darmor lacking the resistance gene Rlm4, which corresponds to AvrLm4. At all temperatures tested, AvrLm4 ascospores produced more lesions than avrLm4 ascospores. The diameters of lesions produced by AvrLm4 ascospores were greater than those of lesions produced by avrLm4 ascospores. At 15–20 °C, more lesions initiated by AvrLm4 ascospores produced pycnidia than did lesions initiated by avrLm4 ascospores. However, there were no differences between AvrLm4 and avrLm4 isolates in incubation period (from inoculation to appearance of lesions) or rate of mycelial growth in leaves from lesions towards the stems. In field experiments with winter oilseed rape cultivars lacking Rlm4, the frequency of AvrLm4 isolates increased from 5.7% at the phoma leaf lesion stage (autumn) to 20.5% at the stem canker stage (summer) during 2002/2003 and from 7.9 to 11.5% during 2003/2004 growing seasons. Results of controlled environment and field experiments indicate that avrLm4 isolates have a fitness cost compared to AvrLm4 isolates.     

Dual control of avirulence in Leptosphaeria maculans towards a Brassica napus cultivar with 'sylvestris-derived' resistance suggests involvement of two resistance genes

Blackleg disease (phoma stem canker) of Brassica napus (canola, oilseed rape) is caused by the fungus Leptosphaeria maculans . In some regions of Australia, resistance in oilseed rape cultivars derived from B. rapa subs . sylvestris (e.g. cv. Surpass 400) became ineffective within three years of commercial release. The genetic control of avirulence in L. maculans towards cv. Surpass 400 is described. When Australian field isolates were screened on this cultivar, three phenotypic classes were observed; virulent, intermediate and avirulent. Analysis of crosses between fungal isolates varying in their ability to infect cv. Surpass 400 demonstrated the presence of two unlinked avirulence genes, AvrLm1 and AvrLmS . Complementation of isolates (genotype avrLm1 ) with a functional copy of AvrLm1 , and genotyping of field isolates using a molecular marker for AvrLm1 showed that virulence towards Rlm1 is necessary, but not sufficient, for expression of a virulent phenotype on cv. Surpass 400. Taken together, these data strongly suggest that cv. Surpass 400, with ' sylvestris -derived' resistance, contains at least two resistance genes, one of which is Rlm1 .     

Analysis of Leptosphaeria maculans Race Structure in a Worldwide Collection of Isolates

ABSTRACT Leptosphaeria maculans, the causal agent of stem canker of oilseed rape, develops gene-for-gene interactions with its hosts. To date, eight L. maculans avirulence (Avr) genes, AvrLm1 to AvrLm8, have been genetically characterized. An additional Avr gene, AvrLm9, that interacts with the resistance gene Rlm9, was genetically characterized here following in vitro crosses of the pathogen. A worldwide collection of 63 isolates, including the International Blackleg of Crucifers Network collection, was genotyped at these nine Avr loci. In a first step, isolates were classified into pathogenicity groups (PGs) using two published differential sets. This analysis revealed geographical disparities as regards the proportion of each PG. Genotyping of isolates at all Avr loci confirmed the disparities between continents, in terms of Avr allele frequencies, particularly for AvrLm2, AvrLm3, AvrLm7, AvrLm8, and AvrLm9, or in terms of race structure, diversity, and complexity. Twenty-six distinct races were identified in the collection. A larger number of races (n = 18) was found in Australia than in Europe (n = 8). Mean number of virulence alleles per isolate was also higher in Australia (5.11 virulence alleles) than in Europe (4.33) and Canada (3.46). Due to the diversity of populations of L. maculans evidenced here at the race level, a new, open terminology is proposed for L. maculans race designation, indicating all Avr loci for which the isolate is avirulent.     

Fitness cost of virulence differs between the <Emphasis Type="Italic">AvrLm1</Emphasis> and <Emphasis Type="Italic">AvrLm4</Emphasis> loci in <Emphasis Type="Italic">Leptosphaeria maculans</Emphasis> (phoma stem canker of oilseed rape)

To investigate whether the reported fitness cost of virulence at the AvrLm4 locus in Leptosphaeria maculans is common to other loci, near-isogenic (NI) isolates differing at AvrLm1 locus were produced in vitro. Fitness of virulent (avrLm1) or avirulent (AvrLm1) isolates on Brassica napus without the corresponding R (resistance) gene Rlm1 was investigated in controlled environment (CE) and field experiments. Results indicate that there is a measurable fitness cost for avrLm1 compared to AvrLm1 isolates in terms of number of lesions, size of lesions, distance grown through leaf tissue towards the petiole in CE experiments and systemic growth from leaf lesions to stems in field experiments. There were differences in fitness cost between the AvrLm1 and AvrLm4 loci. There was a cultivar effect on fitness cost of virulence at the AvrLm1 locus but not at the AvrLm4 locus. In CE experiments, the optimal temperature for leaf infection was greater for AvrLm4 isolates than for AvrLm1 isolates. Field experiment results suggest that on the same host AvrLm4 isolates are more fit than AvrLm1 isolates in warmer seasons. The fitness cost at the AvrLm4 locus was generally greater than at the AvrLm1 locus, suggesting that the corresponding R gene Rlm4 may be more suitable than Rlm1 for redeployment in commercial cultivars after an interval of a few years.     

Genetic Control and Host Range of Avirulence Toward Brassica napus Cultivars Quinta and Jet Neuf in Leptosphaeria maculans

ABSTRACT Leptosphaeria maculans causes blackleg of oilseed rape. Gene-for-gene interactions between race PG3 and Brassica napus cv. Quinta were related to interaction between the fungal avirulence (Avr) gene AvrLm1 and the corresponding resistance gene Rlm1. AvrLm1 isolates were aviru-lent on cvs. Doublol, Vivol, Columbus, and Capitol, and no recombinant phenotypes were observed in the progeny of two AvrLm1 x avrLm1 crosses, suggesting that all of these cultivars may possess Rlm1 or genes displaying the same recognition spectrum, or that a cluster of Avr genes is present at the Avrlm1 locus. In one cross, segregation distortion was observed at the AvrLm1 locus that could be explained by interaction between AvrLm1 and one unlinked deleterious gene, termed Del1. Incompatibility toward cvs. Jet Neuf and Darmor.bzh was governed by a single gene, unlinked to AvrLm1 or Del1. This avirulence gene was termed AvrLm4. Preliminary plant genetic analysis suggested the occurrence of a corresponding dominant resistance gene, termed Rlm4, present in the Quinta line analyzed and linked to Rlm1.     

A 10-year Survey of Populations of Leptosphaeria maculans in France Indicates a Rapid Adaptation Towards the Rlm1 Resistance Gene of Oilseed Rape

Leptosphaeria maculans, the cause of stem canker of oilseed rape (OSR), exhibits gene-for-gene interactions with its host plant. The race structure of L. maculans was assessed on the basis of the analysis of 1011 isolates collected in France between 1990 and 2000, with regards to three AVR genes, AvrLm1, AvrLm2 and AvrLm4. The effect of selection pressure, due to large-scale cropping of Rlm1 cultivars, on the evolution of races of the fungus was also evaluated. The results revealed a scarcity or complete absence of isolates harbouring AvrLm2, whereas isolates harbouring AvrLm4 were present at a variable level, that was as high as 17.2–31.2% depending on the sample year and location. When obtained from rlm1 cultivars, isolates harbouring AvrLm1 always represented more than 83% of the populations until the 1997–1998 growing season. As a consequence, the Rlm1 cultivars had been highly efficient at controlling the disease and were grown on an estimated 43.7% of the total French acreage in OSR in 1998–1999. However, the increased commercial success of Rlm1 cultivars was paralleled by a decrease in the proportion of isolates harbouring AvrLm1 in 1997–1998 and 1998–1999. This resulted in less than 13% of isolates harbouring AvrLm1 in populations being collected from rlm1 cultivars in 1999 and 2000, and contributed to the loss of efficiency of the Rlm1 resistance in the field. The present study is an illustration of one round of a `boom and bust' cycle that occurred for a pathosystem where it has never been reported before. These data and the high evolutionary potential of L. maculans are fully supportive of one pathogen species with a high risk of breaking down resistance genes in OSR and suggest that the development of integrated strategies aiming at maximising the durability of novel resistance is now a priority for this pathosystem.     

Determining frequencies of avirulent alleles in airborne Leptosphaeria maculans inoculum using quantitative PCR

Phoma stem canker (blackleg disease) of Brassica napus (oilseed rape, canola) is caused by the fungus Leptosphaeria maculans. Frequencies of avirulent alleles for loci where virulence can be associated with gene deletion (AvrLm1 and AvrLm6) were determined in samples of L. maculans airborne ascospore inoculum using quantitative PCR. The accuracy, reproducibility and limitations of detection were determined. Changes in the frequency of avirulent alleles were determined for the 2006/2007, 2007/2008 and 2008/2009 growing seasons for winter oilseed rape in the UK. The frequency of AvrLm1 remained small (between 9% and 16%), whilst the frequency of AvrLm6 fluctuated between 35% and 66%. Estimation of frequencies of avirulent alleles in airborne pathogen inoculum gives an efficient and unbiased method to assess the potential of crop cultivars with corresponding resistance genes being at risk of disease.     

Effectiveness of Rlm7 resistance against Leptosphaeria maculans (phoma stem canker) in UK winter oilseed rape cultivars

The Rlm7 gene in Brassica napus is an important source of resistance for control of phoma stem canker on oilseed rape caused by the fungus Leptosphaeria maculans. This study shows the first report of L. maculans isolates virulent against Rlm7 in the UK. Leptosphaeria maculans isolates virulent against Rlm7 represented 3% of the pathogen population when cultivars with the Rlm7 gene represented 5% of the UK oilseed rape area in 2012/13. However, the Rlm7 gene has been widely used since then, representing >15% of the UK oilseed rape area in 2015/16. Winter oilseed rape field experiments included cultivars with the Rlm7 gene, with the Rlm4 gene or without Rlm genes and took place at five sites in the UK over four cropping seasons. An increase in phoma leaf spotting severity on Rlm7 cultivars in successive seasons was observed. Major resistance genes played a role in preventing severe phoma leaf spotting at the beginning of the cropping season and, in addition, quantitative resistance (QR) in the cultivars examined made an important contribution to control of phoma stem canker development at the end of the cropping season. Deployment of the Rlm7 resistance gene against L. maculans in cultivars with QR in combination with sustainable disease management practices will prolong the use of this gene for effective control of phoma stem canker epidemics.     

Race structure and frequency of avirulence genes in the western Canadian Leptosphaeria maculans pathogen population,the causal agent of blackleg in brassica species

Leptosphaeria maculans is the causal agent of blackleg, a serious disease on canola/rapeseed in western Canada, Australia and Europe. Genetic resistance and extended crop rotation provided effective disease control in western Canada for years but the emergence of new pathogen races has reduced the effectiveness of current management strategies. The objective of this study was to analyse L. maculans isolates derived from canola stubble in commercial fields collected in 2010 and 2011 across western Canada for the presence and frequency of avirulence (Avr) genes. A total of 674 isolates were examined for the presence of Avr alleles AvrLm1, AvrLm2, AvrLm3, AvrLm4, AvrLm6, AvrLm7, AvrLm9, AvrLepR1, AvrLepR2 and AvrLmS using a set of differential host genotypes carrying known resistance genes or PCR amplification of AvrLm1, AvrLm6 and AvrLm4–Lm7. Certain alleles were more prevalent in the pathogen population, with AvrLm6 and AvrLm7 present in >85% of isolates, while AvrLm3, AvrLm9 and AvrLepR2 were present in <10% of isolates. A total of 55 races (different combinations of Avr alleles) were detected, with the two most common ones being AvrLm2–Lm4–Lm6–Lm7 and AvrLm2–Lm4–Lm6–Lm7–LmS. Races carrying as many as seven and as few as one known Avr allele were detected. Selection pressure from the race‐specific resistance genes carried in canola cultivars has probably played a significant role in the current Avr profile, which may have also contributed to the recent increase in blackleg observed in western Canada.     

Effect of rotation of canola (Brassica napus) cultivars with different complements of blackleg resistance genes on disease severity

Blackleg disease (phoma stem canker) caused by the fungus Leptosphaeria maculans is a major disease of canola (oilseed rape, Brassica napus) worldwide. Canola plants in pots were exposed to blackleg‐infested stubble of canola with different complements of resistance genes and then assessed for disease. Plant mortality was reduced when plants were exposed to stubble from a cultivar with a different complement of resistance genes compared to stubble of a cultivar with the same resistance gene. These findings were consistent with 7 years of field surveys, which showed that changes in selection pressure as a result of extensive sowing of cultivars with major‐gene resistance, termed ‘sylvestris resistance’, dramatically influenced the frequency of virulent isolates in the population towards particular resistance genes, and therefore disease severity. All these data were supported by PCR‐genotyping surveys of fungal populations whereby the frequency of virulence alleles of avirulence genes AvrLm1 and AvrLm4 changed significantly depending on the resistance gene present in the cultivar from which the isolates were cultured. This is the first example of a study showing that sowing of canola cultivars with different complements of resistance genes in subsequent years, i.e. rotation of resistance genes, minimizes disease pressure by manipulating fungal populations. This approach provides a valuable disease management strategy for canola growers and is likely to be applicable to other plant diseases.     

New Avirulence Genes in the Phytopathogenic Fungus Leptosphaeria maculans

ABSTRACT Leptosphaeria maculans, the causal agent of stem canker of oilseed rape (Brassica napus), develops gene-for-gene interactions with oilseed rape, and four L. maculans avirulence (AVR) genes (AvrLm1, AvrLm2, AvrLm4, and alm1) were previously genetically characterized. Based on the analysis of progeny of numerous in vitro crosses between L. maculans isolates showing either already characterized or new differential interactions, this work aims to provide an overview of the AVR genes that may specify incompatibility toward B. napus and the related species B. juncea and B. rapa. Two novel differential interactions were thus identified between L. maculans and B. napus genotypes, one of them corresponding to a complete resistance to European races of L. maculans. In both cases, a single gene control of avirulence was established (genes AvrLm3 and AvrLm7). Similarly, a single gene control of avirulence toward a B. rapa genotype, also resistant to European L. maculans isolates, was demonstrated (gene AvrLm8). Finally, a digenic control of avirulence toward B. juncea was established (genes AvrLm5 and AvrLm6). Linkage analyses demonstrated that at least four unlinked L. maculans genomic regions, including at least one AVR gene cluster (AvrLm1-AvrLm2-AvrLm6), are involved in host specificity. The AvrLm3-AvrLm4-AvrLm7 region may correspond either to a second AVR gene cluster or to a multiallelic AVR gene.     

Dissemination of Information About Management Strategies and Changes in Farming Practices for the Exploitation of Resistance to Leptosphaeria maculans (Phoma Stem Canker) in Oilseed Rape Cultivars

The management of phoma stem canker (blackleg disease, caused by Leptosphaeria maculans) is an integral component of oilseed rape production. In this paper, we discuss the information about management strategies that is disseminated in Europe and Australia. New cultivars have been introduced with improved resistance to disease, but sometimes this resistance has been overcome as new races of the pathogen have emerged. When cultivars with single major gene resistance have been introduced into areas with high inoculum concentrations, significant economic damage has been caused by new races of L. maculans within 2–3 years. Quantitative or polygenic resistance has also been used successfully against stem canker and offers more durable disease resistance if plant breeders and farmers deploy this resistance more effectively. Strategies to improve the durability of resistance need to be developed and tested in practice. New information on the occurrence of virulence and avirulence genes in populations of Leptosphaeria maculans and modelling of the durability of resistance provide opportunities for plant breeders, specialist technical organisations, cooperatives, advisory services and farmers to collaborate and better exploit cultivar resistance. Changing economic and environmental factors influence cropping practices and, if to be considered successful, management strategies must show clear financial benefits. Technology transfer will need to address all aspects of managing stem canker and other diseases of oilseed rape and using effective written, verbal and electronic methods of communication.     

Major gene resistance in Brassica napus (oilseed rape) is overcome by changes in virulence of populations of Leptosphaeria maculans in France and Australia

Resistance of Brassica napus (oilseed rape, canola) conferred by three different major resistance genes has been overcome by changes in virulence of Leptosphaeria maculans populations in France and Australia. In South Australia where B. napus cultivars with major gene resistance derived from Brassica rapa ssp. sylvestris were grown extensively, resistance was rendered ineffective within 3 years of commercial release of the cultivar. Disease severity was higher on cultivars with sylvestris-derived resistance than cultivars with polygenic resistance. This Australian situation is compared to that in France, where resistance conferred by the Rlm1 gene was overcome nation-wide in 5 years under commercial cropping practices, and also where a source of resistance introgressed into B. napus from B. juncea was rendered inefficient in 3 years in experimental field plots near Rennes.     

The Leptosphaeria maculans – Leptosphaeria biglobosa species complex in the American continent

Stem canker of oilseed rape (canola, Brassica napus ) is associated with a species complex of two closely related fungal species, Leptosphaeria maculans and L. biglobosa . Of these, L. maculans is the most damaging and develops gene-for-gene relationships with the host . Here, a wide scale analysis of the L. maculans - L. biglobosa species complex was performed throughout the American continent (23 locations from Chile to Canada) plus several locations in Western Australia for comparison purposes, based on a collection of 1132 isolates from infected tissues of a susceptible cultivar. Fungal species were discriminated on the basis of morphological, phytopathological and molecular criteria and showed that L. biglobosa was closely associated with L. maculans in most of the locations. Multiple gene phylogeny using sequences of ITS, actin and β-tubulin confirmed the prevalence of the L. biglobosa 'canadensis' sub-clade in Canada, whereas up to three different sub-clades of L. biglobosa were found in Georgia (USA). Race structure of L. maculans was investigated using a combination of pathogenicity tests and PCR amplification of avirulence alleles AvrLm1 , AvrLm4 and AvrLm6 . Three contrasting situations were observed: (i) race structure in Ontario, Chile and Georgia was related to that of European and Western Australian populations, with a low race diversity; (ii) only one race was found in Mexico, and not found outside of this country; (iii) a large diversity of races was observed in central Canada (Manitoba, Alberta and Saskatchewan) with very specific features including maintenance of avirulence alleles absent from Europe, absence of the AvrLm7 allele common in Europe (or eastern Canada) and wide location-to-location variability.     

Distribution ofLeptosphaeria maculans in two fields in southern Ontario as determined by the polymerase chain reaction

The distribution of the highly virulent and weakly virulent types ofLeptosphaeria maculans, causal agent of blackleg of oilseed rape, was studied in two fields in southern Ontario. Using a polymerase chain reaction-based assay with primers specific for these virulence types, plant tissues were directly examined for the occurrence of the pathogen. The highly virulent type was detected in leaf, stem and crown tissue at most of the sampling sites. The weakly virulent type was detected only in leaf lesions at 50% of the sites in field 1 and 30% of the sites in field 2. Of 96 leaf lesions examined, 48 contained the highly virulent type, 12 contained the weakly virulent type and 16 contained both the highly virulent and weakly virulent types.Sclerotinia sclerotiorum was isolated from all leaf lesion that did not react withL. maculans virulence type-specific primers. Coinfection of single blackleg leaf lesions by bothL. maculans virulence types thus occurred in oilseed rape plants in the field. Only the highly virulent type was detected in pseudothecia on stubble. Approximately 1% of the seed collected from these two fields containedL. maculans, and both the highly virulent and weakly virulent types were detected. The highly virulent type was more prevalent and non-specific in the kind of plant tissue it infected, whereas the weakly virulent type appeared to be limited to infecting leaves and seed. This study illustrates an application of polymerase chain reaction with virulence type-specific oligonucleotide primers to study the epidemiology of blackleg of oilseed rape.     

A Cluster of Major Specific Resistance Genes to Leptosphaeria maculans in Brassica napus

ABSTRACT Two types of genetic resistance to Leptosphaeria maculans usually are distinguished in Brassica napus: qualitative, total resistance expressed at the seedling stage and quantitative, partial resistance expressed at the adult plant stage. The latter is under the control of many genetic factors that have been mapped through quantitative trait loci (QTL) studies using 'Darmor' resistance. The former usually is ascribed to race-specific resistance controlled by single resistance to L. maculans (Rlm) genes. Three B. napus-originating specific Rlm genes (Rlm1, Rlm2, and Rlm4) previously were characterized. Here, we report on the genetic identification of two novel resistance genes, Rlm3 and Rlm7, corresponding to the avirulence genes AvrLm3 and AvrLm7. The identification of a novel L. maculans- B. napus specific interaction allowed the detection of another putative new specific resistance gene, Rlm9. The resistance genes were mapped in two genomic regions on LG10 and LG16 linkage groups. A cluster of five resistance genes (Rlm1, Rlm3, Rlm4, Rlm7, and Rlm9) was strongly suggested on LG10. The relation between all these specific resistance genes and their potential role in adult-plant field resistance is discussed. These two Rlm-carrying regions do not correspond to major QTL for Darmor quantitative resistance.     

Effects of fungicides on in vitro spore germination and mycelial growth of the phytopathogens Leptosphaeria maculans and L. biglobosa (phoma stem canker of oilseed rape)

BACKGROUND: Phoma stem canker, caused by the coexisting related fungal pathogens Leptosphaeria maculans (Des.) Ces. & de Not and L. biglobosa Shoemaker & H Brun, is a major disease of winter oilseed rape in the UK. Annually, over 90% of UK crops receive at least one foliar application of fungicide, but little is known about the sensitivity of the more damaging L. maculans and the less damaging L. biglobosa to these fungicides. The effects of flusilazole, tebuconazole and Methyl Benzimidazole Carbamate (MBC) fungicides (benomyl and carbendazim) on the germination of ascospores, conidia and germ tube growth of both species were examined. Isolates collected from different oilseed rape crops in England and Wales were assessed for their mycelial growth on fungicide‐amended medium, and ED₅₀ values were calculated. RESULTS: Leptosphaeria maculans and L. biglobosa differed in their sensitivity to fungicides. Conidial germination of L. maculans was more sensitive to these fungicides than that of L. biglobosa. Isolates of L. maculans had smaller ED₅₀ values for mycelial growth for all fungicides tested than isolates of L. biglobosa. CONCLUSION: These results suggest that fungicide applications might affect the structure of L. maculans/L. biglobosa populations in UK winter oilseed rape crops. Copyright © 2009 Society of Chemical Industry     

World-Wide Importance of Phoma Stem Canker (Leptosphaeria maculans and L. biglobosa) on Oilseed Rape (Brassica napus)

Phoma stem canker is an internationally important disease of oilseed rape (Brassica napus, canola, rapeseed), causing serious losses in Europe, Australia and North America. UK losses of €56M per season are estimated using national disease survey data and a yield loss formula. Phoma stem canker pathogen populations comprise two main species, Leptosphaeria maculans, associated with damaging stem base cankers, and Leptosphaeria biglobosa, often associated with less damaging upper stem lesions. Both major gene and quantitative trait loci mediated resistance to L. maculans have been identified in B. napus, but little is known about resistance to L. biglobosa. Leptosphaeria maculans, which has spread into areas in North America and eastern Europe where only L. biglobosa was previously identified, now poses a threat to large areas of oilseed rape production in Asia. Epidemics are initiated by air-borne ascospores; major gene resistance to initial infection by L. maculans operates in the leaf lamina of B. napus. It is not clear whether the quantitative trait loci involved in the resistance to the pathogen that can be assessed only at the end of the season operate in the leaf petioles or stems. In countries where serious phoma stem canker epidemics occur, a minimum standard for resistance to L. maculans is included in national systems for registration of cultivars. This review provides a background to a series of papers on improving strategies for managing B. napus resistance to L. maculans, which is a model system for studying genetic interactions between hemi-biotrophic pathogens and their hosts.