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.     

Identification and mapping of quantitative resistance to late blight (<Emphasis Type="Italic">Phytophthora infestans</Emphasis>) in <Emphasis Type="Italic">Solanum habrochaites</Emphasis> LA1777

Late blight (Phytophthora infestans) can have devastating effects on tomato production over the whole world. Most of the commercial cultivars of tomato, Solanum lycopersicum, are susceptible. Qualitative and quantitative resistance has been described in wild relatives of tomato. In general qualitative resistance can more easily be overcome by newly evolved isolates. Screening of three S. habrochaites accessions (LA1033, LA2099 and LA1777) through a whole plant assay showed that accession LA1777 had a good level of resistance to several isolates of P. infestans. To explore the potential in this wild species, an introgression line (IL) population of S. habrochaites LA1777 was used to screen individual chromosome regions of the wild species by a detached leaf assay. Two major isolates (T_1,2 and T_1,2,4) were used and two parameters were measured: lesion size (LS), and disease incidence (DI). Substantial variation was observed between the individual lines. QTLs were identified for LS but not for DI. The presence of five QTLs derived from LA1777 (Rlbq4a, Rlbq4b, Rlbq7, Rlbq8 and Rlbq12) results in unambiguous higher levels of resistance. All QTLs co-localized with previously described QTLs from S. habrochaites LA2099 except QTL Rlbq4b, which is therefore a novel QTL.     

Search in <Emphasis Type="Italic">Solanum</Emphasis> (section <Emphasis Type="Italic">Lycopersicon</Emphasis>) germplasm for sources of broad-spectrum resistance to four <Emphasis Type="Italic">Tospovirus</Emphasis> species

The genus Tospovirus was considered as monotypic with Tomato spotted wilt virus (TSWV) being the only assigned species. However, extensive studies with worldwide isolates revealed that this genus comprises a number of species with distinct virulence profiles. The Neotropical South America is one center of Tospovirus diversity with many endemic species. Groundnut ringspot virus (GRSV), TSWV, Tomato chlorotic spot virus (TCSV), and Chrysanthemum stem necrosis virus (CSNV) are the predominant tomato-infecting species in Brazil. Sources of resistance were found in Solanum (section Lycopersicon) mainly against TSWV isolates from distinct continents, but there is an overall lack of information about resistance to other viral species. One-hundred and five Solanum (section Lycopersicon: Solanaceae) accessions were initially evaluated for their reaction against a GRSV isolate by analysis of symptom expression and systemic virus accumulation using DAS-ELISA. A subgroup comprising the most resistant accessions was re-evaluated in a second assay with TSWV, TCSV, and GRSV isolates and in a third assay with a CSNV isolate. Seven S. peruvianum accessions displayed a broad-spectrum resistance to all viral species with all plants being free of symptoms and systemic infection. Sources of resistance were also found in tomato cultivars with the Sw-5 gene and also in accessions of S. pimpinellifolium, S. chilense, S. arcanum, S. habrochaites, S. corneliomuelleri, and S. lycopersicum. The introgression/incorporation of these genetic factors into cultivated tomato varieties might allow the development of genetic materials with broad-spectrum resistance, as well as with improved levels of phenotypic expression.     

Measurement of the field response of <Emphasis Type="Italic">Musa</Emphasis> genotypes to <Emphasis Type="Italic">Radopholus Similis</Emphasis> and <Emphasis Type="Italic">Helicotylenchus Multicinctus</Emphasis> and the implications for nematode resistance breeding

Crop growth and damage parameters (plant growth and yield, root damage and nematode population densities), believed to be associated with resistance of Musa genotypes to nematodes under field conditions, were evaluated in a field trial of 24 Musa genotypes inoculated at planting with a combination of Radopholus similis and Helicotylenchus multicinctus with the objective to identify parameters with strong association with nematode resistance and high heritability. Correlation and path analysis of the association between plant growth, yield, root damage and nematode population densities showed a strong negative association between percentage dead roots, percentage root necrosis, R. similis and H. multicinctus population densities and yield. The strongest negative association was observed between percentage dead roots and yield. Broad-sense genotype heritability estimates demonstrated that heritability estimates for percentage dead roots, number of large lesions and nematode population density were most affected by inoculation with nematodes. These results indicate therefore that effective selection for nematode resistance under field conditions could be obtained by using an index, that includes percentage dead roots, the number of large lesions, and nematode population density.     

Sources of resistance to broomrape (Orobanche crenata Forsk.) in narbon vetch

Little is known about the levels of resistance to Orobanche crenata available in narbon vetch. A germplasm collection of 200 accessions of narbon vetch (Vicia narbonensis L.) originating from different countries was screened for resistance to crenate broomrape under field conditions. Thirteen accessions were selected for more detailed screening under controlled conditions and for additional field testing. Resistance to O. crenata was manifested by lower germination of broomrape seeds, reduced emergence of Orobanche shoots and fewer root attachments per narbon vetch plant. Differences in pod yield among narbon vetch accessions were also found in response to Orobanche infestation. Finally, eight accessions were selected and identified as new sources of narbon vetch resistant to broomrape.     

Genetics of resistance to <Emphasis Type="Italic">Cucumber mosaic virus</Emphasis> in <Emphasis Type="Italic">Cucumis sativus</Emphasis> var. <Emphasis Type="Italic">hardwickii</Emphasis> R. Alef

The genetics of resistance to Cucumber mosaic virus (CMV) in Cucumis sativus var. hardwickii R. Alef, the wild progenitor of cultivated cucumber was assessed by challenge inoculation and by natural infection of CMV. Among the 31 genotypes of C. sativus var. hardwickii collected from 21 locations in India the lowest mean percent disease intensity (PDI) was recorded in IC-277048 (6.33%) while the highest PDI was observed in IC-331631 (75.33%). All the four cultivated varieties (DC-1, DC-2, CHC-1 and CHC-2) showed very high PDI and susceptible disease reaction. Based on mean PDI, 8 genotypes were categorized as resistant, 13 as moderately resistant, 9 as moderately susceptible and one as susceptible. A chi-square test of frequency distribution based on mean PDI in F₂ progenies of six resistant × susceptible crosses revealed monogenic recessive Mendelian ratio 1(R):3(S) to be the best fit. This monogenic recessive model was further confirmed by 1(R):1(S) ratio as the best fit for back cross with resistant parent and no fit for either 3:1 or 1:1 in the back cross with the susceptible parent. The results revealed that CMV resistance in C. sativus var. hardwickii was controlled by a single recessive gene. Considering the cross compatibility between C. sativus var. hardwickii and cultivated cucumber, the resistance trait can be easily transferred to cultivated species through simple backcross breeding.     

Variation of resistance to broomrape (Orobanche ramosa) in tomatoes

Summary Twenty five tomato cultivars and one accession of wild tomato were evaluated for their resistance to branched broomrape (Orobanche ramosa) infestation. Tomato cultivars were found different in their resistance to Orobanche based on different growth parameters. Total Orobanche number, haustoria development and number of emerged Orobanche shoots were all different between tomato cultivars. Differences in the growth and fruit yield among tomato cultivars were also found in response to Orobanche infestation. Ranking tomato cultivars for Orobanche resistance indicates the existence of different resistance mechanisms in these cultivars. Relatively high to moderate levels of resistance were obtained in Tiny Tim, Acora, Castler, Pomodora, Orient, Red Alert and the accession LA 1478 of L. pimpinellifolium. The cultivar Tiny Tim showed the highest level of resistance for all measured parameters and in all experiments.     

Identification of <Emphasis Type="Italic">Ascochyta rabiei</Emphasis> disease resistance in chickpea genotypes

Ascochyta blight (AB) disease, caused by the fungus Ascochyta rabiei, is a major yield limiting factor of chickpea in Australia and around the world. The aggressiveness of six A. rabiei isolates was identified using 3 chickpea varieties (Jimbour, Flipper and Yorker). These AB isolates were isolated from chickpea fields in northern NSW, one of the major chickpea production regions in Australia. Each of the six isolates produced a different aggressiveness pattern and isolate 4859 was found to be the most aggressive. The AB resistance in 16 international and Australian chickpea genotypes was then investigated by inoculating the plants with the most aggressive isolate and a mixture of the other 5 isolates. Resistance to both the most aggressive isolate and the mixed isolates has been identified in 5 genotypes (ICCV 98813, Flipper, ICCV 05111, ICCV 98801, Jimbour #1) while 10 entries (Howzat, ICCV 06108 and ICCV 98818, Jimbour, ICCV 96852, ICCV 06107, ICCV 98816, Yorker, FLIP97-114C, ICCV 96853) were moderately resistant. Only one genotype (Bumper) appeared to be susceptible to both inoculums. There was large variation observed in the pathogenicity of the isolates suggesting that the six AB isolates represent several different pathogen strains. Significant differences in leaf infection rate, plant infection rate, plant death rate and disease development were identified among the chickpea genotypes tested. These findings suggest that these chickpea genotypes carry different resistant genes, which can be exploited in breeding programmes to develop high levels of disease resistance.     

Sources of resistance to <Emphasis Type="Italic">Phytophthora</Emphasis> root rot within the genus <Emphasis Type="Italic">Beta</Emphasis>

Phytophthora root rot caused by Phytophthora drechsleri Tucker is one of the most devastating sugar beet diseases in tropical areas. To identify genetic resources resistant to this disease, an aggressive isolate of P. drechsleri was selected. Then, a screening method was optimized based on the standard scoring scales of 1–9 (1: no symptoms, 9: complete plant death). Finally, 19 sugar beet lines, three cultivars, and 14 accessions of the wild species Beta vulgaris subsp. maritima, B. macrocarpa, B. procumbens, and B. webbiana were evaluated for resistance to the most aggressive isolate of P. drechsleri by using the optimized method (inoculum included 20 g of rice seed together with superficial wound creation). The isolates of P. drechsleri had significant variation in aggressiveness, and Kv10 was the most aggressive isolate on the susceptible variety Rasoul. The lines O.T.201-15, SP85303-0 (resistant check), and S2-24.P.107 had the lowest disease index with scores of 3.09, 3.13, and 3.27 respectively; they were categorized into the resistant group. The interaction between isolates and genotypes was not significant, which indicated the same response of each genotype to different isolates. Investigating the resistance of different generations of sugar beet revealed that progeny selection would be an effective method for increasing the resistance level of breeding materials to P. drechsleri. Among the wild species, the accession 9402 belonging to B. macrocarpa and the accession 7234 of B. vulgaris subsp. maritima had the lowest disease index (2.29 and 2.60, respectively) and were categorized into the resistant group.     

Identification of RAPD markers linked to the rust (<Emphasis Type="Italic">Uromyces fabae</Emphasis>) resistance gene in pea (<Emphasis Type="Italic">Pisum sativum</Emphasis>)

Summary Two RAPD markers linked to gene for resistance (assayed as pustule number cm⁻² leaf area) to rust [Uromyces fabae (Pers.) de Bary] in pea (Pisum sativum L.) were identified using a mapping population of 31 BC₁F₁ [HUVP 1 (HUVP 1 × FC 1] plants, FC 1 being the resistant parent. The analysis of genetics of rust resistance was based on the parents, F₁, F₂, BC₁F₁ and BC₁F₂ generations. Rust resistance in pea is of non-hypersensitive type; it appeared to be governed by a single partially dominant gene for which symbol Ruf is proposed. Further, this trait seems to be affected by some polygenes in addition to the proposed oligogene Ruf. A total of 614 decamer primers were used to survey the parental polymorphism with regard to DNA amplification by polymerase chain reaction. The primers that amplified polymorphic bands present in the resistant parent (FC 1) were used for bulked segregant analysis. Those markers that amplified consistently and differentially in the resistant and susceptible bulks were separately tested with the 31 BC₁F₁ individuals. Two RAPD makers, viz., SC10-82₃₆₀ (primer, GCCGTGAAGT), and SCRI-71₁₀₀₀ (primer, GTGGCGTAGT), flanking the rust resistance gene (Ruf) with a distance of 10.8 cM (0.097 rF and LOD of 5.05) and 24.5 cM (0.194 rF and a LOD of 2.72), respectively, were identified. These RAPD markers were not close enough to Ruf to allow a dependable maker-assisted selection for rust resistance. However, if the two makers flanking Ruf were used together, the effectiveness of MAS would be improved considerably.     

Characterisation of resistance to crenate broomrape (<Emphasis Type="Italic">Orobanche crenata</Emphasis> Forsk.) in <Emphasis Type="Italic">Lathyrus cicera</Emphasis> L.

Field, pots and mini-rhizotrons experiments revealed the existence of useful levels of avoidance and resistance to Orobanche crenata Forsk. within Lathyrus cicera L. accessions, in addition to escape due to precocity or to reduced root biomass. This resistance was characterized by several phenotypic mechanisms of defence which acted alone or in combination, including low stimulation of parasitic seed germination, prevention of the radicle penetration inside the root and prevention of a correct connection by the parasite with the host vascular tissue being visible by reduced development of established parasitic tubercles or abortion of the parasitic tubercles formed.     

Genetics of resistance in <Emphasis Type="Italic">Luffa cylindrica</Emphasis> Roem. against <Emphasis Type="Italic">Tomato leaf curl New Delhi virus</Emphasis>

Sponge gourd is a popular vegetable grown throughout India. Tomato leaf curl New Delhi virus, the causal virus of tomato leaf curl disease, has recently been reported to be associated with sponge gourd, causing up to 100% crop loss under epidemic conditions. We have collected 30 genotypically diverse genotypes of sponge gourd from different parts of India, screened these for resistance under natural epiphytotic conditions, and then confirmed the results through challenge inoculation with a purified strain of the virus under insect-proof greenhouse conditions. The minimum vulnerability index was recorded in genotype DSG-6 (3.33), followed by DSG 7 (6.0) under the challenge (whitefly-populated) inoculation conditions. Two susceptible genotypes (‘Pusa Sneha’ and NSG-1-11), both possessing desirable fruit characters, were crossed with the two most promising resistant lines (DSG-6 and DSG-7) and the disease reaction of segregating and backcross generations studied through challenge inoculation with a purified strain of virus under insect-proof greenhouse conditions. A chi-square (χ²) test of frequency distribution based on the vulnerability index of the F₂ progenies of the two resistant × susceptible crosses revealed monogenic dominant Mendelian ratio 3(R):1(S) to be the best fit in all crosses. This monogenic dominant model was further confirmed by the 1(R):1(S) ratio found to be best fit for the test cross with the susceptible parent. These results reveal that resistance to Tomato leaf curl New Delhi virus associated with yellow mosaic disease of sponge gourd is controlled by a single dominant gene in the genetic background of the resistant parents (DSG-6 and DSG-7) and that these two lines can be effectively utilized for the development of high-yielding and yellow mosaic disease-resistant varieties/hybrids of sponge gourd. This is the first conclusive identification of a resistant source and the inheritance of resistance against Tomato leaf curl New Delhi virus in sponge gourd.     

Identification of quantitative trait loci involved in resistance to <Emphasis Type="Italic">Pseudomonas syringae</Emphasis> pv. <Emphasis Type="Italic">syringae</Emphasis> in pea (<Emphasis Type="Italic">Pisum sativum</Emphasis> L.)

Pseudomonas syringae is the main pathogen responsible for bacterial blight disease in pea and can cause yield losses of 70%. P. syringae pv. pisi is prevalent in most countries but the importance of P. syringae pv. syringae (Psy) is increasing. Several sources of resistance to Psy have been identified but genetics of the resistance is unknown. In this study the inheritance of resistance to Psy was studied in the pea recombinant inbred line population P665 × ‘Messire’. Results suggest a polygenic control of the resistance and two quantitative trait loci (QTL) associated with resistance, Psy1 and Psy2, were identified. The QTL explained individually 22.2 and 8.6% of the phenotypic variation, respectively. In addition 21 SSR markers were included in the P665 × ‘Messire’ map, of which six had not been mapped on the pea genome in previous studies.     

Resistance to <Emphasis Type="Italic">Bemisia tabaci</Emphasis> in tomato wild relatives

Bemisia tabaci is one of the most threatening pests in agriculture, particularly in Solanaceous crops such as tomato and pepper that are cultivated in the open field. Pesticide application is often not effective and hazardous to humans and environment. The exploitation of plant natural defenses that are present in wild relatives of tomato, may offer a solution. To evaluate resistance parameters and to identify plant material with high levels of resistance, we screened a number of accessions of tomato wild relatives using three methods; a free-choice test in a screenhouse in Indonesia, a no-choice test with clip-on cages in a greenhouse and a leaf disc test in a climate-room in the Netherlands. Antibiosis resulting in low adult survival was the major component for resistance in tomato. However, other resistance component(s) may play a role as well. In some accessions there was a change in the resistance level over time. Several resistance parameters used in the different tests were well correlated. The best resistance source was an accession of Solanum galapagense, which had not been identified as being resistant in the past. This is of particular interest as this species is closely related to the cultivated tomato, which may facilitate introgression of the resistance component(s). Whitefly non-preference and resistance were associated with the presence of type IV trichomes. Other mechanisms might be involved since some accessions without type IV trichomes showed low nymphal density. The leaf disc test is a good in vitro alternative for the clip-on cage whitefly resistance screening, as shown by the high correlation between the results obtained with this test and the clip-on cage test. This offers breeders the possibility to carry out tests more efficiently.     

Resistance screening of <Emphasis Type="Italic">Coffea</Emphasis> spp. accessions for <Emphasis Type="Italic">Pratylenchus coffeae</Emphasis> and <Emphasis Type="Italic">Radopholus arabocoffeae</Emphasis> in Vietnam

Coffee varieties with resistance for the plant-parasitic nematodes Pratylenchus coffeae and Radopholus arabocoffeae are limited in Vietnam. A selection of imported varieties and high yield varieties of Arabica coffee in Vietnam were evaluated for resistance to both plant-parasitic nematode species in Northern Vietnam. The same experiments were carried out with hybrid arabica coffee, three selected clones of Coffea canephora and one clone of Coffea excelsa in the Western Highland of Vietnam. The screened coffee accessions from Ethiopia (KH1, KH13, KH20, KH21, KH29, and KH31) were susceptible and good host for P. coffeae. Also accessions 90P4 (Portugal) and Oro azteca (Mexico) had a reproduction factor Rf > 1. Pluma Hidalgo (Mexico), 90/6 (Vietnam), 90P3 (Portugal), 90P2 (Vietnam), Variedad (Mexico), 90T (Portugal), and Garnica (Mexico) were poor hosts (Rf < 1) but not tolerant to P. coffeae, expressed by a reduction of root weight compared to untreated control plants. Most of the coffee accessions tested in Northern Vietnam were intolerant to R. arabocoffeae, except 90T which showed no reduction of root weight, even at high initial nematode densities (4,000/pot). Good hosts for R. arabocoffeae were Variedad, KH1, KH21, KH29, KH20, KH31, and KH13 with Rf > 1. Pluma Hidalgo, 90/6, 90P3, 90P2, 90T, Oro azteca, and Garnica were poor hosts (Rf < 1). In the Western Highland experiment, all arabica coffee accessions were susceptible for P. coffeae with Rf ranging from 1.41 to 1.59. Tolerance to P. coffeae was found in C. liberica var. Dewevrei, Hong34 and Nhuantren. Coffea excelsa, Hong34, Nhuantren, and H1C19 were tolerant to R. arabocoffeae at the highest inoculation density (4,000 nematodes/pot). The most susceptible accessions were Nhuantren and K55. Resistance (Rf < 1) to R. arabocoffeae was found in C. liberica var. Dewevrei and Hong34. This article reports on the first screening for resistance and tolerance to P. coffeae and R. arabocoffeae in coffee accessions in Vietnam and shows promising results for enhanced coffee-breeding.     

Increased resistance to <Emphasis Type="Italic">Ustilago zeae</Emphasis> and <Emphasis Type="Italic">Fusarium verticilliodes</Emphasis> in maize inbred lines bred for <Emphasis Type="Italic">Fusarium graminearum</Emphasis> resistance

Preliminary field observations in our maize breeding nurseries indicated that breeding for improved resistance to gibberella ear rot (Fusarium graminearum) in maize may indirectly select for resistance to another ear disease, common smut (Ustilago zeae). To investigate this, we compared the disease severity ratings obtained on 189 maize inbreds, eight of which included our inbreds developed with selection for gibberella ear rot resistance after field inoculation and breeding for 8–10 years. No correlation was found between disease severities for the 189 inbreds but the eight gibberella-resistant lines were consistently more resistant to smut. To further examine this relationship and to determine if these eight inbreds would be useful for developing inbreds with either common smut or fusarium ear rot (F. verticilliodes) resistance, we conducted a Griffing’s diallel analysis on six inbreds of maize, four with high levels of gibberella ear rot resistance representing all of the pedigree groups in our eight gibberella lines, and two with very low levels. Our most gibberella ear rot resistant inbreds, CO433 and CO441, had the lowest disease ratings for all three diseases, the consistently largest general combining ability effects and several significant specific combining ability effects. It was concluded that some inbreds bred specifically for gibberella ear rot would also be useful in breeding for resistance to common smut and fusarium ear rot.     

Evaluation of seedling and adult plant resistance to <Emphasis Type="Italic">Puccinia hordei</Emphasis> in barley

The responses of 92 barley genotypes to selected Puccinia hordei pathotypes were assessed in greenhouse tests at seedling growth stages and in the field at adult plant growth stages to identify known and/or unknown resistances. On the basis of multipathotype tests, 35 genotypes were postulated to carry Rph1, Rph2, Rph4, Rph5, Rph12, RphCantala, alone or in combination (Rph2 + Rph4 and Rph1 + Rph2), whereas 52 genotypes lacked seedling resistance to P. hordei to the pathotypes used. Five genotypes carried seedling resistance that was effective to all pathotypes, of which four were believed to carry uncharacterised resistance based on pedigree information. Field tests at adult plant growth stages indicated that while 28 genotypes were susceptible, 57 carried uncharacterised APR to P. hordei. Pedigree analysis indicated that APR in the test genotypes could have been derived from three different sources. The resistant responses of seven cultivars at adult plant growth stages were believed to be due to the presence of seedling resistance effective against the field pathotypes.     

Reduction of <Emphasis Type="Italic">Aegilops sharonensis</Emphasis> chromatin associated with resistance genes <Emphasis Type="Italic">Lr56</Emphasis> and <Emphasis Type="Italic">Yr38</Emphasis> in wheat

The Lr56/Yr38 translocation consists primarily of alien-derived chromatin with only the 6AL telomeric region being of wheat origin. To improve its utility in wheat breeding, an attempt was made to exchange excess Ae. sharonensis chromatin for wheat chromatin through homoeologous crossover in the absence of Ph1. Translocation heterozygotes that lacked Ph1 were test-crossed with Chinese Spring nullisomic 6A tetrasomic 6B and nullisomic 6A-tetrasomic 6D plants and the resistant (hemizygous 6A) progeny were analyzed with four microsatellite markers. Genetic mapping suggested general homoeology between wheat chromosome 6A and the translocation chromosomes, and showed that Lr56 was located near the long arm telomere. Thirty of the 53 recombinants had breakpoints between Lr56 and the most distal marker Xgwm427. These were characterized with additional markers. The data suggested that recombinants #39, 157 and 175 were wheat chromosomes 6A with small intercalary inserts of foreign chromatin containing Lr56 and Yr38, located distally on the long arms. These three recombinants are being incorporated into adapted germplasm. Attempts to identify the single shortest translocation and to develop appropriate markers are being continued.     

Resistance to <Emphasis Type="Italic">Cucumber green mottle mosaic virus</Emphasis> in <Emphasis Type="Italic">Cucumis sativus</Emphasis>

Cucumber green mottle mosaic virus (CGMMV) is a severe threat for cucumber production worldwide. At present, there are no cultivars available in the market which show an effective resistance or tolerance to CGMMV infection, only wild Cucumis species were reported as resistant. Germplasm accessions of Cucumis sativus, as well as C. anguria and C. metuliferus, were mechanically infected with the European and Asian strains of CGMMV and screened for resistance, by scoring symptom severity, and conventional RT-PCR. The viral loads of both CGMMV strains were determined in a selected number of genotypes using quantitative RT-PCR. Severe symptoms were found following inoculation in C. metuliferus and in 44 C. sativus accessions, including C. sativus var. hardwickii. Ten C. sativus accessions, including C. sativus var. sikkimensis, showed intermediate symptoms and only 2 C. sativus accessions showed mild symptoms. C. anguria was resistant to both strains of CGMMV because no symptoms were expressed and the virus was not detected in systemic leaves. High amounts of virus were found in plants showing severe symptoms, whereas low viral amounts found in those with mild symptoms. In addition, the viral amounts detected in plants which showed intermediate symptoms at 23 and 33 dpi, were significantly higher in plants inoculated with the Asian CGMMV strain than those with the European strain. This difference was statistically significant. Also, the amounts of virus detected over time in plants did not change significantly. Finally, the two newly identified partially resistant C. sativus accessions may well be candidates for breeding programs and reduce the losses produced by CGMMV with resistant commercial cultivars.