Inheritance of Race-Specific Resistance to Xanthomonas campestris pv. campestris in Brassica Genomes

ABSTRACT The inheritance of resistance to three Xanthomonas campestris pv. campestris races was studied in crosses between resistant and susceptible lines of Brassica oleracea (C genome), B. carinata (BC genome), and B. napus (AC genome). Resistance to race 3 in the B. oleracea doubled haploid line BOH 85c and in PI 436606 was controlled by a single dominant locus (Xca3). Resistance to races 1 and 3 in the B. oleracea line Badger Inbred-16 was quantitative and recessive. Strong resistance to races 1 and 4 was controlled by a single dominant locus (Xca1) in the B. carinata line PI 199947. This resistance probably originates from the B genome. Resistance to race 4 in three B. napus lines, cv. Cobra, the rapid cycling line CrGC5, and the doubled haploid line N-o-1, was controlled by a single dominant locus (Xca4). A set of doubled haploid lines, selected from a population used previously to develop a restriction fragment length polymorphism map, was used to map this locus. Xca4 was positioned on linkage group N5 of the B. napus A genome, indicating that this resistance originated from B. rapa. Xca4 is the first major locus to be mapped that controls race-specific resistance to X. campestris pv. campestris in Brassica spp.     

Identification and Origin of Xanthomonas campestris pv. campestris Races and Related Pathovars

ABSTRACT One hundred sixty-four isolates of Xanthomonas campestris pv. campestris and other X. campestris pathovars known to infect cruciferous hosts (X. campestris pvs. aberrans, raphani, armoraciae, and incanae) were inoculated onto a differential series of Brassica spp. to determine both pathogenicity to brassicas and race. Of these, 144 isolates were identified as X. campestris pv. campestris and grouped into six races, with races 1 (62%) and 4 (32%) being predominant. Other races were rare. The remaining 20 isolates from brassicas and other cruciferous hosts were either nonpathogenic or very weakly pathogenic on the differential series and could not be race-typed. Five of these isolates, from the ornamental crucifers wallflower (Cheiranthus cheiri), stock (Matthiola incana) and candytuft (Iberis sp.), showed clear evidence of pathovar-like specificity to the hosts of origin. A gene-for-gene model based on the interaction of four avirulence genes in X. campestris pv. campestris races and four matching resistance genes in the differential hosts is proposed. Knowledge of the race structure and worldwide distribution of races is fundamental to the search for sources of resistance and for the establishment of successful resistance breeding programs.     

Race-specific reaction of resistance to black rot in Brassica oleracea

Several black rot-resistant varieties of Brassica oleracea showed a race-specific hypersensitive response (HR) to inoculation with Xanthomonas campestris pv. campestris isolates of different races. In progenies of cabbage line PI436606, Portuguese kale ISA454 and Chinese kale SR1 the HR to race 1 of the pathogen was controlled by a dominant gene named R1, when a recessive gene r5 was responsible for the HR to race 5. Genes with a similar race-specific reaction were assumed on the basis of gene-for-gene interaction in black rot-resistant Japanese cabbage cultivars and double haploid lines obtained from them. Homology of gene r5 in cabbage lines PI436606, Fujiwase 01 and kale ISA454 was postulated in crosses between those lines or their progenies. In a cross between SR1 and PI436606, interaction between resistance to race 1 and non-specific resistance localized in the stem vascular system was found. On the basis of pedigree information and homology of resistance genes in the cultivars of East-Asian cabbage and Portuguese kales, the probable origin of race-specific resistance to black rot of cole crops was suggested to be in heading Mediterranean kale. Some evidence was found for a gene conferring resistance to race 4 in B. oleracea.     

Potential for using host resistance to reduce production of pseudothecia and ascospores of Leptosphaeria maculans, the blackleg pathogen of Brassica napus

Pseudothecial density of the blackleg fungus Leptosphaeria maculans and discharge of ascospores was measured from stubble of a range of Brassica species, including Brassica napus (canola) cultivars, with a range of blackleg resistance. Since ascospores are the primary inoculum, these parameters reflect inoculum potential for blackleg. Stubble from a representative line of each of B. carinata , B. nigra , Sinapis alba and B. napus cv. Surpass 400 (incorporates blackleg resistance from B. rapa ssp. sylvestris ) had lower pseudothecial density and discharged fewer ascospores than stubble of other B. napus cultivars (Karoo, Oscar, Emblem, Dunkeld and Columbus). These latter B. napus cultivars and a representative B. juncea line had higher pseudothecial densities and discharged higher numbers of ascospores. If this trait of low blackleg inoculum from stubble could be introgressed into commercial canola cultivars, blackleg disease severity could be substantially reduced, resulting in higher and more stable canola yields. However, the trait of reduced ascospore discharge may not be stable, as demonstrated by the B. rapa ssp. sylvestris -derived resistance already being overcome by the blackleg fungus in Australia.     

Characterization,genetic diversity and distribution of Xanthomonas campestris pv. campestris races causing black rot disease in cruciferous crops of India

Black rot, caused by Xanthomonas campestris pv. campestris (Xcc) is a disease of crucifer crops. The objective of this study was to characterize races of Xcc, their distribution and genetic diversity in India. Two hundred and seventeen isolates of bacteria were obtained from 12 different black rot‐infected crucifer crops from 19 states of India; these were identified as Xcc based on morphology, hrpF gene and 16S rRNA gene based molecular markers and pathogenicity tests. Characterization of races was performed by using a set of seven differential crucifer hosts, comprising two cultivars of turnip (Brassica rapa var. rapa) and cultivars of Indian mustard (B. juncea), Ethiopian mustard (B. carinata), rapeseed mustard (B. napus), cauliflower (B. oleracea) and Savoy cabbage (B. oleracea var. sabauda). Races 1, 4 and 6 of Xcc were identified and, among these races, race 1 followed by race 4 dominated most of the states of India. Genetic diversity of the Indian isolates of Xcc was analysed using repetitive sequence‐based PCR (rep‐PCR) including primers for REP (repetitive extragenic palindromic), ERIC (enterobacterial repetitive intergenic consensus) and BOX (amplifying with BOX A1 R primer) repetitive elements. This method of fingerprinting grouped the isolates into 56 different DNA types (clusters) with a 75% similarity coefficient. Among these clusters, DNA types 22 and 53 contained two different races 1 and 4, whereas DNA type 12 contained races 1, 4 and 6. However, no clear relationship was observed between fingerprints and races, hosts or geographical origin.     

Identification of Isolates that Cause a Leaf Spot Disease of Brassicas as Xanthomonas campestris pv. raphani and Pathogenic and Genetic Comparison with Related Pathovars

ABSTRACT Twenty-five Xanthomonas isolates, including some isolates received as either X. campestris pv. armoraciae or pv. raphani, caused discrete leaf spot symptoms when spray-inoculated onto at least one Brassica oleracea cultivar. Twelve of these isolates and four other Xanthomonas isolates were spray- and pin-inoculated onto 21 different plant species/cultivars including horseradish (Armoracia rusticana), radish (Raphanus sativus), and tomato (Lycopersicon esculentum). The remaining 13 leaf spot isolates were spray-inoculated onto a subset of 10 plant species/cultivars. The leaf spot isolates were very aggressive on several Brassica spp., radish, and tomato causing leaf spots and dark sunken lesions on the middle vein, petiole, and stem. Based on the differential reactions of several Brassica spp. and radish cultivars, the leaf spot isolates were divided into three races, with races 1 and 3 predominating. A differential series was established to determine the race-type of isolates and a gene-for-gene model based on the interaction of two avirulence genes in the pathogen races and two matching resistance genes in the differential hosts is proposed. Repetitive-DNA polymerase chain reaction-based fingerprinting was used to assess the genetic diversity of the leaf spot isolates and isolates of closely related Xanthomonas pathovars. Although there was variability within each race, the leaf spot isolates were clustered separately from the X. campestris pv. campestris isolates. We propose that X. campestris isolates that cause a nonvascular leaf spot disease on Brassica spp. should be identified as pv. raphani and not pv. armoraciae. Race-type strains and a neopathotype strain for X. campestris pv. raphani are proposed.     

Races of Xanthomonas campestris pv. vesicatoria overcoming the gene Bs2 for bacterial spot resistance in pepper, prevalent on Capsicum chinense in Barbados and Grenada and weakly pathogenic on bell pepper and tomato in the field

A total of 404 isolates of Xanthomonas campestris pv. vesicatoria , obtained from Capsicum chinense cv. West Indian Red grown in Barbados and Grenada, were differentiated into pathogenic races, and of these, 96 were tested also for selected taxonomic group phenotypes. The response of C. chinense to infection by several X. campestris pv. vesicatoria races and the contribution of races isolated from this cultivar to severity of bacterial spot of bell pepper and tomato were also investigated. P4T2, P5T2 and P6T2 were the predominant races of X. campestris pv. vesicatoria isolated from C. chinense grown in Grenada, whereas nine races (T1, P4, P6, P0T2, P1T2, P4T1, P4T2, P6T1 and P6T2) were isolated in Barbados. Race P4T2 comprised 46.0 and 71.4% of the isolates from Barbados and Grenada, respectively. The 96 isolates, all of which overcame resistance conferred by the gene Bs2 , shared taxonomic group B strain characteristics, including the presence of the β-protein band, positive amylolytic activity and inability to oxidize cis -aconitate. The C. chinense cv. West Indian Red was susceptible only to races of X. campestris pv. vesicatoria that can overcome Bs2 gene resistance. Of six such races identified in Barbados, only P4T1, P4T2 and P6T1 affected bacterial spot-susceptible bell pepper or tomato in the field, and they amounted to only 1.5–2.1% of each sample of isolates from these plant species. Moreover, they were confined to the smallest bacterial spot lesions. Bell pepper was most severely affected by combinations of races T1 with P3T2 and T2 with P0T1, and tomato by race T1 only and combinations of races P0T1 with P0T2 and P1T1 with P1T0, all of which prevailed in the field despite selection against them by C. chinense cv. West Indian Red.     

Differential Responses to Pea Bacterial Blight in Stems, Leaves and Pods Under Glasshouse and Field Conditions

Resistance to pea bacterial blight (Pseudomonas syringae pv. pisi) in different plant parts was assessed in 19 Pisum sativum cultivars and landraces, carrying race-specific resistance genes (R-genes) and two Pisum abyssinicum accessions carrying race-nonspecific resistance. Stems, leaves and pods were inoculated with seven races of P. s. pv. pisi under glasshouse conditions. For both race-specific and nonspecific resistance, a resistant response in the stem was not always associated with resistance in leaf and pod. Race-specific genes conferred stem resistance consistently, however, there was variability in the responses of leaves and pods which depended on the matching R-gene and A-gene (avirulence gene in the pathogen) combination. R2 generally conferred resistance in all plant parts. R3 or R4 singly did not confer complete resistance in leaf and pod, however, R3 in combination with R2 or R4 enhanced leaf and pod resistance. Race-nonspecific resistance conferred stem resistance to all races, leaf and pod resistance to races 2, 5 and 7 and variable reactions in leaves and pods to races 1, 3, 4 and 6.Disease expression was also studied in the field under autumn/winter conditions. P. sativum cultivar, Kelvedon Wonder (with no R genes), and two P. abyssinicum accessions, were inoculated with the most frequent races in Europe under field conditions (2, 4 and 6). Kelvedon Wonder was very susceptible to all three races, whereas P. abyssinicum was much less affected. The combination of disease resistance with frost tolerance in P. abyssinicum enabled plants to survive through the winter. A breeding strategy combining race-nonspecific resistance derived from P. abyssinicum with race-specific R-genes should provide durable resistance under severe disease pressure.     

Pathogenicity assays restrict the species Xanthomonas campestris into three pathovars and reveal nine races within X. campestris pv. campestris

The species Xanthomonas campestris (Vauterin) groups bacteria associated with cruciferous plants. In order to clarify and refine the pathovar and race structures within X . campestris , 47 representative strains of six pathovars were characterized for their pathogenicity on a large host range of Brassicaceae, including all original hosts. Three diseases were observed on tested plants: (i) black rot disease on cruciferous plants; it was proposed that all strains causing black rot on at least one cruciferous plant be grouped in the single pathovar X . c . pv. campestris ; (ii) leaf spot disease caused by X . c . pv. raphani on hosts belonging to the Brassicaceae and Solanaceae; the sequenced strain 756C identified as X . c . pv. armoraciae was included in this pathovar and the existence of another leaf spot disease caused by X . c . pv. armoraciae was not supported; and (iii) bacterial blight of garden stocks caused by X . c . pv. incanae . No plants susceptible to X . c . pv. barbareae were found. Strains that did not induce any symptom on cruciferous plants tested, including their original hosts, were removed from the pathovar scheme and were named X . campestris only. Three new races were described in addition to the six races previously described within X . c . pv. campestris . The sequenced strains ATCC 33913 (CFBP 5241) and Xcc 8004 (CFBP 6650) belonged to race 3 and to race 9 (one of the new races described), respectively.     

Broadening the Genetic Basis of Verticillium longisporum Resistance in Brassica napus by Interspecific Hybridization

ABSTRACT Verticillium wilt caused by the vascular fungal pathogen Verticillium longisporum is one of the most important pathogens of oilseed rape (Brassica napus sp. oleifera) in northern Europe. Because production of this major oilseed crop is expanding rapidly and no approved fungicides are available for V. longisporum, long-term control of the disease can only be achieved with cultivars carrying effective quantitative resistance. However, very little resistance to V. longisporum is available within the gene pool of oilseed rape, meaning that interspecific gene transfer from related species is the only possibility for broadening levels of resistance in current varieties. The amphidiploid species B. napus can be resynthesized by crossing the two progenitor species Brassica oleracea and Brassica rapa, hence resistant accessions of these two diploid species can be used as resistance donors. In this study a total of 43 potential B. rapa and B. oleracea resistance donors were tested with regard to their reaction to a mixture of two aggressive V. longisporum isolates, and resistances from diverse lines were combined by embryo rescue-assisted interspecific hybridization in resynthesized rapeseed lines. Progenies from crosses of the two B. rapa gene bank accessions 13444 and 56515 to the B. oleracea gene bank accessions BRA1008, CGN14044, 8207, BRA1398, and 7518 showed a broad spectrum of resistance in pathogenicity tests. Of 45 tested resynthesized lines, 41 lines exhibited a significantly higher level of resistance than the moderately V. longisporum-tolerant oilseed rape cultivar Express. These lines represent a promising basis for the combination of different resistance resources in new varieties.     

Identification of the rice blast resistance gene Pib in the National Small Grains Collection

The Pib gene in rice confers resistance to a wide range of races of the rice blast pathogen, Magnaporthe oryzae, including race IE1k that overcomes Pita, another broad-spectrum resistance gene. In this study, the presence of Pib was determined in 164 rice germplasm accessions from a core subset of the National Small Grains Collection utilizing DNA markers and pathogenicity assays. The presence of Pib was evaluated with two simple sequence repeat (SSR) markers and a dominant marker (Pib-dom) derived from the Pib gene sequence. Pathogenicity assays using two avirulent races (IE1k and IB1) and a virulent race (IB54) were performed to verify the resistance responses of accessions. Of the 164 accessions evaluated, 109 contained the Pib gene as determined using both SSR markers and pathogenicity assays, albeit different haplotypes were detected. The remaining 52 germplasm accessions were different in their responses to the blast races IB54, IE1k, and IB1, thus indicating the presence of R gene(s) other than Pib. The accessions characterized in this study could be used for marker-assisted breeding to improve blast resistance in indica and japonica cultivars worldwide.     

Discrimination of <Emphasis Type="Italic">Xanthomonas campestris</Emphasis> pv. <Emphasis Type="Italic">campestris</Emphasis> races among strains from northwestern Spain by <Emphasis Type="Italic">Brassica</Emphasis> spp. genotypes and rep-PCR

Black rot, caused by Xanthomonas campestris pv. campestris (Xcc) is a severe seedborne disease of Brassica crops around the world. Nine races are recognized, being races 1 and 4 the most aggressive and widespread. The identification of Xcc races affecting Brassica crops in a target area is necessary to establish adequate control measures and breeding strategies. The objectives of this study were to isolate and identify Xcc strains from northwestern Spain by using semi-selective medium and pathogenicity tests, determine the existing races of Xcc in this area by differential series of Brassica spp., and evaluate the use of repetitive DNA polymerase chain reaction-based fingerprinting (rep-PCR) to differentiate among the nine existing Xcc races. Seventy five isolates recovered from infected fields were identified as Xcc. Race-typing tests determined the presence of the following seven pathogen races: 1, 4, 5, 6, 7, 8 and 9. Race 4 was the most frequent in Brassica oleracea and race 6 in Brassica rapa crops, therefore breeding should be focussed in obtaining resistant varieties to both races. Cluster analysis derived from the combined fingerprints showed four groups, but no clear relationship to race, crop or geographical origin was found. Rep-PCR analysis was found not to be a reliable method to discriminate among Xcc races, therefore race typing of Xcc isolates should be done by using the differential series of Brassica spp. genotypes or another alternative approach.     

Characterisation of Xanthomonas campestris pv. campestris isolates from South Africa using genomic DNA fingerprinting and pathogenicity tests

The genetic diversity of Xanthomonas campestris pv. campestris isolates from South Africa was evaluated using 28 isolates obtained from the Johannesburg Fresh Produce Market. Samples were collected from cabbage supplies from farms in Gauteng, Mpumalanga and North West Provinces. Strains were isolated from small sections of infected cabbage leaf samples and cultured on Yeast Dextrose Agar. Isolates identity was confirmed by ELISA and Pathogenicity test. Pathogenicity tests were performed by inoculating leaves of known susceptible cabbage seedlings. Infection symptoms induced could be categorized into three groups, ranging from typical to non-typical black rot symptoms. Four differential Brassica cultivars with known avirulence genes were used for race typing done by spray inoculation. Four races, namely 1, 3, 4 and 6, were identified. Of the 28 isolates, four were identified as race 1, two as race 3, 19 as race 4 and three as race 6. Repetitive DNA polymerase chain reaction-based fingerprinting using Eric- and Box-primers was used to assess the genetic diversity. Generated fingerprints of X. c pv. campestris were relatively similar. Cluster analysis could not strictly group isolates by their geographical origin, suggesting limited diversity of Xanthomonas campestris pv. campestris strains within cabbage producing regions in South Africa.     

Winter oilseed rape with high levels of resistance to Pyrenopeziza brassicae derived from wild Brassica species

Oilseed rape cultivars possess inadequate levels of resistance to light leaf spot disease, caused by the ascomycete Pyrenopeziza brassicae Sutton & Rawlinson. High levels of resistance to this disease were found within wild accessions of Brassica oleracea and B. rapa . This resistance was introgressed into agronomically acceptable winter oilseed rape breeding lines. Seedling resistance was determined by two genes. One of these, derived from B. rapa and positioned on linkage group N1, resulted in no apparent symptoms following infection, while the other, derived from B. oleracea and positioned on N16, resulted in black necrotic flecks and a reduced amount of sporulation compared with standard cultivars. Several agronomically acceptable double haploid lines were developed which expressed very high levels of adult plant resistance.     

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.     

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.     

The dynamics of homologous and heterologous interactions between rice and strains of Xanthomonas campestris

Lesion development, bacterial multiplication and spread were measured in leaves of cultivars of rice containing different Xa (resistance) genes, following inoculation with different races of the bacterial leaf blight pathogen. Xanthomonas campestris pv. oryzae. Both compatible and incompatible races possessed the ability to colonize rice plants. The difference between compatible and incompatible host pathogen combinations appeared to be mainly in symptom production since multiplication rates and spread were very similar until after the onset of symptoms. No form of HR (hypersensitive response) was observed. The ability of incompatible races to modify host reaction in dual-inoculation was dependent on the genotype of the host plant. The heterologous non-pathogen of rice X. campestris pv. campestris produced few symptoms, failed either to multiply or spread within rice leaves and was unable to induce any marked cross-protection against homologous pv. oryzae strains in dual-inoculation experiments.     

Phage sensitivity in relation to pathogenicity and virulence of the cotton bacterial blight pathogen of Sudan

At least two pathotypes of Xanthomonas campestris pv. malvacearum are now known to exist in Sudan. The pre-Barakat (race 1) and post-Barakat (race 2) pathogens have been shown to exhibit different host specificity. The former is pathogenic and highly aggressive on only cultivars with no resistance genes or with the B₂ and/or B₃ resistance factors, while the latter can infect the B₆ cultivars also. Race 2 in Sudan, which was previously reported to infect all the standard differentials, produced milder angular leaf spot symptoms and occasionally restricted vein infection. Moreover, it exhibited reduced growth in planta compared with race 1.
Bacteriophage studies revealed that the two races are quite distinct in their phage sensitivity. Race 1 can be lysed by only three, or rarely four, of the six phages used for typing, while race 2 is sensitive to all of them. The present study suggests that phage 7 may be the type-determining phage for race 2. Race 2 strain mutants resistant to phage 3 or 4 were found to be sensitive to phage 7 and pathogenic to both Acala and Barakat, although showing marked attenuation of virulence. However, mutants resistant to phage 2 or 7 were insensitive to all the phages and although they retained their pathogenicity to Acala, they either lost the ability to infect Barakat or produced a hypersensitive reaction. The resistance of all mutants was found to be due to failure to adsorb the homologous phage, indicating a change in the cell wall. The association of this with the attenuation of virulence suggests that bacterial wall components may function as virulence determinants in Xanthomonas campestris pv. malvacearum.     

Genetics of resistance to race TTKSK of Puccinia graminis f. sp. tritici in Triticum monococcum

Race TTKSK (or Ug99) of Puccinia graminis f. sp. tritici possesses virulence to several stem rust resistance genes commonly present in wheat cultivars grown worldwide. New variants detected in the race TTKSK lineage further broadened the virulence spectrum. The identification of sources of genetic resistance to race TTKSK and its relatives is necessary to enable the development and deployment of resistant varieties. Accessions of Triticum monococcum, an A-genome diploid wild and cultivated wheat, have previously been characterized as resistant to stem rust. Three resistance genes were identified and introgressed into hexaploid wheat: Sr21, Sr22, and Sr35. The objective of this study was to determine the genetic control and allelic relationships of resistance to race TTKSK in T. monococcum accessions identified through evaluations at the seedling stage. Generation F(2) progeny of 8 crosses between resistant and susceptible accessions and 13 crosses between resistant accessions of T. monococcum were evaluated with race TTKSK and often with North American races, including races QFCSC, TTTTF, and MCCFC. For a selected population segregating for three genes conferring resistance to race TTKSK, F(2:3) progeny were evaluated with races TTKSK, QFCSC, and TTTTF. In that population, we detected two genes conferring resistance to race TTKSK that are different from Sr21, Sr22, and Sr35. One of the new genes was effective to all races tested. The identification of these genes will facilitate the development of varieties with new resistance to race TTKSK.     

Identification of Lentil Germ Plasm Resistant to Colletotrichum truncatum and Characterization of Two Pathogen Races

ABSTRACT A total of 1,771 lentil accessions from the U.S. lentil collection (U.S. Department of Agriculture-Agricultural Research Service, Pullman, WA) and the Institut für Pflanzengenetik und Kulturpflanzenforschung (Gatersleben, Germany) were screened for resistance to Colletotrichum truncatum, the cause of anthracnose. About 95% of the accessions were susceptible when inoculated with a single isolate in the field. Retesting, under controlled conditions, of accessions rated as resistant or moderately resistant in the field resulted in identification of anthracnose resistance in four accessions from the U.S. collection (PI 320937, PI 320952 [cv. Indianhead], PI 345629, and 468901), and 12 accessions from the German collection (Lens 3, 102, 104, 106, 107, 119, 122, 134, 135, 177, 195, and 209). Seven of the accessions were used as host differentials to characterize pathogenic variability of 50 single-spore isolates collected in Manitoba and Saskatchewan, Canada. The presence of two distinct races was demonstrated. Isolates of C. truncatum avirulent on cv. Indianhead, PI 320937, PI 345629, PI 468901, Lens 102, Lens 104, and Lens 195 were designated race Ct1. Isolates that were virulent on these seven entries were designated race Ct0, indicating their lack of avirulence genes. Race Ct0 was isolated more frequently from commercial seed samples than race Ct1, but the two races were isolated with similar frequency from plants in commercial fields planted to susceptible cultivars. Race Ct0, to which no resistance has yet been identified, presents a high risk to lentil production in Canada and potentially worldwide.