Stable resistance in barley to Pyrenophora teres f. teres isolates from the Nordic-Baltic region after increase on standard host genotypes

Results from tests of a mixture of Finnish net blotch, Pyrenophora teres Drechs. f. teres Smedeg., isolates on a differential series of barley seedlings, comprising 17genotypes, indicated that patterns of infection response (IR)and percentage leaf area damaged (PLAD) were unaffected by differences in seedling size. Variation of the concentration of inoculum between 1,250 conidia ml^-1 and 20,000 conidia ml^-1 produced similar patterns of IR and PLAD on the differential series. IR and PLAD scored on the second seedling leaf differentiated resistance to P. teres f. teresamong the genotypes better than on the first seedling leaf. Ina second experiment, 120 single-spore P. teres f. teres isolates from Finland, Sweden, Norway, Latvia, Estonia and Ireland were used in tests conducted in the greenhouse to differentiate them in terms of virulence reaction on seedlings of six differential barley genotypes. Each isolate was tested directly following isolation from the leaf material and after having passaged each through barley cvs. Arve or Pohto, to produce 360 isolates in total. Virulence of the isolates differed significantly on the members of the differential series, but differences associated with country of origin and passaging, and interactions, were small. It is concluded that little variation between virulence of P. teres f. teres isolates is evident over a large geographic area, incorporating Nordic and Baltic countries, and Ireland. Barley genotype response to P. teres f. teres appeared to be of more significance than relative virulence of the pathogen isolates. This could simplify breeding barley for improved resistance to this phytopathogen. This revised version was published online in August 2006 with corrections to the Cover Date.     

Diversity among Finnish net blotch isolates and resistance in barley

Summary Seedlings of a differential barley (Hordeum vulgare L.) series (21 genotypes) and of six check genotypes were used in the greenhouse to assess variation in virulence among 20 single-spore isolates of the net blotch pathogen. Pyrenophora teres Drechs. f.teres Smedeg., collected from various sites in Finland. The experiment was run twice and symptom expression was recorded on the first three leaves. Analysis of second leaf symptom scores from Run 1 indicated differences in virulence between isolates, all of which were pathogenic, and differential resistance among the barleys. The virulence of P. teres isolates appeared to be conditioned by the host barley from which the isolate derived; the average virulence of isolates collected from a susceptible host was greater than that of isolates collected from a less susceptible host. Results from Run 2 were similar regarding resistance within the barleys, but variation in virulence among the P. teres isolates was not consistent with that from Run 1. CI 9819 caries duplicate genes for resistance to P. teres (Rpt1b and Rpt1c), and CI 7548 possesses Rpt3d. Both genotypes were highly resistant to all isolates. The Rpt1a gene of Tifang (CI4407) conferred resistance to all isolates in Run 2, but only to about half of the isolates in Run 1. The checks, including two of the symptomatically most resistant Nordic barley genotypes, were universally susceptible during these stringent tests. No selective pressure has been placed on Finnish isolates of P. teres through previous deployment of major resistance genes, and it is speculated that any variation in virulence among isolates is likely to be due to a combination of evolutionary forces including, natural selection, random genetic drift and gene flow.     

Generation Mean Analysis of Inheritance of Resistance to Pyrenophora teres in Barley

F1, F2, Fl × parentl (BC1), and Fl × parent2 (BC2) generations resulting from four crosses among seven cultivars of barley used in national and international breeding programmes were tested at the seedling stage for their resistance to a mixture of five isolates of Pyrenophora teres. Four methods were used to assess disease resistance: infection type (IT), average lesion size (ALS), number of lesions per unit leaf area (NL) and percent leaf area infected (PLAI). Gene actions were estimated by generation mean analysis on each of the four crosses and on each of the evaluation methods. Significant additive and additive × additive epistatic effects were found. Infection type and percent leaf area infected were found to be highly correlated in all four crosses. These results suggest that barley breeders could improve the level of resistance to P. teres by making appropriate crosses between highly susceptible barley cultivars.     

Inheritance of Partial Resistance to Net Blotch in Barley

Reciprocal crosses were made between 9 different barley genotypes with high genetic variability for net blotch resistance. Parents and 72 F₁ plants were used to determine the inheritance of partial resistance to net blotch. Four experiments, one in a growth chamber on seedlings and 3 others in the field on adult plants, were undertaken using a randomised complete block design. An isolate of net blotch from local cultivars was used for inoculation. Non-inoculated plants of one of the field experiments were used for the detached leaf test in petri dishes. Results show that the cultivars ‘Banteng’, and ‘Arrivate’, the Ethiopian line CI 5791, and the Syrian line 79 SIO-10, had a high partial resistance in all experiments. Diallel analysis showed high significant general and specific combining ability when maternal and reciprocal effects were not significant. As the resistance genotypes have a high additive genetic effect, they could be successfully used for breeding purposes.     

Virulence studies of Swedish net blotch isolates (Drechslera teres) and identification of resistant barley lines

Six Swedish and one Canadian single spore isolate of Drechslera teres f. teres were used to screen 109 barley lines for disease resistance and to select a differential set of barley lines for use in assessing pathogen virulence. A large variation for net blotch resistance was found among the 109 barley lines which were classified into four groups, those showing: 1) only resistant reactions; 2) differential reactions; 3) only intermediate reactions and 4) only susceptible reactions. The European commercial varieties included, showed susceptibility to all Swedish isolates, but a few were resistant to the Canadian isolate. The 18- member differential set separated 25 Swedish and two Canadian isolates of D. teres into 14 pathotypes, three of which made up 59% of the isolates. Only one barley differential (CI 9776) was resistant to all net form isolates. Host selection on the pathogen seems to be present as all six isolates obtained from cv. Golf belonged to the same pathotype and 4 of 5 isolates from cv. Karin shared the same virulence pattern. The net form of net blotch (D. teres f. teres) predominated in the sampled regions and only one of 26 Swedish isolates was of the spot form (D. teres f. maculata). This revised version was published online in July 2006 with corrections to the Cover Date.     

The Yd2 gene and enhanced resistance to barley yellow dwarf virus (BYDV) in winter barley

A breeding programme was developed to obtain barley yellow dwarf virus (BYDV)-resistant winter genotypes using the Yd2 gene. The aim was to incorporate the Yd2 allele into the new high-yielding genotypes to release cultivars that allow barley cultivation in areas where BYDV is endemic. The resistant lines were developed using pedigree selection. An ICARDA resistant line (83RCBB130) carrying the Yd2 gene was crossed with three susceptible, high-yielding winter varieties and their F₁ lines were either selfed or backcrossed to the matching susceptible parent. The best lines selected from subsequent selfing generations were evaluated in replicated trials in the presence or absence of BYDV, starting from F₆ and BC₁F₅ to F₈ and BC₁F₇ generations. Four genotypes with superior agronomic traits and BYDV resistance were selected.     

Genetic Relationships Among Four Pepper Genotypes Resistant to Phytophthora capsici

According to our previous investigations, resistance to Phytophthora capsid in Capsicum annuum genotypes, ‘Line 29’, ‘PI201232’, ‘PI201234’ and Serrano Criollo de Morelos 334 (‘SCM334’), seems to be controlled by three genes. In order to determine the genie relationships between these four sources of resistance, three experiments were conducted which included the four genotypes, their F₁s, F₂s, F3s and BC₁ generations together with the susceptible pepper genotype ‘Morron INI A 224’. Inoculations were made, when plants had 4—6 leaves, by irrigating the culture substrate with a zoospore suspension of P. capsici isolate ‘Bl’. Though the four genotypes showed percentages of resistance close to a 100%, none of them actually reached this level in the three experiments. ‘SCM334’ was the most resistant genotype, transmitting a high level of resistance to its F₁, F₂ and BQ generations. ‘Line 29’ was more resistant than ‘PI201232’ and ‘PI201234’. However, the F₁ F₂ and BQ generations of these three lines showed similar degrees of resistance. The four genotypes seem to have one of the three genes postulated for their resistance in common. All genes displayed a similar level of resistance, except the specific genes of ‘SCM334’, the effect of which was slightly higher. Several working procedures are suggested for breeding programmes.     

Genetic analysis of seedling resistance to Stagonospora nodorum blotch in selected tetraploid and hexaploid wheat genotypes

Stagonospora nodorum blotch (SNB), caused by Phaeosphaeria nodorum, is a major component of the leaf‐spotting disease complex of wheat (Triticum aestivum L.) in the northern Great Plains of North America. This study was conducted, under controlled environmental conditions, to determine the inheritance of resistance to SNB in a diverse set of hexaploid and tetraploid wheat genotypes and to decipher the genic/allelic relationship among the resistance gene(s). Plants were inoculated at the two to three‐leaf stages with a spore suspension of P. nodorum isolate Kelvington‐SK and disease reaction was assessed 8 days after inoculation based on a lesion‐type scale. Tests of the F₁ and F₂ generations and of F_{2 : 3} or F_{2 : 5} families indicated that a single recessive gene controlled resistance to SNB in both hexaploid and tetraploid resistance sources. Lack of segregation in intra‐specific and inter‐specific crosses between the hexaploid and the tetraploid resistant genotypes, indicated that these genetically diverse sources of resistance possess the same gene for resistance to SNB. Results of this study suggest that the wheat‐P. nodorum interaction may follow the toxin model of the gene‐for‐gene hypothesis.     

Inheritance of resistance to the bipartite Tomato chlorotic mottle begomovirus derived from Lycopersicon esculentum cv. ‘Tyking’

Severe outbreaks of bipartite begomoviruses (family Geminiviridae) have been observed on tomatoes after the introduction of the whitefly Bemisia tabaci (biotype B) in Brazil. The Lycopersicon esculentum line ‘TX 468-RG’ was identified as one of the best sources of broad-spectrum resistance to species comprising the tomato-infecting Begomovirus complex in Brazil. The genetic basis of resistance to one Begomovirus isolate was investigated using populations from the cross between ‘TX 468-RG’ (P₁) and the susceptible line ‘Ohio 8245’ (P₂). Parental lines, F₁, backcross (BC) to P₁ and BC to P₂ and F₂ generations were inoculated at the two true-leaf stage using 20 viruliferous whiteflies per plant. Assessment was done two weeks after inoculation based upon visual analysis of symptom expression. The ratio of resistant to susceptible plants closely fit to a single recessive gene (locus) model. The sequence analysis indicated that the Begomovirus isolate used in this assay was closely related to the bipartite Tomato chlorotic mottle virus. Therefore, this gene/locus, was tentatively named tcm-1 (tomato chlorotic mottle virus resistance-1). This locus has been transferred to distinct tomato cultivars and levels of resistance similar to that of ‘TX 468-RG’ were observed in advanced (F₈ and F₉) generations. In addition, breeding lines carrying the tcm-1 locus were also resistant to other Brazilian bipartite tomato-infecting Begomovirus species.     

Genetics of disease resistance in urdbean (Vigna mungo (L.) Hepper) to the leaf spots caused by Colletotrichum truncatum (Schw.) Andrus & Moore

Summary The resistance sources among various test cultivars of urdbean to Colletotrichum truncatum, a leaf spotting pathogen, were identified and genetics of resistance was worked out by studying F₁, F₂ and F₃ generations of crosses between resistant cultivars and the susceptible cv. Kulu 4 and of those among the resistant parents. The resistance was found to be controlled by single dominant genes and the resistance genes were non-allelic.     

Estimates of additive and dominance effects for Fusarium head blight resistance of winter triticale

The most effective strategy to control Fusarium head blight (FHB), a devastating disease of small‐grain cereals, is breeding resistant cultivars. This resistance study of F₁ crosses, F₂ and backcross generations of triticale estimates heterosis, general and specific combining ability (GCA, SCA), additive and dominance effects and compares parents with segregating generations. The genetic material consisted of 10 parents with their 45 F₁ crosses and of six parents with their 15 F₂ progeny and backcrosses to each parent. Genotypes were grown in various environments and artificially inoculated with an aggressive isolate of F. culmorum. FHB was assessed, by visual rating, as the mean of four to five individual ratings of disease development. Heterosis for FHB was of little importance. The correlation between the FHB rating of F₁ crosses and their mid‐parent performance was close. GCA was the predominant source of variation, although the significance of the SCA variance also implied non‐additive allelic interaction. The preponderance of additive gene effects is encouraging for increasing resistance by a recurrent selection programme. The relationship between the GCA effect of a parent and its per se performance was close, which gives the possibility of predicting FHB resistance in F₁ crosses. Additive effects were predominant in the F₂ progeny and also in the backcrosses. Transgressive segregants could not be detected. Searching for them should be postponed to the F₃ or later generations.     

Inheritance of partial resistance to spot blotch in barley

Diallel crosses (without reciprocals) were made among 10 different barley genotypes with genetic variability for spot blotch resistance. Forty‐five F₁ hybrids and their parents were assessed for their combining abilities for the disease resistance. Three experiments, two in a growth chamber on detached leaf and seedlings tests and one in the field on adult plant stages, were undertaken using a randomized complete block design with five replicates. A mixed conidial suspension of nine virulent isolates of the pathogen was used for inoculation. Statistical analysis showed genetic variability for spot blotch resistance. Results showed that the cultivar Banteng, the Ethiopian line CI‐5791 and the Syrian line 79‐SIO‐9 had partial resistance in all experiments. General combining ability was significant, with either positive or negative values. Resistant genotypes show favourable GCA‐effects, and they could therefore be successfully used for breeding purposes.     

Molecular breeding for the BaMMV/BaYMV resistance gene ym4 in winter barley

The aim of this investigation was to develop a procedure for the largescale molecular breeding for ym4, allowing resistance to BaMMV/BaYMV to be fixed in early breeding generations of winter barley. A codominant STS marker derived from the restriction fragment length polymorphism marker MWG838 for the ym4 resistance gene was combined with a new and easy procedure for preparing leaf samples for polymerase chain reaction (PCR), theoretically allowing one person to extract DNA from 5000 samples in a single day. In the procedure for molecular breeding for ym4, all steps, including leaf sampling, DNA extraction, PCR amplification and digestion with restriction enzyme were assembled in microtitre plates allowing multipipetting throughout the procedure, including the loading of gels. The method is amenable to further automation with the aid of a robot arm. Double haploid (DH) lines, as well as F₂ and F₄ breeding lines were analysed and, based on markers, homozygous and heterozygous BaMMV/BaYMV resistant plants were identified for further breeding. The winter barley breeding programmes were modified to include marker-based selection for BaMMV/BaYMV resistance on DH or on F₂ individuals, which had been preselected for mildew and leaf rust resistance.     

Mapping seedling resistance to net form of net blotch (Pyrenophora teres f. teres) in barley using detached leaf assay

To develop molecular markers against Pyrenophora teres f. teres in barley, a detached leaf assay was conducted on two DH populations with a set of 11 single conidial lines (SCLs). Out of these, three showed different reactions in the DH population ‘Uschi × HHOR3073’ and two in ‘(P x V) × HHOR9484’. For SCL ‘QLB’, a 1r : 1s (χ² = 2.78) segregation was observed in the population ‘Uschi × HHOR3073’. In contrast to this, a continuous variation was observed for the SCL ‘WvB’ and ‘d8_4’ in the DH population ‘Uschi × HHOR3073’ and for ‘AR’ and ‘net1840’ in ‘(P × V) × HHOR9484’. With respect to resistance to the SCL ‘QLB’, a single major gene was located on chromosome 7H, and for resistance against SCL ‘WvB’, two QTLs were detected on chromosome 3H and 7H, and against SCL ‘d8_4’, two loci were mapped on chromosome 3HS in ‘Uschi × HHOR3073’. In the DH population ‘(P x V) × HHOR9484’, one locus conferring resistance to the SCL ‘AR’ was located on chromosome 3H. For resistance to SCL ‘net1840’, two QTLs were mapped on chromosome 4H and 5H.     

Identification of a novel recessive gene for resistance to powdery mildew (Blumeria graminis f. sp. hordei) in barley (Hordeum vulgare)

One of the most important diseases of barley (Hordeum vulgare) is powdery mildew, caused by Blumeria graminis f. sp. hordei. Spring barley line 173-1-2 was selected from a Moroccan landrace and revealed broad-spectrum resistance to powdery mildew. The objective of this study was to map and characterize the gene for seedling powdery mildew resistance in this line. After crossing with the susceptible cultivar ‘Manchuria’, genetic analysis of F₂ and F₃ families at the seedling stage revealed powdery mildew resistance in line 173-1-2 conditioned by a single recessive gene. Molecular analysis of non-segregating homozygous resistant and homozygous susceptible F₂ plants conducted on the DArTseq platform (Diversity Arrays Technology Pty Ltd) identified significant markers which were converted to allele-specific PCR markers and tested among 94 F₂ individuals. The new resistance gene was mapped on the long arm of chromosome 6H. No other powdery mildew recessive resistance gene has been located on 6H so far. Therefore, we concluded that the 173-1-2 barley line carries a novel recessive resistance gene designated as mlmr.     

QTL mapping of net blotch resistance genes in a doubled-haploid population of six-rowed barley

Net blotch, caused by Pyrenophora teres f. teres, is a damaging foliar disease of barley worldwide. It is important to identify resistance germplasm and study their genetics. 'Chevron', a six-rowed barley used as a parent for the production of a doubled haploid (DH) population for mapping of Fusarium head blight (FHB) resistance, was also found to be resistant to net blotch. To map the resistance genes, the population was evaluated for resistance at the seedling stage in a greenhouse. The resistance data showed a two-peak distribution. Through linkage mapping, one resistance gene, tentatively called Rpt, was located on chromosome 6HS flanked by Xksua3b-Xwg719d, which was also detected by QTL mapping. This QTL explained 64% of the phenotypic variance for the resistance in this DH population. In addition, a minor QTL was found on chromosome 2HS defined by Xcdo786-Xabc156a. 'Chevron' and 'Stander' contributed the resistant alleles of Rpt and the 2HS QTL, respectively. Both QTLs together explained nearly 70% of the phenotypic variance. The markers for these QTLs are useful for marker-assisted selection of net blotch resistance in barley breeding.     

Genetic analysis of karnal bunt resistance in 14 Mexican bread-wheat genotypes

Fourteen Mexican genotypes of bread wheat (Triticum aestivum L.) with good to moderate levels of resistance to Karnal bunt (Tilletia indica (Mitra)) were crossed with the highly susceptible cultivar WL711 to determine the genetic basis of resistance. The parents, F₁ F₂ and backcross populations of the 14 crosses were evaluated under artificial epiphytotic conditions during the 1993–94 season for Karnal bunt resistance. The F₁ data suggested that the resistance was dominant to partially dominant over susceptibility. The F₂ analysis of the segregation ratios in the F₂ and backcross generations indicated that the resistance in the wheat genotypes Luan, Attila, Vee #7/Bow, Star, Weaver, Milan, Sasia and Turacio/Chil is controlled by two genes. The resistance in genotypes Cettia, Irena, Turaco, Opata, Picus, and Yaco was found to be conditioned by a single dominant gene. The genotypes with two genes for resistance expressed a higher level of resistance than those with a single gene and, therefore, are better sources of resistance to Karnal bunt.     

Barley Yellow Dwarf Virus Resistance in Triticum aestivum×Leymus angustus Hybrids

The enzyme-linked immunosorbent assay (ELISA) was used to determine the concentration of barley yellow dwarf virus (BYDV) in leaves of spring wheat Triticum aestivum L. cv. ‘Fukuhokomugi’, Leymus angustus (Trin.) Pilger (Altai wild rye) and their F₁ hybrids inoculated at the 2- to 3-leaf stage. The BYDV isolate “Cloutier” (serotype PAV) was used. The results showed that L. angustus confers immunity or strong resistance to the virus. Significant multiplication of the virus occurred in ‘Fukuhokomugi’ which expressed an intermediate level of tolerance to the disease under field conditions. The F₁ haploid hybrids of ‘Fukuhokomugi’×L. angustus expressed a level of resistance almost equal to that of their wild parent, with a low concentration of virus appearing 16 days after inoculation and no more afterwards. The discovery of BYDV resistance in L. angustus and the expression of this character in the F₁ hybrids provide new opportunities for the enlargement of the gene pool for BYDV resistance in wheat.     

Dominant gene for common bean resistance to common bacterial blight caused by <Emphasis Type="Italic">Xanthomonas</Emphasis><Emphasis Type="Italic">axonopodis</Emphasis> pv. <Emphasis Type="Italic">phaseoli</Emphasis>

The common bacterial blight pathogen [Xanthomonas axonopodis pv. phaseoli (Xap)] is a limiting factor for common bean (Phaseolus vulgaris L.) production worldwide and resistance to the pathogen in most commercial cultivars is inadequate. Variability in virulence of the bacterial pathogen has been observed in strains isolated from Puerto Rico and Central America. A few common bean lines show a differential reaction when inoculated with different Xap strains, indicating the presence of pathogenic races. In order to study the inheritance of resistance to common bacterial blight in common bean, a breeding line that showed a differential foliar reaction to Xap strains was selected and was crossed with a susceptible parent. The inheritance of resistance to one of the selected Xap races was determined by analysis of segregation patterns in the F₁, F₂, F₃ and F₄ generations from the cross between the resistant parent PR0313-58 and the susceptible parent ‘Rosada Nativa’. The F₁, F₂ and F₃ generations were tested under greenhouse conditions. Resistant and susceptible F_3:4 sister lines were tested in the field. The statistical analysis of all generations followed the model for a dominant resistance gene. The resistant phenotype was found to co-segregate with the SCAR SAP6 marker, located on LG 10. These results fit the hypothesis that resistance is controlled by a single dominant gene. The symbol proposed for the resistance gene is Xap-1 and for the bacterial race, XapV1.     

The inheritance of resistance to blue mold (Peronospora tabacina Adam) in two cultivars of tobacco (Nicotiana tabacum L.)

Summary Two blue-mold resistant cultivars of Nicotiana tabacum L. were crossed with each other and with the susceptible Israeli local cultivar Mikhal. F₁, F₂ and F₃ progenies of these crosses, F₁ and F₂ of backcrosses and the parental varieties were grown in a series of experiments, in which the seedlings were exposed to heavy natural infection with Peronospora tabacina Adam. The genetic basis of resistance was found to be identical in the two resistant strains Bel-61-10 and Chemical mutant. A single dominant major gene determined the segregation of resistance versus susceptibility in the crosses of these strains with Mikhal. The level of resistance of resistant segregates was shifted by environment and by quantitatively modifying genes. The index of resistance, which was calculated as a weighted mean of the degree of blue-mold expression of the resistant segregates, differed in the generations of cross progeny. This could be explained by the different expected levels of the modifying genes in these generations. Heritability of the index of resistance was calculated by parent-offspring regression and it was found to be 0.542 in the F₃ and 0.227 in F₂ backcross progenies.