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.     

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.     

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.     

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.     

Frequency of Avirulence Alleles in Field Populations of Leptosphaeria maculans in Europe

This paper describes the first large-scale Europe-wide survey of avirulence alleles and races of Leptosphaeria maculans. Isolates were collected from the spring rape cultivar Drakkar, with no known genes for resistance against L. maculans, at six experimental sites across the main oilseed rape growing regions of Europe, including the UK, Germany, Sweden and Poland. Additionally in Poland isolates were collected from cv. Darmor, which has resistance gene, Rlm9. In total, 603 isolates were collected during autumn in 2002 (287 isolates from Germany and the UK) and 2003 (316 isolates from Poland and Sweden). The identity of alleles at eight avirulence loci was determined for these isolates. No isolates had the virulence allele avrLm6 and three virulence alleles (avrLm2, avrLm3 and avrLm9) were present in all isolates. The isolates were polymorphic for AvrLm1, AvrLm4, AvrLm5 and AvrLm7 alleles, with virulence alleles at AvrLm1 and AvrLm4 loci and avirulence alleles at AvrLm7 and AvrLm5 loci predominant in populations. Virulent avrLm7 isolates were found at only one site in Sweden. Approximately 90% of all isolates belonged to one of two races (combinations of avirulence alleles), Av5-6-7 (77% of isolates) or Av6-7 (12%). Eight races were identified, with four races at frequencies less than 1%. The study suggested that Rlm6 and Rlm7 are still effective sources of resistance against L. maculans in oilseed rape in Europe. The results are comparable to those of a similar survey done in France in autumn 2000 and 2001.     

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.     

禾谷类白粉菌无毒基因研究进展

基因对基因假说阐明了病原菌无毒基因(avirulence gene,Avr gene)与寄主植物抗性基因(resistance gene,Rgene)相互识别和互作的关系。禾谷类白粉菌无毒基因产物作为重要的激发子,能够与其寄主R基因产物发生特异性互作,诱导植物细胞防卫反应。为了更加深入地了解这些无毒基因的作用,作者总结了最近关于AVRa1、AVRa10、AVRa13、AVRk1、AvrPm2和AvrPm3a2/f2等已克隆无毒基因的研究进展,讨论了它们作为效应蛋白(effector)或激发子的双重功能,在毒性菌株中的变异规律,与其对应R基因之间的互作模型,以及与转座子等重复序列之间的关联等。本文还对无毒基因研究方法的改进和未来的研究方向提出了建议。     

The inheritance of virulence of Phytophthora infestans to potato

Of 31 matings between isolates of P. infestans from several countries, six yielded enough progeny for analysis of inheritance of the virulence phenotype. Virulence was determined in vitro after inoculation of detached leaflets of nine differential lines of potato, each carrying a different gene for resistance. Parents of three matings carried an isozyme marker (glucosephosphate isomerase) which allowed the hybridity of most progeny to be confirmed. Apparently non-hybrid progeny from all three matings were probably selfs or apomicts; these were discarded. The inheritance of virulence in two sib-cross and one backcross family was determined. Patterns of inheritance in F₁ and F₂ indicated the presence of a gene-for-gene interaction in which alleles of a single locus in the pathogen conditioned virulence or avirulence on each differential. Although the hypothesis that avirulence alleles were dominant and virulence alleles were recessive was supported by many of the data, unexpected segregations were obtained. Alternative hypotheses to explain the latter included low aggressiveness in a proportion of the progeny, a second locus inhibiting avirulence in one parent, a different locus in each parent determining avirulence/virulence on one R-gene, and dominance of some alleles determining virulence. Avirulent field isolates appeared to be heterozygous ( A vravr ) rather than homozygous ( Avr Avr ) at avirulence loci. A somatic segregation from avirulence to virulence at three avirulence loci was postulated for one parental isolate. Evidence for linkage of these three loci suggested that the observed somatic segregation resulted from mitotic crossing-over.     

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.     

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.     

A Gene-for-Gene Relationship Underlying the Species-Specific Parasitism of Avena/Triticum Isolates of Magnaporthe grisea on Wheat Cultivars

ABSTRACT To elucidate genetic mechanisms of the species-specific parasitism of Magnaporthe grisea, a Triticum isolate (pathogenic on wheat) was crossed with an Avena isolate (pathogenic on oat), and resulting F(1) progeny were subjected to segregation analyses on wheat cvs. Norin 4 and Chinese Spring. We found two fungal loci, Pwt3 and Pwt4, which are involved in the specific parasitism on wheat. Pwt3 operated on both cultivars while Pwt4 operated only on 'Norin 4'. Using the cultivar specificity of Pwt4, its corresponding resistance gene was successfully identified in 'Norin 4' and designated as Rmg1 (Rwt4). The presence of the corresponding resistance gene indicated that Pwt4 is an avirulence locus. Pwt3 was assumed to be an avirulence locus because of its temperature sensitivity. We suggest that gene-for-gene interactions underlie the species-specific parasitism of M. grisea.     

Genetic Analysis and Identification of Amplified Fragment Length Polymorphism Markers Linked to the alm1 Avirulence Gene of Leptosphaeria maculans

ABSTRACT A gene-for-gene interaction was previously suggested by mapping of a single major locus (LEM 1) controlling cotyledon resistance to Leptosphaeria maculans isolate PHW1245 in Brassica napus cv. Major. In this study, we obtained further evidence of a gene-for-gene interaction by studying the inheritance of the corresponding avirulence gene in L. maculans isolate PHW1245. The analysis of segregating F(1) progenies and 14 test crosses suggested that a single major gene is involved in the interaction. This putative avirulence gene was designated alm1 after the resistance locus identified in B. napus. Amplified fragment length polymorphism (AFLP) markers were used to generate a rudimentary genetic linkage map of the L. maculans genome and to locate markers linked to the putative avirulence locus. Two flanking AFLP markers, AC/TCC-1 and AC/CAG-5, were linked to alm1 at 3.1 and 8.1 cM, respectively. Identification of markers linked to the avirulence gene indicated that the differential interaction is controlled by a single gene difference between parental isolates and provides further support for the gene-for-gene relationship in the Leptosphaeria-Brassica system.     

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.     

Genetics of avirulences in Erysiphe graminis f.sp. hordei

The genetics of avirulences towards barley mildew resistances were analysed in crosses of the Ervsiphe graminis f.sp. hordei isolate DH14 with CC107 and with CC138. Nine avirulences, Av r_a9, Avr _a10, Avr _a11, Avr _a12, Avr _Ab, Avr _CP, Avr _h, Avr _k and Avr _La, segregated as single genes in one or other cross. However. F₁ segregation data were consistent with avirulence matching the Mla7 resistance gene being controlled by two genes, designated Avr _a7 1 and Avr _a7 2. Infection types of avirulent isolates differed on varieties in which Mla7 had been derived from each of the four known sources of that resistance. Linkage was detected between Avr _a7 1 and Avr _h in the cross CC107 × DH14, and between Avr _a10 and Avr _k, Avr _a11 and Avr _La, and Avr _h and the triadimenol response gene Tdl2 in CC138 × DH14.     

番茄与叶霉菌互作的分子机理

 番茄和叶霉菌(Cladosporium fulvum Cooke)系统是研究植物和病原物互作分子机理的模式系统。4个叶霉菌无毒基因已被克隆。这些无毒基因编码含偶数个半胱氨酸的小分子量蛋白,在叶霉菌毒性菌株中的存在与否及存在方式各不相同。Avr4具有几丁质结合活性;而Avr9可能在叶霉菌氮素代谢中起调节作用。7个具有功能的番茄抗叶霉菌Cf基因已被克隆。它们与其同源基因一起以基因簇形式存在于复合基因座中,其成员称为Hcr (homologues of C. fulvum resistance gene Cf)基因。Cf蛋白定位于细胞质膜,但主体在膜外,主要结构域为富含亮氨酸重复(LRR)和跨膜结构域。LRR重复单元数目以及N端序列决定了Cf蛋白对Avr的识别特异性。Cf对Avr的识别遵守"保卫"假说。在Cf-2对Avr2的识别系统中,蛋白酶Rcr3为"被保卫"蛋白。Cf识别Avr后迅速激活下游信号传导和防卫反应,包括过敏性反应的产生和氧化迸发,以及K⁺离子通道、各类蛋白激酶、SGT1、硫氧还蛋白及磷脂酸途径的活化。温度、湿度和光照等环境条件显著影响Cf介导的过敏性反应和抗病性。不同Cf对Avr的识别机理及其下游信号传导途径有显著差别。     

Genetics of Host-Pathogen Relationships Between Venturia inaequalis Races 6 and 7 and Malus Species

ABSTRACT Resistance to scab originating from Malus floribunda clone 821 is the most widely form of resistance used in apple breeding programs. A dominant gene, named Vf, was introgressed from this clone into recent cultivars, although the genetic determinants of the resistance of M. floribunda 821 are apparently more complex than a single gene. The appearance of new races overcoming the resistance of cultivars with the Vf gene, the parental clone, or both made it possible to undertake a genetic analysis of host-pathogen interactions. The segregation of resistance in progenies of crosses from 'Golden Delicious' x M. floribunda 821 and 'Golden Delicious' x 'Idared' into five strains of Venturia inaequalis-races 1 (strains 104, 1093, and 301), 6 (strain 302), and 7 (strain 1066)-demonstrated the existence of a second dominant gene in M. floribunda 821. This gene, independent of Vf, was named Vfh because it seemed to induce a hypersensitive reaction. The results obtained with strain 1066, virulent to M. floribunda 821, allowed identification of another dominant gene, Vg, responsible for the resistance of 'Golden Delicious' to this strain. Vg is also carried by 'Florina', which was selected for its Vf resistance. The pathogenicity of a progeny originating from a cross between V. inaequalis strains 1066 and 301, characterized in vitro on leaf disks of differential genotypes, revealed two independent avirulence genes involved in the pathogenicity toward the Vg and Vf genes, respectively. These avirulence genes were named Avr Vg and Avr Vf. The host-pathogen interactions detected are consistent with a gene-for-gene relationship.     

小麦白粉菌无毒位点的遗传特性

 通过小麦白粉菌菌株之间的有性杂交,获得1个白粉菌杂交分离群体。对该群体中各个体在鉴别寄主上进行了无毒性/毒性反应的测定,结果表明:对应于小麦抗病基因Pm 4a、Pm4b和品种白免3号、Era抗性的小麦白粉菌无毒性分别由1对基因控制;对应于Pm 3b的无毒性/毒性分离在杂交群体中偏离1对或2对基因的分离比例。对应于Pm4a、Pm 4b和白免3号抗性的3个无毒位点之间完全连锁,在杂交群体中共分离;该3个位点与Era抗性对应的无毒位点之间不具有连锁性。     

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.     

Genetic Analysis of Avirulence/Virulence of an Isolate of Magnaporthe grisea from a Rice Field in Texas

ABSTRACT An isolate of Magnaporthe grisea, Tm4, from a rice field in Texas was crossed with a fertile laboratory strain, 70-6. The progenies showed segregation of avirulence/virulence on rice cvs. Newbonnet, Lemont, Lebonnet, Leah, and Katy. The avirulent/virulent segregation ratios were 29:6 on Newbonnet, Lemont, and Lebonnet; 28:7 on Leah; and 33:2 on Katy. There was cosegregation on the first three cultivars. Several avirulent progenies were backcrossed to virulent parent 70-6. Three generations of backcrossing avirulent progenies to 70-6 led to segregation ratios that suggested certain strains had only one avirulence gene. Strains avirulent only on cv. Katy or only on cvs. Newbonnet, Lemont, and Lebonnet were test crossed with virulent siblings. Strains that gave progeny ratios approximating 1 avirulent:1 virulent when crossed with virulent siblings were selected for further test crossing. Intercrosses between strains with possible single avirulence genes were made to determine whether these strains had the same or different avirulence genes. Many lines still segregated two genes for avirulence after three generations of backcrossing. This is based on the recovery of virulent progenies from crossing two avirulent siblings.