Barley yellow dwarf virus resistance in the genus Avena

Summary The barley yellow dwarf virus resistance level was evaluated in acessions of most species of the genus Avena. Highest levels of resistance were found in A. sterilis and A. occidentalis. High resistance levels were also found in A. barbata, A. fatua, A. hybrida, A. macrostachya, diploid A. nuda and A. strigosa. Results are discussed in relation to breeding for resistance.Contribution No. 229.     

Inheritance of resistance to the Moroccan watermelon mosaic virus in the cucumber line TMG-1 and cosegregation with zucchini yellow mosaic virus resistance

Inbred lines derived from the Chinese cucumber (Cucumis sativus L.) cultivar, ‘Taichung Mou Gua’ (TMG), have been shown to be resistant to several potyviruses including: zucchini yellow mosaic virus (ZYMV), zucchini yellow fleck virus (ZYFV), watermelon mosaic virus (WMV) and the watermelon strain of papaya ringspot virus (PRSV-W). Recently, an additional virus that infects cucurbits, the Moroccan watermelon mosaic virus (MWMV), has been determined to be a distinct member of the potyvirus group. This study demonstrates that TMG-1 possesses resistance to MWMV. Rub or aphid inoculated TMG-1 seedlings remain free of symptoms. Progeny analyses of the F₁, F₂ and backcross generations show that resistance to MWMV is conferred by a single recessive gene (proposed designation, mwm). Sequential inoculation of progeny possessing resistance to ZYMV followed by MWMV (or MWMV followed by ZYMV) and analysis of F₃ families derived from F₂ individuals selected for resistance to ZYMV indicate that both resistances are conferred by the same gene, or two tightly linked genes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Screening the Spanish Barley Core Collection for disease resistance

The Spanish Barley Core Collection comprises 159 landrace-derived inbred lines and 16 cultivars adapted to Southern European conditions. The collection was screened for resistance to powdery mildew ( Blumeria graminis ), scald ( Rhynchosporium secalis ), leaf rust ( Puccinia hordei ), net blotch ( Pyrenophora teres f. teres ), Barley yellow dwarf virus (BYDV) and Barley mild mosaic virus (BaMMV). Resistance to powdery mildew was outstanding, with 58 lines presenting mean overall resistance, among them seven landrace-derived lines resistant to all seven isolates tested. About 26% of the Spanish lines were resistant to scald. Resistance to leaf rust and to net blotch was scarce, though a few accessions showed resistance levels as good as the checks. Thirteen accessions (12 Spanish) were totally resistant to BaMMV and c. 20% of accessions showed moderate tolerance to BYDV. Landrace-derived lines from the Mediterranean Coast and Southern regions of Spain were the most resistant to powdery mildew and leaf rust, but the most susceptible to viruses. Potential sources of resistance might be preserved in some accessions subjected to selective pressure in the region of origin.     

Combination of resistances to barley yellow mosaic virus and Rhynchosporium secalis by recurrent selection with repeated haploid steps

The aim of this investigation was the incorporation of resistance to BaYMV and Rhynchosporium secalis into German winter barley varieties together with the maintenance of their high agronomic characters. The agronomic value of DH-lines after two backcrosses and three haploid steps were compared with the original parents. Lines with similar or superior results could be selected for all the agronomic features evaluated. This method of recurrent selection with repeated haploid steps can therefore lead to the development of lines useful for practical purposes.     

A new resistance gene to PepYMV (Pepper yellow mosaic virus) in Capsicum annuum L.

Pvr4 locus, commonly associated with a co-dominant CAPS marker, confers monogenic dominant resistance to potyvirus complex in Capsicum spp. Aiming to investigate whether the resistance found in resistant genotypes not bearing Pvr4 marker is due to allelism in the Pvr4 locus, or due to a new locus of the pvr series, segregation analyses of an F₂ population obtained from a cross between two pepper lines “Myr-29-10” (P₁) (resistant to PepYMV, showing a single band of 444 bp, Pvr4/Pvr4) and “PIM-025” (P₂) (resistant to PepYMV, showing a single band of 458 bp, Pvr4⁺/Pvr4⁺) were performed. According to the results, there is strong evidence that the locus controlling PepYMV resistance in PIM-025 (P₂) is not Pvr4. These results provide evidence that those resistant genotypes, bearing a susceptible band pattern in the Pvr4 locus (458 bp, Pvr4⁺/Pvr4⁺), carry a different gene from those described in the literature up to the present time.     

Pyramiding genes for resistance to bean common mosaic virus

Summary Gene pyramiding in Phaseolus vulgaris is being utilized to develop more effective resistance to the temperature-insensitive-necrosis-inducing (TINI) strains of Bean Common Mosaic Virus (BCMV) present in the USA. Our data indicate that contrary to previous work, the bc-3 gene is effective against these strains in the absence of the strain unspecific bc-u gene in genotypes possessing the I gene. The epistatic bc-3 gene interferes with traditional efforts to pyramid the other recessive resistance genes by masking their activity. Indirect selection based on markers linked to the other recessive resistance genes would likewise be ineffective without the ability to also select for the bc-u gene which is required for expression of the bc-2 ² gene in germplasm carrying the I gene. Because the most resistant genotype (I, bc-u, bc-I ², bc-2², bc-3) can only be introduced into a wide range of germplasm through the use of molecular markers linked to the different resistance genes, the search for a marker linked to the strain unspecific bc-u gene should also be given priority.     

Single-gene resistance to zucchini yellow mosaic virus in Cucurbita moschata

Summary The mode of inheritance for resistance to zucchini yellow mosaic virus (ZYMV) in Cucurbita moschata was determined from F₁, F₂, and backcross progenies of the cross between the susceptible cultivar Waltham Butternut from the U.S.A. and a resistant inbred line of the Menina cultivar from Portugal. Resistance to ZYMV in Menina was conferred by a single dominant gene designated Zym.     

Major genes for resistance to beet necrotic yellow vein virus (BNYVV) in Beta vulgaris

Summary Inheritance of resistance to beet necrotic yellow vein virus (BNYVV) was studied in segregating F2 and backcross families obtained from crosses between resistant plants of the sugar beet selection Holly-1-4 or the wild beet accession Beta vulgaris subsp. maritima WB42 and susceptible parents. Greenhouse tests were carried out, in which seedlings were grown in a mixture of sand and infested soil. Virus concentrations of BNYVV in the rootlets were estimated by ELISA. To discriminate resistant and susceptible plants, mixtures of normal distributions were fitted to log₁₀ virus concentrations, estimated for segregating F1, F2 and BC populations of both accessions. The hypothesis that Holly-1-4 contained one single dominant major gene was accepted. For WB42, results fitted with the hypotheses that resistance was based on either one (or more) dominant major gene(s) showing distorted segregation, or two complementary dominant genes, which are both required for resistance. Resistance from WB42 appeared to be more effective against BNYVV than resistance from Holly-1-4.This research was carried out as part of a PhD study at the Graduate School Experimental Plant Sciences (EPS), Department of Virology, Wageningen, The Netherlands     

Evaluation of tomato entries with different combinations of resistance genes to tomato yellow leaf curl disease in Tunisia

Tomato yellow leaf curl virus (TYLCV) is one of the most widespread begomoviruses transmitted by the whitefly Bemisia tabaci that cause tomato yellow leaf curl virus diseases (TYLCDs). TYLCD losses can be especially severe in open‐field tomato (Solanum lycopersicum L.) crops. TYLCV is particularly well known and widespread in the Mediterranean (Med) countries where TYLCD control is mostly based on insecticidal control of B. tabaci populations. Unfortunately, Tunisian B. tabaci populations include Middle East‐Asia Minor I and the Med species that have developed resistance to many classes of insecticides. Therefore, TYLCD‐resistant cultivars are essential for sustainable disease management. Six TYLCD resistance genes (Ty) have been introgressed from wild species into cultivated tomato and are available for breeding. Information on the Ty genes or gene combinations is useful for breeding resistant cultivars. To this end, 14 tomato lines carrying different Ty gene combinations and two susceptible tomato entries were evaluated for TYLCD incidence and severity in two field trials during late season in Tunisia. Entries with Ty‐1/Ty‐3 + Ty‐2 offered the highest levels of resistance in Tunisia.     

Role of male-sterile cytoplasm in resistance to barley yellow mosaic virus and Fusarium head blight in barley

Two types of male‐sterile cytoplasm, designated msm1 and msm2, in barley were investigated to determine whether these cytoplasms confer resistance to barley yellow mosaic virus (Ba YMV) and Fusarium head blight (FHB). Alloplasmic lines and isogenic lines of two cultivars showed the same reaction to each Ba YMV as that of their euplasmic lines. This demonstrates that the barley male‐sterile cytoplasms msm1 and msm2 have no effect on resistance to BaYMV. No significant difference in reactions to FHB was recognized among fertile alloplasmic lines of ‘Adorra’, but the difference in reactions to FHB between fertile and sterile isogenic lines of ‘Adorra’ was significant. The damage caused by FHB in the male‐sterile lines that produced sterile pollen was significantly greater than the damage in a sterile line that did not produce pollen. These results suggest that pollen or anthers are important factors in infection with or spread of FHB. For production of hybrid seeds, male‐sterile lines with no pollen production, such as those with msm1 male‐sterile cytoplasm, would reduce FHB infestation.     

Inheritance of zucchini yellow mosaic virus resistance in Cucumis melo L.

Summary Resistance to zucchini yellow mosaic virus has been found in the muskmelon line PI 414723 from India. This resistance is effective against the ZYMV strains E15 and 1318 belonging respectively to the NF and F pathotypes. Resistance to E15 (no vein clearing and yellowing symptoms) is governed by one dominant gene (symbol Zym) according to segregations observed in F₁, F₂ and BC₁ progenies. This gene is epistatic dominant over Fn, which induces wilting and necrosis after inoculation with F pathotype. Linkage studies suggest that Zym inherits independently from Fom-1, Fom-2, Vat, Wmv and Fn but is linked with a (13.1 ±2.4 units).     

Strategies using genes for non-durable disease resistance

Strategies to deploy non-durable `major' or `R' genes for resistance to plant diseases are reviewed. The reasons for continuation of deployment of R genes singly despite its obvious failure are discussed. Alternative deployment strategies are described and the potential impact of plant biotechnology on the deployment of resistance genes is discussed. A strategy focusing on the crop phenotype, taking into account production practices, the nature of the pathogen and alternative control measures to determine the most appropriate deployment of host resistance genes is advocated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Identification and characterization of a RAPD/SCAR marker linked to a resistance gene for soybean mosaic virus in soybean

Soybean line `ICGR95-5383' [Glycinemax (L.) Merr.] is a newly releasedgermplasm from China and is resistant (R)to soybean mosaic virus (SMV). ICGR95-5383was crossed to the susceptible (S)cultivars `HB1', `Tiefeng21', `Amsoy', and`Williams' to investigate the inheritanceof SMV resistance. The F<sub>1</sub> and F<sub>2</sub>plants were inoculated with SMV-3 (the mostvirulent) strain from Northeast China. Theresults showed that F<sub>1</sub> plants from thefour R × S crosses were necrotic (N) andall F<sub>2</sub> populations segregated in a3(R+N):1S ratio, indicating thatICGR95-5383 carries a single gene withincomplete dominance for resistance to SMV. In a bulked segregant analysis (BSA) of theF<sub>2</sub>population from ICGR95-5383 × HB1, a codominant RAPD marker,OPN11<sub>980/1070</sub>, was found to be linkedto the resistance gene in ICGR95-5383. The980-base pair (bp) fragment OPN11<sub>980</sub>was amplified in the R parent ICGR95-5383,R bulk, and resistant F<sub>2</sub> plants. Theother 1070-bp fragment OPN11<sub>1070</sub> wasamplified in the S parent HB1, S bulk, andsusceptible F<sub>2</sub>plants.OPN11<sub>980/1070</sub> was amplified in theF<sub>1</sub> plants and the necroticF<sub>2</sub> plants from the R×S cross.Segregation analysis of the RAPD marker inthe F<sub>2</sub> population revealed that themarker OPN11<sub>980/1070</sub> is closely linkedto the resistance gene with a map distanceof 3.03 cM. OPN11<sub>980/1070</sub> was clonedand sequenced, and specific PCR primerswere designed to convertOPN11<sub>980/1070</sub> into sequencecharacterized amplified region (SCAR) makerSCN11<sub>980/1070</sub>. SCAR analysis of theF<sub>2</sub> population confirmed thatOPN11<sub>980/1070</sub> and SCN11<sub>980/1070</sub> areat the same locus linked to the SMVresistance gene. The RAPD markerOPN11<sub>980</sub> was used as RFLP probefor southern hybridization to soybeangenomic DNA. Southern analysis showed thatsoybean genome contains low-copy sequenceof OPN11<sub>980</sub>. Using a recombinant inbredmapping population of `Kefeng No.1' (R) ×Nannong1138-2'(S), OPN11_980/1070 was mapped to thesoybean molecular linkage group (MLG) Fbetween the restriction fragment lengthpolymorphism (RFLP) markers B212 (0.7 cM) and K07 (6.7 cM) and 3.03 cM apart from theSMV resistance gene.     

Barley yellow dwarf virus in relation to the breeding and assessment of herbage grasses for yield and uniformity

Summary Ryegrass plants (Lolium spp.) rarely developed obvious foliar symptoms when infected with barley yellow dwarf virus, but their yield was often substantially reduced. Perhaps of even greater importance, the virus modified their morphological development and seasonal growth rhythm. Consequently, the disease introduces considerable non-heritable variation into populations of grass plants so its possible presence may introduce difficulties into selecting varietal mother plants for specific growth characteristics and when assessing bred varieties for uniformity.Virus infection caused the greatest deviation in growth morphology and the smallest reduction in yield in early-flowering hay type varieties: the converse was true of late-flowering pasture type varieties. Unless a source of immunity or extreme resistance to the virus can be found therefore, it may be difficult to combine minimum deviation in growth morphology and minimum yield loss in a single variety.     

Resistance of Cucurbita moschata to watermelon mosaic virus type 2 and its genetic relation to resistance to zucchini yellow mosaic virus

Summary Cucurbita moschata Menina originating from Portugal is known to carry a single dominant gene, Zym, for zucchini yellow mosaic virus (ZYMV) resistance. Resistance to watermelon mosaic virus type 2 (WMV2) was found in the same cultivar. In resistant plants, WMV2 migration from inoculated leaves or cotyledons seems to be limited and blocked. Resistance to WMV2 is conferred by one dominant gene and is effective against eight strains from different geographic origins. Results of linkage studies, based on co-inoculation of plants with WMV2 and ZYMV, indicate that resistance to these two viruses is conferred by the same gene, probably Zym, or perhaps by two closely linked genes.     

Double dose efficiency of the yellow rust resistance gene Yr17 in bread wheat lines

Yellow rust, caused by Puccinia striiformis f. sp. tritici, is one of the most severe wheat disease worldwide. Crop losses have ranged from 10% to 70% and up to 100% in extreme conditions. Eighty-two resistance genes, designated Yr, have been identified. Among them, Yr17 derived from Aegilops ventricosa and located on chromosome 2A has been widely used in wheat breeding. However, it had been overcome already. Through recombination of the Ae. ventricosa Yr17-carrying 6N^v chromosome with 2D of wheat, we introduced Yr17 onto chromosome 2D. Then, lines carrying Yr17 on both 2A and 2D were generated. Seedlings of the latter, as well as those carrying a single dose of Yr17 either on 2A or on 2D, were inoculated with virulent or avirulent strains on wheat seedlings. The different genotypes were fully susceptible for the two pathotypes that are virulent on Yr17. In the case of avirulent pathotypes, the Yr17 double dose lines were fully resistant, while those with the Yr17 gene only on either 2A or 2D had intermediate resistance reactions towards one or the other or both pathotypes.     

Genetic analysis and identification of two soybean mosaic virus resistance genes in soybean [Glycine max (L.) Merr]

Soybean mosaic virus (SMV) commonly affects soybean production worldwide, and the SC18 strain has been widespread in China. This study aimed to characterize and map the SC18 resistance genes present in soybean cultivars ‘Kefeng No. 1’ and ‘Qihuang 22’. Inheritance analysis revealed that two independent single dominant genes in Kefeng No. 1 and Qihuang 22 confer resistance to SC18. Using simple sequence repeat (SSR) markers and bulked segregant analysis, the Kefeng No. 1 and Qihuang 22 resistance genes were located on soybean chromosomes 2 and 13, respectively. We further screened two populations of recombinant inbred lines with 32 SSR markers in the target region, where the resistance gene in Kefeng No. 1 was fine mapped to an 80‐kb region containing six putative genes. Sequence and expression analyses of these genes revealed that SMV resistance in Kefeng No. 1 was probably attributable to three of the candidate genes (i.e. Glyma.02G127800, Glyma.02G128200 and Glyma.02G128300). Collectively, the results of this study will greatly facilitate the cloning of SC18 resistance genes and marker‐assisted breeding of SMV‐resistant soybean cultivars.     

Inheritance and linkage analysis of resistance to zucchini yellow mosaic virus, watermelon mosaic virus, papaya ringspot virus and powdery mildew in melon

A melon (Cucumis melo L.) breeding line derived from PI 414723 is resistant to three potyviruses,watermelon mosaic virus (WMV), zucchini yellow mosaic virus (ZYMV), papaya ringspot virus (PRSV), and to powdery mildew (PM). The inheritance and linkage relationships of these four resistances were studied in a segregating F2 population and derived F3 families from a cross between cultivar Top Mark and the resistant breeding line. Dominant monogenic inheritance of all four resistances was observed. We report that line 414723-4S3, which was initially selected as a source of ZYMV and WMV resistance, is also a source of dominant monogenic resistances to PRSV and PM race 1. We also report on genetic linkage (significant departure from independent segregation, χ² = 58.1, p≪ 0.0001) between resistance to WMV and ZYMV. The map distance between these loci was estimated to be 7.5 cm. The genes for resistance to PM and PRSV segregated independently from each other, and from ZYMV and WMV resistance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Potential and limitations of isozymes for chromosomal localization of resistance genes against barley mild mosaic virus (BaMMV)

Summary To assess the possibilities offered by isozymes to locate resistance genes against barley mild mosaic virus (BaMMV), the isozyme patterns of 19 barley (Hordeum vulgare L.) genotypes carrying genes different from ym4 were determined. Of the 15 isozyme systems tested, only three were polymorphic, namely aconitate hydratase, esterase, phosphogluconate dehydrogenase, providing markers on four of the seven barley chromosomes. Studies of F₂ progenies derived from three crosses between resistant genotypes and susceptible varieties failed to reveal linkage between resistance genes and isozymes. Another goal of the experiment was to study the linkage relationships between ym4 and the esterase locus (Est1-Est2-Est4). Our estimates of the recombination rate between these two loci (3.41 and 8.32%) were in the range of those reported between these esterases and one of the resistance genes of the Chinese variety Mokusekko 3.