Scab Resistance Research Articles

Generation of advanced fire blight-resistant apple (Malus\u2009\xd7\u2009domestica) selections of the fifth generation within 7\xa0years of applying the early flowering approach

Main conclusionThe approach presented here can be applied to reduce the time needed to introduce traits from wild apples into null segregant advanced selections by one-fourth.Interesting traits like resistances to pathogens are often found within the wild apple gene pool. However, the long juvenile phase of apple seedlings hampers the rapid introduction of these traits into new cultivars. The rapid crop cycle breeding approach used in this paper is based on the overexpression of the birch (Betula pendula) MADS4 transcription factor in apple. Using the early flowering line T1190 and ‘Evereste’ as source of the fire blight resistance (Fb_E locus), we successfully established 18 advanced selections of the fifth generation in the greenhouse within 7 years. Fifteen individuals showed the habitus expected of a regular apple seedling, while three showed very short internodes. The null segregants possessing a regular habitus maintained the high level of fire blight resistance typical for ‘Evereste’. Using SSR markers, we estimated the percentage of genetic drag from ‘Evereste’ still associated with Fb_E on linkage group 12 (LG12). Eight out of the 18 selections had only 4% of ‘Evereste’ genome left. Since genotypes carrying the apple scab resistance gene Rvi6 and the fire blight resistance QTL Fb_F7 were used as parents in the course of the experiments, these resistances were also identified in some of the null segregants. One seedling is particularly interesting as, beside Fb_E, it also carries Fb_F7 heterozygously and Rvi6 homozygously. If null segregants obtained using this method will be considered as not genetically modified in Europe, as is already the case in the USA, this genotype could be a very promising parent for breeding new fire blight and scab-resistant apple cultivars in European apple breeding programs.

A Genome-Wide Association Study of Apple Quality and Scab Resistance.

The apple ( × Borkh.) is an economically and culturally important crop grown worldwide. Growers of this long-lived perennial must produce fruit of adequate quality while also combatting abiotic and biotic stress. Traditional apple breeding can take up to 20 yr from initial cross to commercial release, but genomics-assisted breeding can help accelerate this process. To advance genomics-assisted breeding in apple, we performed genome-wide association studies (GWAS) and genomic prediction in a collection of 172 apple accessions by linking over 55,000 single nucleotide polymorphisms (SNPs) with 10 phenotypes collected over 2 yr. Genome-wide association studies revealed several known loci for skin color, harvest date and firmness at harvest. Several significant GWAS associations were detected for resistance to a major fungal pathogen, apple scab ( [Cke.] Wint.), but we demonstrate that these hits likely represent a single ancestral source. Using genomic prediction, we show that most phenotypes are sufficiently predictable using genome-wide SNPs to be candidates for genomic selection. Finally, we detect a signal for firmness retention after storage on chromosome 10 and show that it may not stem from variation in , a gene repeatedly identified in bi-parental mapping studies and widely believed to underlie a major QTL for firmness on chromosome 10. We provide evidence that this major QTL is more likely due to variation in a neighboring ethylene response factor (ERF) gene. The present study showcases the superior mapping resolution of GWAS compared to bi-parental linkage mapping by identifying a novel candidate gene underlying a well-studied, major QTL involved in apple firmness.

Effect of polyploidisation on the response of apple (Malus × domestica Borkh.) to Venturia inaequalis infection

Development of new and durable strategies to improve the apple plant’s resistance against apple scab (Venturia inaequalis) remains a major challenge. Polyploids or organisms with three or more complete chromosome sets often possess properties superior to their diploid counterparts. Studies reported that polyploidy confers an increased resistance to biotic and abiotic stress factors. However, the potential use of polyploidy to increase disease resistance is still insufficiently investigated. We determined the influence of artificial genome doubling on the response of three Malus × domestica genotypes, with a variable level of susceptibility to apple scab. Based on visual symptom evaluation and real-time PCR quantification of the V. inaequalis DNA in apple leaves, an increased resistance was observed in the neotetraploid form of the monogenic resistant genotype compared to its diploid progenitor. No pronounced effects were observed comparing ploidy levels of the susceptible genotypes. These results suggest a potential role for polyploidisation in apple scab resistance, but it seems to depend on the degree of disease susceptibility of the genotype. Still, further research on the effects of polyploidy might offer potential in the development of new and durable resistance strategies.

Genetics of resistance in apple against Venturia inaequalis (Wint.) Cke

Apple (Malus × domestica) is the third important fruit in terms of production and consumption worldwide. Apple scab caused by Venturia inaequalis is the most devastating disease of apple. In the apple-growing regions, many fungicides are sprayed to control the disease leading to increase in the production cost. Development of scab-resistant cultivars is the long-lasting solution to control the disease. In apples, more than 20 major scab resistance genes have been identified in various cultivars and few wild relatives. Of all these genes, Rvi6 derived from Malus floribunda has been most extensively used in different breeding programs. Gene for gene interactions of these resistance genes with the avirulence genes from V. inaequalis have been understood in many cases. QTL-based polygenic resistance has also been characterized in apple. Nucleotide Binding Site Leucine-Rich Repeats (NBS-LRR) have been identified from the apple genome and many of them have been characterized from the scab resistance region. Molecular markers associated with most of the major scab resistance genes have been identified and their position has been mapped on different linkage groups. Marker-assisted selection (MAS) can be helpful in speeding up and accurately identifying the scab-resistant parents and progeny. Pyramiding of several major resistance genes can be undertaken for more durable resistance against apple scab. The present paper reviews the Malus-Venturia pathosystem, current status of knowledge about scab resistance genes, and their application in breeding against apple scab.

Population structure of Venturia inaequalis, a causal agent of apple scab, in response to heterogeneous apple tree cultivation

BackgroundTracking newly emergent virulent populations in agroecosystems provides an opportunity to increase our understanding of the co-evolution dynamics of pathogens and their hosts. On the one hand host plants exert selective pressure on pathogen populations, thus dividing them into subpopulations of different virulence, while on the other hand they create an opportunity for secondary contact between the two divergent populations on one tree. The main objectives of the study were to explore whether the previously reported structure between two Venturia inaequalis population types, virulent or avirulent towards Malus x domestica cultivars carrying Rvi6 gene, is maintained or broken several years after the first emergence of new virulent strains in Poland, and to investigate the relationship between ‘new’ and ‘native’ populations derived from the same commercial orchards. For this purpose, we investigated the genetic structure of populations of the apple scab fungus, occurring on apple tree cultivars containing Rvi6, Rvi1 or Rvi17 resistance gene or no resistance at all, based on microsatellite data obtained from 606 strains sampled in 10 orchards composed of various host cultivars.ResultsApplication of genetic distance inferring and clustering methods allowed us to observe clear genetic distinctness of the populations virulent towards cultivars carrying Rvi6 gene from the Rvi6-avirulent populations and substructures within the Rvi6-group as a consequence of independent immigration events followed by rare, long-distance dispersals. We did not observe such a structuring effect of other genes determining apple scab resistance on any other populations, which in turn were genetically homogenous. However, in two orchards the co-occurrence of strains of different virulence pattern on the same trees was detected, blurring the genetic boundaries between populations.ConclusionsAmong several resistance genes studied, only Rvi6 exerted selective pressure on pathogens populations: those virulent toward Rvi6 hosts show unique and clear genetic and virulence pattern. For the first time in commercial Malus x domestica orchards, we reported secondary contacts between populations virulent and avirulent toward Rvi6 hosts. These two populations, first diverged in allopatry, second came into contact and subsequently began interbreeding, in such way that they show unambiguous footprints of gene flow today.

Searching for resistance genes to Venturia inaequalis in wild and domestic apples in Iran

Abstract Scab caused by the fungus Venturia inaequalis (Cook) G. Winter is the most devastating disease of the apple worldwide. To control the disease most efforts have been directed toward the breeding programmes by introgression of scab resistance genes from wild apples into commercial cultivars. Although, various apple genotypes exist in Iran, no comprehensive data regarding the possible presence of scab resistance genes is available. In this research the presence of five R genes (Rvi2, Rvi4, Rvi6, Rvi8 and Rvi11) was assessed in 28 wild and domestic apple genotypes using 13 SCAR and SSR markers. Sequencing and blastN results confirmed the identify of Rvi2, Rvi6, Rvi8 and Rvi11 genes. Based on the results, wild apples had more resistance genes and among foreign and autochthonous apples, ‘Golden Delicious’, ‘Malayer-1’ and ‘Shahroud-6’ carrying at least three R genes categorized as almost tolerant cultivars. However, ‘Gala’, ‘Boshghabi-e Damavand’, ‘Manouchehri’ and ‘Golab’ had just one R gene and classified as susceptible cultivars. The most frequent scab resistance breeding programmes rely on Rvi6 gene (HcrVf2 gene), which was also detected in this study in crabapple Malus floribunda providing a rich resource for developing resistant apple cultivars in Iran.

Ascorbic acid content in apple pulp, peel, and monovarietal cloudy juices of 64 different cultivars

ABSTRACTThe present work was designed to compare the vitamin C (ascorbic acid, AsA) content of pulp, peel, and juice of 64 apple cultivars. These cultivars were carefully identified as ‘true to type’ by molecular genetic tools, grown in the same site under identical conditions and processed by a standardized protocol. Twenty-one of them, accounting for more than 95% of the apple production of South Tyrol, were chosen to represent the current market, 16 were old or local cultivars formerly grown in the area, and 27 were new cultivars, including 15 with scab resistance and 12 with red flesh fruit. For the determination of the AsA content, a new High-Performance Liquid Chromatography with Diode-Array Detection method was developed and validated. While old cultivars stood out for their high AsA content in pulp and peel, the red-fleshed cultivars are the ones maintaining most of their AsA content during processing. Our data thus suggest a potential for old and red-fleshed cultivars for healthy juices or further processed food components.

ESTIMATES OF GENETIC PARAMETERS AND SELECTION GAINS TO SCAB RESISTANCE IN SOUR PASSION FRUIT

ABSTRACT Passion fruit stands out as one of the most cultivated fruits in Brazil. As the culture expanded, an increase in phytosanitary problems was observed. Fungal diseases like scab are among phytosanitary problems. The aim of this study was to quantify scab intensity on leaves, and to estimate genetic parameters and selection gains to scab resistance in sour passion fruit full related families (FRF). Two experiments were conducted to evaluate the reaction of FRF, the first period from January to April (environment 1) and the second from May to August 2014 (environment 2). Experiments were conducted in greenhouse. Randomized block design for both experiments with FRF in two seasons was used, with three plants per plot and three repetitions. The evaluated characteristics were: score within 42 days after inoculation (DAI), area under the disease progress curve of severity (AUDPCS) and incidence (AUDPCI). Individual and joint variance trait analyses were carried out, estimating genetic parameters and gains per direct and indirect selection. Heritability estimates in a broad sense were higher than 89.9% for all traits in the individual and joint analysis. Selection gains were high for the evaluated traits in both environments and the greatest genetic progress was obtained by the direct selection in each feature. Of FRF selected, 92% and 36% were classified as moderate for environments 1 and 2, respectively. The highest gains in direct and indirect selection were obtained for AUDPCS in the individual analyses of environments and in the joint analysis. FRF 68, 46, 66, 75, 47, 51, 52, 54 and 77 were moderately resistant, which allows their use in a scab resistance breeding program.

Comparative proteomics analysis of young spikes of wheat in response to Fusarium graminearum infection

To investigate signaling events and defense responses in wheat–Fusarium graminearum interaction, the proteomes of young spikes with and without 24 h of F. graminearum inoculation were investigated in the resistant wheat variety Sumai 3 and the susceptible Huaimai 27. Totally 96 protein spots displayed differential expressions in one or two materials after inoculation. 65 of them were identified through mass spectrometry analysis. These proteins can be functionally classified into 12 categories and most of them participate in defense response and signaling. Our study revealed an accumulation of defense/stress response proteins, suppression of protein synthesis and photosynthesis, and changes in basal and energy metabolisms during compatible and incompatible interactions. However, differential regulation of hormone signaling, proteasome-mediated proteolysis, nitrogen metabolism, transcription, and protein modification was observed between two interactions. This result was further confirmed by analyzing the corresponding genes of 19 important proteins from six functional groups at the mRNA level. In addition, the resistant and susceptible wheat varieties showed differences in antioxidant enzymes activities and malondialdehyde contents 24-h postinoculation. Overall, the current study allows us to further dissect the possible metabolism and regulation progresses activated in F. graminearum-treated wheat spikes, and offers new understanding of mechanisms underlying wheat scab resistance.

Apple cultivar Regia possessing both Rvi2 and Rvi4 resistance genes is the source of a new race of Venturia inaequalis

The apple cultivar Regia, bred in Dresden-Pillnitz, Germany, is one of the few cultivars originating from a different scab (Venturia inaequalis) resistance background than Malus floribunda 821, the source of Rvi6. Cv. Regia is a descendant of Russian apple R12740-7A and has been proved to contain the two scab resistance genes Rvi2 and Rvi4. The cultivar itself has been grown in Dresden-Pillnitz since the early 1970s, while seedling populations derived from cv. Regia have been raised in fungicide-free plots since at least 1982. In 2011, small scab lesions were found for the first time on leaves of cv. Regia trees in an experimental unsprayed orchard in Dresden-Pillnitz. Single spore isolate Regia2 (R2) was cultured from these scab lesions and propagated, and then inoculated on scions of the VINQUEST scab differential host set comprising host (0) to host (15), host (17) and cv. Regia, grafted on rootstock M9, in a greenhouse. Strong sporulation was observed on hosts (0), (1), (2), (8), (9), (10) and cv. Regia, and weak sporulation on hosts (3), (4), (13) and (17), suggesting that the new isolate possesses the respective virulences. The implications of these results are discussed in relation to the natural infection of the differential hosts in the orchard and the artificial inoculation of the grafted differential hosts with inoculum gained from in vitro culture of R2.

Apple genes involved in the response to Venturia inaequalis and salicylic acid treatment

Abstract Apple scab, caused by the apoplastic fungal pathogen Venturia inaequalis , is one of the most damaging apple diseases worldwide and scab resistance is still one of the most important goals in apple breeding programmes. Improving knowledge of defence systems against pathogens through the identification of resistance and defence-related genes enables the development of molecular tools that can be used for assisted-selection of elite varieties. The apple scab Rvi6 (formerly HcrVf2 ) resistance gene has been extensively used to breed resistant varieties, and several defence-related genes activated downstream have recently been identified. In this work, subtractive hybridisation was performed in order to collect cDNA differentially expressed in the resistant Rvi6 cultivar ‘Florina’ after challenge with the fungus and treatment with salicylic acid. A total of 97 unique sequences were identified, and most of them were successfully annotated and located on the apple genome in silico . The expression profiles of twelve candidate genes were analysed using quantitative real-time PCR in both incompatible and compatible Malus - V.inaequalis interactions. The responses induced by the two treatments differed significantly, suggesting that salicylic acid might be not involved in apple scab defence mechanisms. Some candidate genes are involved in well-known defence strategies against apoplastic fungal pathogens, such as the reinforcement and maintenance of cell wall integrity and fungal growth hindrance. Others are presumed to play a role in the signal transduction pathway, in a similar way to that known for other pathosystems. Finally, potential mechanisms involved in the defence response against apple scab are proposed.

Realization of the genetic potential of frost hardiness in apple hybrids of different ploidy

The results of a study of resistance to unfavorable winter conditions of new selection seedlings of apple developed in the Institute as part of priority directions of apple breeding are presented. The goal of the work is to estimate winter hardiness of apple genotypes developed by combining scab resistance in a triploid genotype (RVi6+3х). Methods of a winter hardiness study in the field and laboratory conditions were used by modelling damage factors in a “ESPEC” PSL-2 KPN deep-freezer . It was determined that by early December the genotypes gained a good hardening and displayed the resistance of vitally important tissues to early winter frosts up to 30 °C (frost resistance component I). The genotypes were revealed that under artificial freezing bore critical temperatures –38 and –40 °C as well as –42 °C with damages of vitally important tissues at the level of Antonovka Obyknovennaya (component II). The modelling of a three-day artificial thaw (+2 °C) with the following temperature lowering to –30 °C showed that in all studied apple seedlings the damages of bark, cambium and wood were reversible and did not exceed 2.0 points while in immune, triploid forms 31-2-130 and ELS 31-36-149, the damages did not exceed 1.2 points. The winter hardiness study of selection apple seedlings allowed revealing the most winter hardy and promising genotypes: scab resistant and triploid forms 31-2-15, 31-2-115, 31-2-130 (Afrodita×13-6-106) and 31-36-149 [Veniaminovskoye×25-35-144 (Wealthy tetraploid× Papirovka tetraploid)] as well as scab resistant diploids 32-35-58 (Yubiley Moskvy×Krasa Sverdlovska) and 31-15-126 [23-16-96 (seedling 814 – open pollination)×Gulliver)]. Those genotypes displayed the resistance of vitally important tissues to early winter frosts, frosts in the middle of winter up to –40 °C at the level of Antonovka Obyknovennaya as well as to the frosts after thaws and retained the ability to restore resistance during repeated hardening after thaws.

Practical DNA markers to Estimate Apple (Malus x domestica Borkh.) Skin Color, Ethylene Production and Pathogen Resistance

Determination of agronomically important traits with DNA markers in genetically segregating populations is a key to high-throughput breeding of apple trees. In the present report, we first estimated usefulness of four PCR-based DNA markers of apple in a set of 22 apple cultivars, which include popular cultivars in Japan such as ‘Fuji’, ‘Orin’, and ‘Shinano Gold’. The four DNA markers are reported to determine fruit storage life (ethylene production), fruit skin color, Alternaria resistance, and scab resistance. The target genes/locus of these DNA markers are MdACS1 gene, MdMYB1 gene, MdAlt locus, and MfVf gene. The types of these DNA markers are SSLP, dCAPS, allele-specific, and gene-specific, thus the markers were designated MdACS1-SSLP, MdMYB1-dCAPS(BstEII), MdAlt-AS, and MfVf-GS, respectively. DNA band patterns after gel electrophoresis were reasonable and consistent with previous reports for MdACS1-SSLP and MfVf-GS markers. On the contrary, band patterns of the MdAlt-AS marker were not necessarily consistent with Alternaria tolerance of the apple cultivars. It was also quite difficult to determine genotypes from the band patterns of the BstEII-based dCAPS marker MdMYB1-dCAPS(BstEII). Attachment of DS6 adaptor sequence to the forward primer improved genotype determinations, by making clear difference between the sizes of allelic DNA bands. Further improvement of this DNA marker was also achieved by the MdMYB1-CAPS(PmlI) marker, which is a PmlI-based CAPS marker. MdMYB1 genotypes determined by the MdMYB1-CAPS(PmlI) marker was consistent with apple skin colors and their genetic segregations.

Retrotransposon-based sequence-specific amplified polymorphism markers for the analysis of genetic diversity and phylogeny in Malus Mill. (Rosaceae)

Sequence-Specific Amplified Polymorphism (S-SAP)-analysis of 131 accessions of different species from five sections of the genus Malus was carried out to study genetic diversity, and to clarify phylogenetic and taxonomic issues. S-SAP-markers, based on long terminal repeat (LTR)-retrotransposons TRIM2 and dem1, were developed, which identified 679 polymorphic fragments in the studied Malus species. S-SAP technique proved to be effective for taxonomic studies in Malus. The obtained results generally support the existing sectional taxonomy in the genus Malus and allowed to determine the taxonomic status of some Russian landraces. The genetic diversity and taxonomic status of the Russian apple Antonovka landraces, widely used in breeding programs for their increased adaptation to abiotic stress and scab resistance, were clarified. All of them belong to M. domestica, section Malus. However, Antonovka Olginskaya might have a hybrid origin with some contribution from Gymnomeles species according to PCO analysis data. The taxonomic status was resolved for another Russian landrace, Yakutskaya, which exhibits increased winter hardiness and drought resistance; it belongs to section Gymnomeles, with a high resemblance to M. baccata.

Genetic diversity of genus Malus Mill. for scab resistance genes

The results of molecular genetic analysis of wild species and varieties of genus Malus Mill. for loci of monogenic scab resistance are presented. Dominant allele of gene Rvi6 was identified in varieties of M. robusta 43199, M. hupehensis, and M. floribunda. Locus Rvi4 was detected in genotypes of M. robusta v. persicifolia, M. asiatica 2343, M. sieversii 13280, M. niedzwetzkyana 13279, M. purpurea 2392, M. orientalis 41623, M. orientalis 29476, M. orientalis 29 460, M. spectabilis v. albi plena, and M. spectabilis v. rubra plena. Gene Rvi5 in the test collection of wild species is absent.

Scab Susceptibility of a Provenance Collection of Pecan in Three Different Seasons in the Southeastern United States

Pecan scab (caused by Fusicladium effusum) is the most economically destructive disease of pecan in the Southeast United States. Wet, humid conditions typical of the Southeast are known to provide conditions conducive to epidemics. A provenance collection of pecan from 19 locations representing the native range of the tree is located in Byron, Georgia, and was assessed for pecan scab severity in 1998, 2013, and 2014. There were significant differences among the 19 provenances (F = 5.6 to 62.5, P < 0.0001). Provenances from wetter locations (generally north of Texas) had the greatest proportion of scab resistant trees, while provenances from the drier southern areas (Texas and Mexico) tended to be the most susceptible to scab. The association with rainfall was borne out by correlation analysis (r = -0.625 to -0.823 [P < 0.0001 to 0.004]). Other factors consistently associated with scab severity included leaflet tilt and droop angle (r = -0.533 to -0.883 [P < 0.0001 to 0.02]). Multiple regression analysis demonstrated that leaflet droop angle was a particularly good predictor of provenance susceptibility. Leaflet characteristics vary with provenance location, and whether there is a direct relationship between scab severity and leaflet characteristics is not established. Estimates of heritability were not entirely consistent among years, but different methods were used to assess scab severity in 1998 (a 1 to 5 category scale) compared with 2013 and 2014 (the percent ratio scale). Despite using different methods, there was generally good agreement among years in regard to severity of disease on individual trees. In conclusion, trees from more northern populations (in areas with greater annual rainfall) are most likely to provide valuable and diverse sources of resistance to scab. The provenance collection contains a range of scab-resistant genotypes from diverse locations that can contribute to genetic improvement regarding scab resistance.

Occurrence of Venturia inaequalis races in Poland able to overcome specific apple scab resistance genes

The aim of this study was to confirm the presence of races in populations of the fungus Venturia inaequalis that are able to overcome specific apple scab resistance gene(s) within the major apple-growing areas of Poland. The monitoring was conducted in six orchards located in the north, centre and south of Poland. The study involved the use of 16 differential genotypes for pathogenicity testing conducted under both greenhouse and orchard conditions. In addition, the occurrence of apple scab on 10 apple cultivars containing the Rvi6 gene was assessed in four organic orchards in central Poland. Apple scab was found on their leaves for the first time in Poland in 2010. The use of differential genotypes containing specific resistance genes suggested that 10 apple scab resistance genes had been overcome by V. inaequalis in the orchards monitored in this study.

Characterization of Volatile Compounds in New Cider Apple Genotypes Using Multivariate Analysis

Gas chromatography combined with solid-phase microextraction has been used for the identification of the aromatic profiles of new cider apple genotypes, and a chemometric characterization of these new cider apple genotypes has been carried out using exploratory and modelling techniques. Three breeding targets have been explored: (1) regular bearing and scab resistance, (2) resistance to bio-aggressors and (3) high polyphenol content and late ripening. Exploratory techniques established two genotype groups: those that come from breeding towards targets 1 and 2 with low polyphenol contents and those that come from breeding towards target 3 with high polyphenol contents. Alcohols were related to the genotypes with breeding towards target 3, and compounds such as esters were related to the genotypes with breeding towards targets 1 and 2. Models computed using the soft independent modelling of class analogy (SIMCA) technique presented good sensitivity (93 %), specificity (91 %) and classification hits (96 %). However, the predictions computed by SIMCA (70 %) and the artificial neural network (ANN) (76 %) were low.

Heritability of powdery mildew and scab resistance within apple progenies

Blažek J., Zelený L., Křelinova J. (2016): Heritability of powdery mildew and scab resistance within apple progenies. Plant Protect. Sci., 52: 123–127. The occurrence of both diseases was evaluated for four years in a selected orchard where 22 different apple progenies, totalling 699 seedlings were planted. During this period, no chemical protection against the diseases was applied in the orchard, and the incidence of the diseases was rated using a 9-point scale (from 9 = not infected to 1 = very highly infected). For the finalisation of the results, each seedling was characterised by the highest rate of infection from all four years of the evaluation. The lowest mean level of mildew infection equal to 7.47 was observed in the progeny of Resista × McIntosh Wijcik. Concerning the scab, the best was the progeny of Resista × Karmina having a rating equal to 8.27. The highest level of segregation of seedlings having joint resistance to both diseases was found out in the progenies obtained by crossings of HL665 × HL782 and Resista × HL447.

Identifying Rare FHB-Resistant Segregants in Intransigent Backcross and F2 Winter Wheat Populations.

Fusarium head blight (FHB), caused mainly by Fusarium graminearum Schwabe [telomorph: Gibberella zeae Schwein.(Petch)] in the US, is one of the most destructive diseases of wheat (Triticum aestivum L. and T. durum L.). Infected grain is usually contaminated with deoxynivalenol (DON), a serious mycotoxin. The challenge in FHB resistance breeding is combining resistance with superior agronomic and quality characteristics. Exotic QTL are widely used to improve FHB resistance. Success depends on the genetic background into which the QTL are introgressed, whether through backcrossing or forward crossing; QTL expression is impossible to predict. In this study four high-yielding soft red winter wheat breeding lines with little or no scab resistance were each crossed to a donor parent (VA01W-476) with resistance alleles at two QTL: Fhb1 (chromosome 3BS) and QFhs.nau-2DL (chromosome 2DL) to generate backcross and F2 progeny. F2 individuals were genotyped and assigned to 4 groups according to presence/ absence of resistance alleles at one or both QTL. The effectiveness of these QTL in reducing FHB rating, incidence, index, severity, Fusarium-damaged kernels (FDK) and DON, in F2-derived lines was assessed over 2 years. Fhb1 showed an average reduction in DON of 17.5%, and conferred significant resistance in 3 of 4 populations. QFhs.nau-2DL reduced DON 6.7% on average and conferred significant resistance in 2 of 4 populations. The combination of Fhb1 and QFhs.nau-2DL resistance reduced DON 25.5% across all populations. Double resistant lines had significantly reduced DON compared to double susceptible lines in 3 populations. Backcross derived progeny were planted in replicated yield trials (2011 and 2012) and in a scab nursery in 2012. Several top yielding lines performed well in the scab nursery, with acceptable DON concentrations, even though the average effect of either QTL in this population was not significant. Population selection is often viewed as an “all or nothing” process: if the average resistance level is insufficient, the population is discarded. These results indicate that it may be possible to find rare segregants which combine scab resistance, superior agronomic performance and acceptable quality even in populations in which the average effect of the QTL is muted or negligible.