Melon Lines Research Articles

Identification of sources of seedling resistance to Phytophthora capsici in Cucumis melo

The employment of genetic resistance to minimize yield losses induced by Phytophthora capsici remains unexplored in melon (Cucumis melo). A diverse collection of melon accessions was evaluated against P. capsici isolates at the seedling stage. In the first screening assay, 105 accessions were evaluated using isolate PCpe-04 obtained from cucumber (Cucumis sativus). In a second assay, 31 accessions displaying high levels of resistance in the first assay were challenged with a distinct isolate (PCpe-09 also from cucumber). In a third assay, a subset of 14 selected accessions was re-evaluated using isolates PCpe-09 and PCmo-07 (from strawberry). In the last screening, seven accessions with high levels of resistance across all assays were challenged with five isolates from representative host species [PC-Vagem (snap bean), PCp-129 (Capsicum chinense), PCp-155 (C. annuum), PCpe-09 and PCmo-07] to assess their reaction against a varied sample of P. capsici isolates. For two accessions (CNPH-093 and L040), all plants remained free of symptoms after inoculation with all five isolates. Accessions WMR-29, CNPH 084, CNPH 088 and CNPH 092 were also free of symptoms to all isolates, except PCmo-07. These large-spectrum resistance sources might be useful for breeding programs aiming to incorporate resistance against P. capsici in elite melon lines.

Aroma volatiles as biomarkers of textural differences at harvest in non-climacteric near-isogenic lines of melon

The texture and aroma volatiles of two non-climacteric near-isogenic lines (NILs) of melon (Cucumis melo L.), SC10-2 and SC7-1, containing introgressions of the Korean cultivar ‘Shongwan Charmi’ accession PI161375 (SC) in the Spanish cultivar ‘Piel de Sapo’ (PS) were studied. Data were examined using different supervised and unsupervised univariate and multivariate statistical techniques in order to determine the aroma volatiles most closely associated with differences in melon texture. SC10-2 showed 65% higher flesh firmness, a higher level of pellet juice content and lower flesh juiciness than PS and was harvested at least 7days later. In SC7-1, only the textural trait, whole fruit hardness, was lower (by 34%) than in PS. About eleven quantitative trait loci for aroma volatiles were mapped in linkage group VII and other nineteen in group X. The aroma compounds discriminated SC10-2 from PS better than SC7-1, as revealed by Partial least squares-discriminant analysis and, to a lesser extent, by Random forest analysis. Around seven aroma volatile compounds, mostly ketones, aldehydes and alcohols, consistently lent support to the discrimination irrespective of the methodology used. Higher levels of several aldehydes (e.g. 2,4-dimethylbenzaldehyde, 2-methylpropanal and 2-methylbutanal), not present in PS, discriminated SC10-2 from PS. Also, SC10-2 lacked some ketones (e.g. 1-phenylethanone), alcohols (e.g. 1-phenylethanol) and one acid (e.g. 2-phenylpropanoic acid) compared with higher content in PS. The former aldehydes and other compounds, such as 3-methylsulfanylpropanal, were the volatiles with the closest association with flesh firmness and harvest, and form part of the phenylalanine, leucine, valine, isoleucine or methionine metabolism and the benzenoid biosynthetic pathway. A negative correlation with the former discriminative compounds of SC7-1 and PS and whole fruit hardness was established. SC7-1 stood out mainly for its higher content of one sulfur-derived compound (S-methyl ethanethioate), one acetate ester (benzyl acetate) and two alcohols (octan-1-ol and (2R, 3S)-3-ethylheptan-2-ol) compared with PS, and one ketone (pentane-2,3-dione), which was absent in PS, but none of these were associated with flesh textural traits.

Genetic and chemical characterization of an EMS induced mutation in Cucumis melo CRTISO gene

In order to broaden the available genetic variation of melon, we developed an ethyl methanesulfonate mutation library in an orange-flesh ‘Charentais’ type melon line that accumulates β-carotene. One mutagenized M2 family segregated for a novel recessive trait, a yellow–orange fruit flesh (‘yofI’). HPLC analysis revealed that ‘yofI’ accumulates pro-lycopene (tetra-cis-lycopene) as its major fruit pigment. The altered carotenoid composition of ‘yofI’ is associated with a significant change of the fruit aroma since cleavage of β-carotene yields different apocarotenoids than the cleavage of pro-lycopene. Normally, pro-lycopene is further isomerized by CRTISO (carotenoid isomerase) to yield all-trans-lycopene, which is further cyclized to β-carotene in melon fruit. Cloning and sequencing of ‘yofI’ CRTISO identified two mRNA sequences which lead to truncated forms of CRTISO. Sequencing of the genomic CRTISO identified an A–T transversion in ‘yofI’ which leads to a premature STOP codon. The early carotenoid pathway genes were up regulated in yofI fruit causing accumulation of other intermediates such as phytoene and ζ-carotene. Total carotenoid levels are only slightly increased in the mutant. Mutants accumulating pro-lycopene have been reported in both tomato and watermelon fruits, however, this is the first report of a non-lycopene accumulating fruit showing this phenomenon.

Breeding melon for resistance to Fusarium wilt: recent developments

AbstractMelon Fusarium wilt (MFW), caused byFusarium oxysporumf. sp.melonis(Fom), is one of the most destructive diseases of melon (Cucumis meloL.). The development and deployment of resistant cultivars is generally considered to be the best approach to control MFW. Based on the host resistance genes associated with variants of this pathogen,Fomisolates were classified into four physiological races designated 0, 1, 2, and 1,2. Two dominant resistance genes,Fom-1andFom-2, control resistance to races 0 and 2, and 0 and 1, respectively.Fomisolates classified as race 1,2 are able to induce disease in melon lines carrying the above resistance genes. Many sources of resistance toFomraces 0, 1, and 2 have been reported. Partial resistance to race 1,2 controlled by polygenic recessive genes was only detected in a few Far Eastern melon accessions, except for the breeding line BIZ where complete resistance was described. Identification of DNA markers tightly linked to genes conferring resistance toFomhas immediate application in MFW resistance breeding programs. TheFom-2gene has been cloned, and it encodes a protein with a nucleotide binding site (NBS) and leucine-rich repeats domain (LRR). Based on the sequence of this domain, some molecular markers linked to this gene were developed. Several DNA markers linked toFom-1have also been described. However, the usefulness of these markers was variety-dependent. Therefore, their combined use would be very useful in marker assisted selection for introducing resistance toFomraces 0 and 2 in melon. Recently, these markers were used for the positional cloning of this gene, which encoded a protein with a NBS–LRR domains that shows similarity to the toll and interleukin-1 receptores (TIR). RegardingFomrace 1,2, nine QTL were detected on five linkage groups by composite interval mapping. In this paper we review the current knowledge of MFW disease, and focus on genetic resistance toFomand marker-assisted selection for resistance.

Transgenic lines of melon (Cucumis melo L. var. makuwa cv. ‘Silver Light’) expressing antifungal protein and chitinase genes exhibit enhanced resistance to fungal pathogens

The oriental melon (Cucumis melo L. var. makuwa cv. ‘Silver Light’) is an important fruit crop in the tropical and subtropical regions. However, oriental melon production is severely decreased by fungal diseases. In this study, antifungal protein (AFP) and chitinase (CHI) fusion genes were introduced into oriental melons to control fungal diseases caused by Rhizoctonia solani and Fusarium oxysporum. Transformation of oriental melon (Cucumis melo L. var. makuwa cv. ‘Silver Light’) with Agrobacterium tumefaciens strain LBA4404 containing antifungal protein (AFP) and chitinase (CHI) fusion genes under the control of the cauliflower mosaic virus (CaMV) 35S promoter and neomycin phosphotransferase (nptII) gene as a selectable marker was performed. Cotyledon explants of oriental melon were inoculated by Agrobacterium suspensions with pBI121–AFP–CHI and cultured in a regeneration medium. After regeneration, genomic DNA polymerase chain reaction (PCR) was conducted to confirm the presence of putative transgenic shoots. Southern blot analysis confirmed that the AFP–CHI fusion gene was incorporated into the genomic DNA of the PCR-positive lines. RT-PCR analysis showed that the AFP–CHI fusion gene was expressed in the individual transgenic lines. Western blot analysis revealed the accumulation of CHI protein in leaves. A segregation analysis of the T1 generation confirmed the inheritance of the transgene. Our results demonstrated that the AFP–CHI fusion gene was effective in protecting the transgenic melon plants against fungal disease caused by Rhizoctonia solani and Fusarium oxysporum.

Cucumis melo MicroRNA Expression Profile During Aphid Herbivory in a Resistant and Susceptible Interaction

Aphis gossypii resistance in melon (Cucumis melo) is due to the presence of a single dominant virus aphid transmission (Vat) gene belonging to the nucleotide-binding site leucine-rich repeat family of resistance genes. Significant transcriptional reprogramming occurs in Vat(+) plants during aphid infestation as metabolism shifts to respond to this biotic stress. MicroRNAs (miRNAs) are involved in the regulation of many biotic stress responses. The role of miRNAs was investigated in response to aphid herbivory during both resistant and susceptible interactions. Small RNA (smRNA) libraries were constructed from bulked leaf tissues of a Vat(+) melon line following early and late aphid infestations. Sequence analysis indicated that the expression profiles of conserved and newly identified miRNAs were altered during different stages of aphid herbivory. These results were verified by quantitative polymerase chain reaction experiments in both resistant Vat(+) and susceptible Vat(-) interactions. The comparative analyses revealed that most of the conserved miRNA families were differentially regulated during the early stages of aphid infestation in the resistant and susceptible interactions. Along with the conserved miRNA families, 18 cucurbit-specific miRNAs were expressed during the different stages of aphid herbivory. The comparison of the miRNA profiles in the resistant and susceptible interactions provides insight into the miRNA-dependent post-transcriptional gene regulation in Vat-mediated resistance.

Temperature-dependent enhancement of pollen tube growth observed in interspecific crosses between wild Cucumis spp. and melon (C. melo L.)

Wild Cucumis spp., which have resistance to various diseases, have the potential to be used for melon (C. melo L.) breeding. Nevertheless, few interspecific hybrids between melon and wild Cucumis spp. have been obtained because of strong reproductive barriers such as pollen–pistil incongruity and hybrid seed abortion. For this study, we examined the temperature conditions enhancing pollen tube growth and fruit set in interspecific crosses between melon and wild Cucumis spp. Before temperature-conditioned pollination, four wild Cucumis spp. were pollinated with the melon line ‘MR-1’ and two lines of C. anguria, PI 147065 and PI 320052, and a line of C. metuliferus, PI 526242, were selected for later experiments. These three lines were crossed reciprocally with ‘MR-1’ under controlled temperatures of 24–34°C at 2°C intervals. When ‘MR-1’ pistils were pollinated with three lines of wild Cucumis spp., pollen tube growth was arrested after germination at all temperatures. However, in the pistil of wild Cucumis spp., pollen tube growth of ‘MR-1’ was significantly greater at 32°C and at 34°C than that under 30°C. Furthermore, pollen tube length in pistils of PI 320052 and PI 526242 at 32°C was comparable to that of ‘MR-1’ self-pollination at 28°C. Fruit set frequency was also improved at 32°C. These results demonstrated that the pollen tube inhibition of Cucumis spp. interspecific crosses might be partially or fully recoverable in higher-temperature conditions. Moreover, 32°C is apparently the threshold temperature for promoting pollen tube growth in these cross combinations.

Chromosomal location and mode of inheritance of a gene conferring resistance to fusarium wilt in Cucumis anguria L.

SummaryFusarium wilt of melon (Cucumis melo L.) is caused by Fusarium oxysporum f. sp. melonis and is considered a severe disease, worldwide. Resistant cultivars are effective in controlling such soil-borne diseases in melon. However, high resistance to F. oxysporum f. sp. melonis race 1,2y has not been found in any melon line, except recently, when it was reported in a wild Cucumis species which was regarded as a valuable genetic resource for breeding melon cultivars with high resistance. To plan an inter-specific breeding programme and to select resistant lines using marker-assisted selection, information on the mode of inheritance of resistance and linked markers are required. In this study, we developed F1, F2, F3, and BC1F1 lines from crosses between a highly resistant (R) and a susceptible (S) line of C. anguria. We evaluated the segregation of R and S phenotypes and constructed a linkage map using these phenotypes and simple sequence repeat (SSR) marker analysis. High resistance to F. oxysporum f. sp. melonis race 1,2y in C. anguria was proposed to be controlled by a single dominant gene named F1,2y for Fusarium oxysporum f. sp. melonis race 1,2y resistance. F1,2y was located on linkage group (LG) XI between the SSR markers ECM164 and CMN04_03. This is the first genetic study to report high resistance to F. oxysporum f. sp. melonis race 1,2y.

Water-Soluble Antioxidant Potential of Melon Lines Grown in Turkey

The antioxidant potential of 42 melon (Cucumis melo) lines including six cultivars grown in Turkey was assessed by measuring total water-soluble antioxidant capacity, phenolic and vitamin C contents. The lines showed significant variability for all three antioxidant parameters with breeding lines having higher antioxidant capacity and phenolic content than some popular cultivars. Different types of melons also showed significantly different antioxidant potentials. Thus, galia and ananas types showed a higher mean antioxidant capacity and phenolic content than the other tested types (yuva, kislik, canary, and charentais). Correlation analysis between antioxidant parameters showed a significant correlation between water-soluble antioxidant capacity and phenolic content.

A High Level of Transgenic Viral Small RNA Is Associated with Broad Potyvirus Resistance in Cucurbits

Gene-silencing has been used to develop resistance against many plant viruses but little is known about the transgenic small-interfering RNA (t-siRNA) that confers this resistance. Transgenic cucumber and melon lines harboring a hairpin construct of the Zucchini yellow mosaic potyvirus (ZYMV) HC-Pro gene accumulated different levels of t-siRNA (6 to 44% of total siRNA) and exhibited resistance to systemic ZYMV infection. Resistance to Watermelon mosaic potyvirus and Papaya ring spot potyvirus-W was also observed in a cucumber line that accumulated high levels of t-siRNA (44% of total siRNA) and displayed significantly increased levels of RNA-dependent RNA (RDR)1 and Argonaute 1, as compared with the other transgenic and nontransformed plants. The majority of the t-siRNA sequences were 21 to 22 nucleotides in length and sense strand biased. The t-siRNA were not uniformly distributed throughout the transgene but concentrated in "hot spots" in a pattern resembling that of the viral siRNA peaks observed in ZYMV-infected cucumber and melon. Mutations in ZYMV at the loci associated with the siRNA peaks did not break this resistance, indicating that hot spot t-siRNA may not be essential for resistance. This study shows that resistance based on gene-silencing can be effective against related viruses and is probably correlated with t-siRNA accumulation and increased expression of RDR1.

Reaction of families and lines of melon to powdery mildew

Powdery mildew, caused by Podosphaera xanthii, is an important disease of melon in the Brazilian Northeast. The objective of this work was to evaluate the reaction of families, lines and cultivars of melon to P. xanthii. Two experiments in simple lattice were carried out under field conditions in the municipalities of Mossoró and Baraúna to evaluate 144 families. In a greenhouse, an experiment was carried out in a completely randomized design to evaluate 19 lines and the cultivars Yellow Queen, Chilton and Hy Mark. Disease was assessed by a severity scale. There was genetic variability among families. The families 4, 5, 11, 14, 18, 23, 33, 42, 101, 112 and 140 were identified in both sites as resistant. The lines LOF-027, LOF-105, LPS-124, MEL-008 and cultivar Chilton showed resistance to race 1 of the fungus.

Divergência genética entre linhagens de melão do grupo Inodorus

O presente trabalho teve como objetivo estimar a divergência genética entre linhagens Pele de Sapo e entre linhagens Honey Dew. Foram avaliadas dez linhagens de melão Pele de Sapo e dez linhagens de melão Honey Dew, em experimentos separados, conduzidos em blocos casualizados com três repetições. A divergência genética foi estimada pela distância de Mahalanobis. Foram utilizados os métodos de Tocher e UPGMA para o agrupamento das linhagens. Para as linhagens Pele de Sapo, constatou-se divergência genética com a formação de quatro e cinco grupos conforme os métodos de agrupamento de Tocher e UPGMA, respectivamente. Sugere-se cruzamento das linhagens PS-01, PS-05 e PS-7 entre si ou com as demais linhagens do grupo I. Para as linhagens Honey Dew, verificou-se a formação dos mesmos três grupos nos métodos de agrupamento de Tocher e UPGMA. O grupo II formado pelas linhagens OF-01 e OF-02; o grupo III pela linhagem OF-03 e o primeiro grupo pelas demais linhagens. Com relação às linhagens Honey Dew, recomenda-se os cruzamentos das linhagens OF-01 ou OF-02, com as demais linhagens avaliadas. A linhagem OF-03, com características semelhantes às linhagens OF-01 ou OF-02 pode ser cruzada com as linhagens do grupo I.

Engineering Melon Plants with Improved Fruit Shelf Life Using the TILLING Approach

BackgroundFruit ripening and softening are key traits that have an effect on food supply, fruit nutritional value and consequently, human health. Since ethylene induces ripening of climacteric fruit, it is one of the main targets to control fruit over ripening that leads to fruit softening and deterioration. The characterization of the ethylene pathway in Arabidopsis and tomato identified key genes that control fruit ripening.Methodology/Principal FindingsTo engineer melon fruit with improved shelf-life, we conducted a translational research experiment. We set up a TILLING platform in a monoecious and climacteric melon line, cloned genes that control ethylene production and screened for induced mutations that lead to fruits with enhanced shelf life. Two missense mutations, L124F and G194D, of the ethylene biosynthetic enzyme, ACC oxidase 1, were identified and the mutant plants were characterized with respect to fruit maturation. The L124F mutation is a conservative mutation occurring away from the enzyme active site and thus was predicted to not affect ethylene production and thus fruit ripening. In contrast, G194D modification occurs in a highly conserved amino acid position predicted, by crystallographic analysis, to affect the enzymatic activity. Phenotypic analysis of the G194D mutant fruit showed complete delayed ripening and yellowing with improved shelf life and, as predicted, the L124F mutation did not have an effect.Conclusions/SignificanceWe constructed a mutant collection of 4023 melon M2 families. Based on the TILLING of 11 genes, we calculated the overall mutation rate of one mutation every 573 kb and identified 8 alleles per tilled kilobase. We also identified a TILLING mutant with enhanced fruit shelf life. This work demonstrates the effectiveness of TILLING as a reverse genetics tool to improve crop species. As cucurbits are model species in different areas of plant biology, we anticipate that the developed tool will be widely exploited by the scientific community.

GENETIC BACKGROUND OF QUALITY AND CELL WALL CHANGES IN FRESH-CUT MELONS

Fresh-cut melon is a growing and convenient segment ot melon products. Melon cultivars were not bred specifically for fresh-cut processing, and the genetic bases for fresh-cut quality are unknown. We examined the link between fresh-cut quality, cell wall changes, and genetics in three near-isogenic lines (NILs) of melon (Cucumis melo L.) and in a 'Piel de Sapo' (PS) cultivar during a 9-day storage period at 4°C. The three NILs differed in initial firmness and differences persisted throughout storage of fresh-cut melon. Juice efflux was negatively correlated with flesh firmness. The climacteric NIL SC3-5-1 was softer and had higher juice efflux than the remaining NILs of the parental line PS. Fresh-cut prepared from NILs SC7-2 and SC10-2 were firmer and generally released less juice than SC3-5-1 and PS. Chelator-soluble pectins of SC3-5-1 exhibited a marked molecular size downshift during a 6-day storage period in SC3-5-1, but no significant differences in molecular size distribution were observed in the other NILs. Apparently two different quantitative trait loci (QTL) with pleiotropic effect were affecting flesh firmness and juice efflux. One QTL located in the linkage group (LG) III increased pectin depolymerisation, which was probably associated with climacteric behavior. Another two QTLs in LG VII and X were associated with initial firmness levels.

Double-Virus Resistance of Transgenic Oriental Melon Conferred by Untranslatable Chimeric Construct Carrying Partial Coat Protein Genes of Two Viruses

Production of oriental melon (Cucumis melo var. makuwa) in Asia is often limited by two potyviruses, the watermelon infecting type of Papaya ringspot virus (PRSV W) and Zucchini yellow mosaic virus (ZYMV). In order to engineer transgenic resistance to these two viruses, an untranslatable chimeric DNA comprising partial coat protein (CP) sequences of ZYMV and PRSV W was constructed and used to transform the elite cultivar of oriental melon, Silver Light, by Agrobacterium. Greenhouse evaluation by mechanical challenges with ZYMV and PRSV W, alone or together, identified transgenic lines exhibiting different levels of resistance or complete immunity to ZYMV and PRSV W. Molecular analyses of transgenic lines revealed random insertion of transgene into the host genome, with insert numbers differing among transformants. There was no correlation between transgene insert numbers and the degree of resistance expressed by transgenic lines. The levels of accumulation of transgene transcript varied among transgenic lines. However, an inverse correlation was observed between the level of accumulation of transgene transcripts and the degree of virus resistance. Moreover, small interfering (si)RNA was readily detected from the immune and highly resistant lines, but not from the weakly resistant and susceptible lines. Altogether, our results indicated that RNA-mediated post-transcriptional gene silencing (PTGS) was the underlying mechanism of double-virus resistance of the transgenic melon lines. The segregation analysis of the R1 progeny of the immune line ZW-1 indicated that the single inserted transgene is associated with the resistance phenotype and is inherited as a dominant trait. These transgenic melon lines with high degrees of resistance to ZYMV and PRSV W have great potential for the control of ZYMV and PRSV W in C. melo in Asia and elsewhere.

PRELIMINARY STUDY ON MAPPING OF GENES CONTROLLING STAMINATE FLOWER EXPRESSION AND QTL ANALYSIS OF THE RATIO OF PISTILLATE FLOWERS IN MELON (CUCUMIS MELO L.)

ISHS IV International Symposium on Cucurbits PRELIMINARY STUDY ON MAPPING OF GENES CONTROLLING STAMINATE FLOWER EXPRESSION AND QTL ANALYSIS OF THE RATIO OF PISTILLATE FLOWERS IN MELON (CUCUMIS MELO L.)

Differential colonization and defence responses of resistant and susceptible melon lines infected by Fusarium oxysporum race 1·2

A green fluorescent protein (GFP)‐expressing strain of Fusarium oxysporum f.sp. melonis race 1·2 (FOM 1·2‐GFP) was used to visualize infection of a susceptible melon cultivar, Ein Dor (ED). At 1–2 days post‐inoculation (d.p.i.), the fungus grew on the root epidermis and adhered to epidermal cell borders. By day 4, the mycelium crossed the cortex and endodermis through narrow pores in cell walls and reached xylem vessels, where it sporulated and produced secondary hyphae that grew upwards. Colonization dynamics of ED seedlings were compared with those of a genetically resistant line, BIZ. FOM 1·2‐GFP colonized the resistant plant’s vascular system, but the incidence of seedling infection was lower than in ED, suggesting stronger defence responses in BIZ expressed at the pre‐xylem stage of infection. Infection of the vascular system of BIZ was slower: at 11 d.p.i., FOM 1·2‐GFP only colonized the lower hypocotyl sections of BIZ, whilst the upper hypocotyls of ED were already infected, indicating that BIZ also restricted FOM 1·2 movement in the xylem. The expression patterns of three defence genes were compared between the resistant and susceptible genotypes using real‐time PCR. Transcript levels of phenylalanine ammonia lyase (PAL), chitinase (CHI) and hydroperoxide lyase (HPL) were induced to a greater extent in BIZ than in ED. A constitutive two‐ to fourfold difference between BIZ and ED in the basal levels of all three transcripts was also apparent. Both the constitutive and inducible defence responses could contribute to reduced vascular colonization of the resistant genotype.

Performance of melon hybrids derived from parents of diverse geographic Origins

Genetic relatedness and phenotype are important factors that govern the expression of heterosis in hybrid progeny of many cross-pollinating plant species. Since this relationship is important but not well understood in melon (Cucumis melo L.), one monoecious and two andromonoecious melon lines of diverse Chinese [Peoples Republic of China (PRC)] origin were crossed to the andromonoecious U.S. Western Shipping market type ‘Top Mark’ (TM) and the andromonoecious, highly branched line H-16 to determine parental combining ability and heterosis for five yield component traits in three test environments [open-field (USA), and energy-saving (PRC) and plastic greenhouses (PRC)]. Random amplified polymorphic DNA (RAPD)- and simple sequence repeat (SSR)-based genetic distances (GD) among and between parents (5) and their hybrids (6) were calculated and compared to phenotypic trait values. These germplasms were evaluated for lateral branch number (LBN), days to 50% flower (DF), fruit number and weight per plant, and fruit length:diameter (L:D) ratio in each of three test environments. General combining ability was significant for all characters, except for L:D in all locations, and LBN and DF in the plastic greenhouse environment. Both descriptors of difference (genetic marker and phenotype) were discriminatory, and provided similar assessments of relationships among parents and hybrids. Although dramatic performance differences were detected between parents and among F1 hybrid progeny, a strong relationship between GD and heterotic effects was not consistently detected.

Uncommon disorders and decay in near-isogenic lines of melon and reference cultivars

Postharvest disorders and rots can produce important economic losses in fruits stored for long time for exportation. The genetic and physiological basis of some disorders in melon (Cucumis melo L.) are unknown and particularly the possible relation with climacteric behavior. A collection of melon near-isogenic lines (NILs) (SC3-5 and seven more showing climacteric and two non-climacteric ripening pattern) were analyzed to study genetic and physiological aspects of fruit disorders and rots. Two non-climacteric (Nicolás; Inodorus Group; and Shongwan Charmi PI161375, Conomon Group) and two climacteric cultivars (Fado, Reticulatus Group; Védrantais, Cantaloupensis Group) were used as reference. The field was divided in eight blocks containing one three-plant replication for each NIL, two for the parental cultivar Piel de Sapo and one or two for the reference cultivars. Replications evaluated were more than six in the cultivars studied. Plant problems included aphids, powdery mildew, and leaf wind injury. Preharvest fruit disorders included whole fruit cracking in cultivar Védrantais and NIL 5M2, and stylar-end cracking in cultivar Fado. Climacteric NILs with yellow skin were particularly affected by over-ripening, stylar-end cracking, and sunburn during cultivation. At harvest, two NILs showed slight placental tissue necrosis which was inherited from SC and were also detected after storage. Other uncommon disorders seen at harvest or 30 days after storage at 8ºC included warted skin (scarring), flesh discoloration (light brown or translucent areas), hollow flesh disorder, and deep furrow netting inherited from SC. Less common rots included grey mould, bacterial soft rot, Penicillium rot, cottony leak and internal Cladosporium rot. Stylar-end hardness below 20 N·mm-1 was associated with cracking and softening. The incidence of the disorders and rots was too low to confirm that the genetic component played a role in their development.

‘Piel de Sapo’ Breeding Lines Tolerant to Melon Vine Decline

Melon vine decline (MVD) results in severe economic losses in melon crops (Cucumis melo L.) throughout the Mediterranean area and in the United States. MVD is a complex disease produced by many soilborne pathogens such as Monosporascus cannonballus Pollack et Uecker, Acremonium cucurbitacearum Alfaro-Garcia, W. Gams et Garcia-Jimenez, Macrophomina phaseolina (Tassi) Goidanich, and Rhizopycnis vagum DF Farr (Bruton et al., 1995; Chilosi et al., 2008; Fita et al., 2007a; Garcia-Jimenez et al., 2000). Occurrence and pathogenicity of fungi associated with MVD is variable depending on the climate area and the cultural practices (Aegerter et al., 2000; Bruton et al., 1999; Makam et al., 2005, Martyn, 2007). Despite this complex situation, M. cannonballus is the pathogen most frequently associated with severe outbreaks of MVD. It has great virulence and produces severe root damage in artificial inoculation alone or combined with other fungi, comparable to that found in naturally infested roots (Armengol et al., 2003; Beltran, 2006; Fita et al., 2008; Iglesias et al., 2000a). Also, realtime polymerase chain reaction has demonstrated that the roots of genotypes reported to be resistant to MVD have lower levels of M. cannonballus in comparison with susceptible genotypes (Pico et al., 2008). M. cannonballus causes brown discoloration of the roots, which can evolve into rot and even lead to necrosis of the whole root system. Affected roots are unable to supply the vine with the necessary nutrients and water, leading to plant wilt and collapse. Vine decline and/or collapse are usually observed in the late season when the level of water required by fruit increases (Fita et al., 2007a). The occurrence and intensity of these aboveground symptoms are highly dependent on the environmental conditions and cultural practices such as temperature, fruit load, water management, and so on. Consequently, disease assessment has been specifically centered on the extent of root damage. This methodology requires root extraction from the soil, which makes breeding programs highly difficult and time-consuming because all the plants must first of all be artificially pollinated and then, at the adult stage, evaluated by their root systems (Fita et al., 2007a). The limited number of sources of resistance to MVD reported to date (Cohen et al., 1996; Crosby, 2001; Esteva and Nuez, 1994; Wolff and Miller, 1998) is also a major obstacle in the development of resistant cultivars. One of the most promising accessions for its tolerance to MVD was ‘Pat 81’, which belongs to the subsp. agrestis of C. melo (Pitrat, 2008). ‘Pat 81’ displayed very low levels of vine collapse in field assays and exhibited root lesions that were less widespread and less severe than susceptible cultivars after artificial inoculations with M. cannonballus and A. cucurbitacearum (Dias et al., 2002, 2004; Iglesias et al., 2000a, 2000b, 2000c). In addition, ‘Pat 81’ develops a highly branched root system with long laterals and a high regeneration ability, which has proven to be crucial in overcoming the disease. The performance of melon plants against MVD is highly dependent on two factors: the intrinsic tolerance to the fungus infection and the architecture of the root system before and after infection (Fita et al., 2006, 2008).‘Pat 81’ has these two features and has proven to be a satisfactory rootstock for the grafting of melons in the battle against MVD (Fita et al., 2007b). For these reasons, it was selected as a donor parent to breed melon lines resistant to MVD (Dias, 2003). The ‘Piel de Sapo’ melon is very popular in the Mediterranean area, especially in Spain, and is becoming increasingly important in the European market. To cover the market demand during winter, Brazil is also producing this melon type. Unfortunately, ‘Piel de Sapo’ melons are highly susceptible to MVD, which seriously limits output in the main producing areas of the world.