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Jack E. Staub, Vladimir Meglic, and James D. McCreight

Nineteen polymorphic and eleven monomorphic isozyme loci were identified in thirteen enzyme systems in a survey of four-hundred melon (Cucumis melo L.) accessions. Segregation of allozymes in F2 and backcross (BC) families for isozyme loci agreed with the expected 1:2:1 and 1:1 segregation ratios (P <0.01). Eleven isozyme loci were linked and were integrated to form a map containing two linkage groups spanning 98 cM with a mean linkage distance of ≈9 cM. Linkage groups (A and B) contain the following loci in the order: A Fdp-2, Pgd, Pgm, Mpi-1, Idh, and Ac, and B Pep-gl, Mdh-2, Mdh-4, Mdh-5, Mdh-6. The remaining eight loci (Acp-1, Acp-4, Ak-4, Fdp-1, Gpi, Mpi-2, Pep-la, and Pep-pap) segregated independently. The isozyme map constructed in this study provides genomic information for future linkage studies with economically important traits and concensus map construction through map merging.

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Liangli Yu, Denys J. Charles, Amots Hetzroni, and James E. Simon

The volatiles of muskmelon (Cucumis melo L. reticulatis cv. Mission) were sampled by dichloromethane extraction and dynamic headspace methods and analyzed by gas chromatography (GC) and GC–mass spectroscopy (MS). A total of 34 constituents were identified, with esters contributing 8%–92% of the total volatiles. Butyl propionate, ethyl 3-methylpentanoate, hexadecanoic acid, methyl (methylthio)acetate, propyl butyrate, phenylpropyl alcohol, and vanillin, were recovered only by solvent extraction, while hexanal was only detected using dynamic headspace sampling. Methyl butyrate 35.2%, ethyl acetate 17.1%, butyl acetate 11.6%, ethyl propionate 8.3%, and 3-methylbutyl acetate 6.3% were the major constituents by solvent extraction sampling method. Butyl acetate 35.5%, 3-methylbutyl acetate 20.9%, ethyl acetate 7.3%, 2-butyl acetate 5.6%, and hexyl acetate 3.8% were the major constituents recovered by headspace sampling. Fruit tissue was also separated into five layers (exocarp, outer mesocarp, middle mesocarp, inner mesocarp, and seed cavity) and the volatile constituents differed significantly in content and composition by tissue.

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N. Boissot, D. Lafortune, C. Pavis, and N. Sauvion

The whitefly Bemisia tabaci (Gennadius) B-Biotype is a major pest on cucurbits in the Caribbean islands. Five field trials were conducted to identify resistance among 80 genotypes of Cucumis melo L. from diverse geographic origins. We focused on resistant rather than tolerant genotypes by counting adults on the abaxial side of two leaves of each plant at least three times in each trial, and larval density of under leaf disk samples at least twice in each trial. On the basis of insect density, three Indian accessions, PI 414723, PI 164723, and 90625, and one Korean accession, PI 161375, had field resistance. On those accessions, we observed 3.6 to 6 times fewer adults than on the most susceptible genotypes (`AR Top Mark' or `Délice de Table') and 11 to 29 times fewer larvae than on `AR Top Mark' or B66-5. Those levels of resistance may be sufficient to significantly reduce pesticide use in Guadeloupe (Lesser Antilles) where B. tabaci populations are lower than in the Southern United States or in the Mediterranean Basin. Higher levels of resistance are needed for genetic control, and may be achieved by a combination of different partial resistance genes.

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Jiwon Jeong, James Lee, and Donald J. Huber

Cantaloupe ( Cucumis melo L.) melons of the Reticulatus Group, commonly known as cantaloupes or muskmelons, are climacteric fruits in which ripening is highly coordinated by ethylene and have a relatively short storage life ( Seymour and

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A.I. López-Sesé and M.L. Gómez-Guillamón

Forty-four accessions of Cucumis melo L. and related wild species were tested for reaction to a yellowing disease, incited by the recently identified cucurbit yellowing stunting disorder virus (CYSDV), under natural and controlled-inoculation conditions. The C. melo TGR-1551 accession and one Cucumis metuliferus Naud. accession were asymptomatic. The segregation ratios obtained following controlled inoculations of the family produced by crossing TGR-1551 with the susceptible Spanish cv. Piel de Sapo revealed that the resistance to CYSDV in TGR-1551 is conditioned by a dominant allele at one locus. The name Cucurbit yellow stunting and symbol Cys is proposed for this locus.

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Yosef Burger, Uzi Sa'ar, Asaph Distelfeld, Nurit Katzir, Yelena Yeselson, Shmuel Shen, and Arthur A. Schaffer

The sweet cultivars of Cucumis melo are characterized by high sucrose levels, together with low acid levels in the mature fruit flesh. The trait of high sugar accumulation in C. melo fruit is determined by a single recessive gene, suc. High acid content, conferred by a single dominant gene, So, is found only in C. melo varieties that do not accumulate high levels of sugar and are used for nondessert purposes. We combined the genetic traits of high acid content (low pH) and high sugar levels by crossing the nonsweet, high acid C. melo var. flexuosus, `Faqqous' (So/So, Suc/Suc), with high sugar, low acid C. melo genotypes (so/so, suc/suc) and generating the recombinant genotype So/—, suc/suc. Segregating F2 populations derived from the cross between `Faqqous' and a standard high sugar, low acid line showed that the traits of high sugar and low pH were inherited independently of each other. The accumulation of acid and sugar in the developing fruit of a recombinant high acid, high sugar breeding line, A6, were also temporally independent, with acid accumulation preceding the rise in sucrose levels. The low pH of A6 was correlated with the developmental increase in titratable acidity and particularly of citric acid levels. The combination of increased acidity and high sugar provides the melons with a unique taste due to a sugar to acid ratio not present in sweet C. melo cultivars. These results are discussed in terms of the evolution under domestication of C. melo.

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Yiping Zhang, Molly Kyle, Konstantinos Anagnostou, and Thomas A. Zitter

Greenhouse and field evaluations of melon (Cucumis melo L.) for resistance to gummy stem blight, caused by the fungus Didymella bryoniae (Auersw.) Rehm, were conducted on 798 U.S. Dept. of Agriculture Plant Introduction (PI) accessions and 24 related Cucumis species. Plants were inoculated at the three to four true-leaf stage with a virulent isolate of D. bryoniae collected from Onondaga County, N.Y., and disease indices were calculated based on foliar and stem symptoms. In greenhouse screens, 43 C. melo accessions showed a high level of resistance. Results were consistent between the optimized greenhouse screening procedure described and inoculated replicated field tests. Of these accessions, a Chinese group, PIs 157076, 157080, 157081, 157082, 157084; another group from Zimbabwe, PIs 482393, 482398, 482399, 482402, 482403, 482408; and some others from different origins, PI 255478 (Korea) and PI 511890 (Mexico), showed high levels of resistance, at least equal to that in PI 140471, the leading source of resistance to date.

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Yosef Burger, Uzi Saar, Nurit Katzir, Harry S. Paris, Yelena Yeselson, Ilan Levin, and Arthur A. Schaffer

Fruit sweetness is the major determinant of fruit quality in melons (Cucumis melo L.) and reflects the concentration of the three major soluble sugars, sucrose, glucose, and fructose, present in the fruit flesh. Of these three sugars, sucrose is the prime factor accounting for both the genetic and the environmental variability observed in sugar content of C. melo fruit. Faqqous (subsp. melo var. flexuosus), a cultivar having a low sucrose and total sugar content, was crossed with Noy Yizre'el (subsp. melo var. reticulatus), a cultivar having a high sucrose and total sugar content. F1 plants had a sucrose content averaging slightly higher than that of the low-sucrose parent, indicating that low sucrose content is nearly completely dominant. Segregation in the F2 and backcross progenies indicated that high sucrose accumulation in melon fruit flesh is conferred by a single recessive gene herein designated suc. When the high-sucrose parent was crossed with the moderate-sucrose landrace known as Persia 202 (subsp. melo var. reticulatus), the segregation in the filial and backcross progenies suggested that additional genetic factors affect the amount of sucrose accumulation.

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James D. McCreight, Jack E. Staub, Anabel López-Sesé, and Sang-Min Chung

Genetic variation among 378 melon (Cucumis melo L.) germplasm accessions collected in India in 1992 and 26 accessions in China in 1994 was evaluated with 19 isozyme loci. `Top Mark' and `Green Flesh Honeydew', which represented two distinct C. melo ssp. melo L. groups, Cantalupensis and Inodorus, respectively, were used as reference cultivars. Genetic distances among accessions were calculated, and an initial cluster analysis using these distances resulted in 148 groups of varying size, ranging from two to 47 accessions. One accession from each of the 148 groups was chosen at random and used in a second cluster analysis that identified 11 accession groups. Group 1 was unique and consisted of only two C. melo ssp. agrestis (Naudin) Pangalo accessions. Two large branches were detected at cluster node 2. One branch was comprised of three groups of 3, 12, and 34 accessions, while the other branch contained seven groups of 2, 3, 14, 16, and 47 accessions, and the reference cultivars. Of the 148 accessions, 132 were from 41 sites in Rajasthan and Madhya Pradesh, India, which were distributed unequally across the 11 groups. The 14 Chinese accessions originating from seven provinces were also dispersed unequally in the four major cluster groups. `Top Mark' and `Green Flesh Honeydew' were genetically distinct and uniquely clustered in the same group. These results indicate that additional collections of melon germplasm should be made in eastern and southern India.

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C.E. Thomas

Field evaluations for resistance against downy mildew, incited by Pseudoperonospora cubensis (Berk. & M.A. Curtis) Rostovzev were conducted on 1076 U.S. Plant Introductions (PI) of Cucumis melo L. (melon). A disease index (DI) was calculated for each entry that had one or more resistant plants. Based on DI, PIs 271329 and 401644 were the most resistant overall (DI = 2.6 and 2.8, respectively). However, resistant plants exhibiting reaction type (RT) 3 were identified in 68 accessions, and 110 accessions had moderately resistant (RT 2) plants.