BackgroundGenomic libraries of introgression lines (ILs) consist of collections of homozygous lines with a single chromosomal introgression from a donor genotype in a common, usually elite, genetic background, representing the whole donor genome in the full collection. Currently, the only available melon IL collection was generated using Piel de sapo (var. inodorus) as the recurrent background. ILs are not available in genetic backgrounds representing other important market class cultivars, such as the cantalupensis. The recent availability of genomic tools in melon, such as SNP collections and genetic maps, facilitates the development of such mapping populations.ResultsWe have developed a new genomic library of introgression lines from the Japanese cv. Ginsen Makuwa (var. makuwa) into the French Charentais-type cv. Vedrantais (var. cantalupensis) genetic background. In order to speed up the breeding program, we applied medium-throughput SNP genotyping with Sequenom MassARRAY technology in early backcross generations and High Resolution Melting in the final steps. The phenotyping of the backcross generations and of the final set of 27 ILs (averaging 1.3 introgressions/plant and covering nearly 100 % of the donor genome), in three environments, allowed the detection of stable QTLs for flowering and fruit quality traits, including some that affect fruit size in chromosomes 6 and 11, others that change fruit shape in chromosomes 7 and 11, others that change flesh color in chromosomes 2, 8 and 9, and still others that increase sucrose content and delay climacteric behavior in chromosomes 5 and 10.ConclusionsA new melon IL collection in the Charentais genetic background has been developed. Genomic regions that consistently affect flowering and fruit quality traits have been identified, which demonstrates the suitability of this collection for dissecting complex traits in melon. Additionally, pre-breeding lines with new, commercially interesting phenotypes have been observed, including delayed climacteric ripening associated to higher sucrose levels, which is of great interest for Charentais cultivar breeding.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0842-0) contains supplementary material, which is available to authorized users.

The fungal species Monosporascus cannonballus and M. eutypoides have been described as the causal agents of Monosporascus root rot and vine decline disease (MRRVD), which mainly affects melon and watermelon crops. Resistance to M. cannonballus has been reported in some melon cultivars (ssp. melo). Moreover, melon ssp. agrestis accessions have proven to be better resistance sources. This is the case of the Korean accession 'Pat 81', highly resistant under field and artificial inoculation. The objective of the work here presented was the evaluation of the resistance to MRRVD of different accessions representing the variability of Cucumis melo ssp. agrestis, against both, M. cannonballus and M. eutypoides, in a multiyear assay under different infection conditions. In general, M. eutypoides was less aggressive than M. cannonballus in the different environmental conditions. There was a strong influence of temperature on MRRVD, with more severe symptoms with higher temperatures and with variable effect of infection on plant development depending on the fungal species considered. Resistance to MRRVD has been confirmed in 'Pat 81' and in its derived F1 with a susceptible Piel de Sapo melon.Among the new germplasm explored, African accessions (both wild agrestis and exotic cultivated acidulus) showed good performance in artificial inoculation assays and in field conditions. These sources do not present compatibility problems with commercial melons, so they can be introduced in backcrossing programs. The accession assayed of the wild relative Cucumis metuliferus, also resistant to Fusarium wilt and to root-knot nematode, was highly resistant to MRRVD. The interest of this accession mainly relies in its advantages as a rootstock for melon.

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Grafting plants onto resistant rootstocks is a cultural practice spread worldwide to cope with biotic and abiotic stresses, such as fungal diseases, nematodes, drought, salinity, and extreme temperatures. For Cucurbitaceae crops, the most common rootstocks are interspecific hybrids between Cucurbita maxima Duchesne and Cucurbita moschata Duchesne (Davis et al., 2008). Besides their tolerance to biotic and abiotic stresses, hybrid Cucurbita rootstocks are preferred because they show good emergence performance and develop long and thick hypocotyls that facilitate grafting. These hybrids have however some important shortcomings. Their excess of vigor can cause a delay in the flowering and ripening processes in grafted plants (Soteriou et al., 2016), they are not resistant to nematodes (Cohen et al., 2014;€ Ozarslandan et al., 2011), and often they have negative impacts on fruit quality (Guan et al., 2015;Rouphael et al., 2010;Soteriou et al., 2014). These effects on quality are dependent on the rootstock-scion interactions. In melon (Cucumis melo L.), for example, an increase of fruit size and seed cavity (Verzera et al., 2014) or modifications of flesh firmness (Colla et al., 2006;Zhao et al., 2011) as a result of grafting are common. Flesh color variations (Colla et al., 2006), vitrescence (Jang et al., 2014, and changes in pH and soluble solids content (Colla et al., 2006;Verzera et al., 2014) have also been reported.