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D.T. Ray and J.D. McCreight

A new chlorophyll-deficient mutant is the first cytoplasmically inherited trait described in melon. This mutant is characterized by yellow apices with the leaves and stems progressively turning green in color as the branches mature. A protocol is proposed for naming and symbolizing cytoplasmic traits in melon. This mutation is named yellow-tip and symbolized cyt-Yt.

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J. Staub, Felix Sequen, and J.D. McCreight

Genetic variation in cucumber (Cucumis sativus L.) accessions from India was assessed by examining variation at 21 polymorphic isozyme loci. Forty-six accessions acquired by the U.S. National Plant Germplasm System (NPGS) before 1972 were compared with 146 accessions collected during a 1992 U.S.–India expedition to the states of Rajasthan, Madhya Pradesh, and Uttar Pradesh, India. Two distinct groups (Group 1 and Group 2) were identified within accessions collected in 1992 (0.025 < P < 0.01). Variation at Ak-2, Fdp-2, Gr, Mdh-2, Mpi-1, Per, Pgm, and Skdh was important in the detection of this difference. Group 1 contained 37 (27 Madhya Pradesh + 10 Uttar Pradesh) accessions and Group 2 contains 102 (84 Rajasthan + 18 Madhya Pradesh) accessions. Seven accessions (5 Madhya Pradesh + 2 Rajasthan) were not associated with either group. Isozymic variation in U.S. NPGS accessions acquired before 1972 differed significantly (P < 0.005) from those collected during 1992. When Indian accessions taken collectively (collected before 1972 and in 1992) were compared with an array of 707 C. sativus U.S. NPGS accessions examined previously, relationships differed between accessions grouped by country or subcontinent.

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Edward J. Ryder, William Waycott, and James D. McCreight

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J. Staub, J. Box, V. Meglic, T.F. Horejsi, and J.D. McCreight

Principal component analyses of variation at 21 isozyme and 43 random amplified polymorphic DNA (RAPD) loci in eight cucumber (GY-14a, G421, H-19, WI 2757, and PIs 432860, 458845, and 183967) and seven melon [Top Mark (TM), Doublon, Green Flesh Honeydew (GFH), Juane Canari (JC), Freeman cucumber (FC), Snakemelon (SM), and PI 124111] cultigens were used to determine the use of these markers for assessing genetic variation among and within populations of each species (outgroup = Cucumis metuliferus). RAPD and isozyme marker variation was related to previous taxonomic classification and available pedigree information. Although dendrograms derived from cluster analyses using species' variation at marker loci were dissimilar, these disparities were consistent with differences in the pedigrees and/or other information (e.g., morphological) known about each accession and species. Elite U.S. processing cucumbers (G421, GY-14a, and H-19) shared distinctive biochemical affinities. Doublon was differentiated from TM, GFH, and JC. Doublon had biochemical affinities with FC, SM, and PI 124111.

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V. V. Meglic, T. F. Horejsi, J. E. Staub, and J. D. McCreight

The genetic diversity of 400 U.S. melon germplasm plant introductions was assessed using 35 enzyme systems. Polymorphisms were observed at 24 putative loci (Ac, Acp1, Acp4, Ak2, Ak3. Ak4, Fdp1, Fdp2, Fdp4, Gpi, Idh, Mdh2, Mdh4, Mdh5, Mdhb, Mpi1, Mpi2, Pgd1, Pgd2, Pgm, Pep-g1, Pep-1a, Pep-pap, Skdh) representing 17 different enzymes. Sixteen loci demonstrated simple Mendelian inheritance. Multivariate analyses aided in reduction of data using 16 loci and linkage relationships were observed among the plant introductions. Two of 16 loci (Pgd1 and Acp1) segregated independently. Fourteen loci were assigned into three linkage groups (A-C): A Fdp1, Fdp2, Acp4, Skdh; B Mdh2, Mdh4, Mdh5, Mdh6, Pep-g1, Pgm; C Mpi2, Ac, Idh.

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Albert N. Kishaba, Steven J. Castle, Donald L. Coudriet, James D. McCreight, and G. Weston Bohn

The spread of watermelon mosaic virus by the melon aphid (Aphis gossypii Glover) was 31%, 74%, and 71% less to a melon aphid-resistant muskmelon (Cucumis melo L.) breeding line than to the susceptible recurrent parent in a field cage study. Aphid-resistant and susceptible plants served equally well as the virus source. The highest rate of infection (97.9%) was noted when target plants were all melon-aphid susceptible, least (26.7%) when the target plants were all melon-aphid resistant, and intermediate (69.4%) when the target plants were an equal mix of aphid-resistant and susceptible plants. The number of viruliferous aphids per plant required to cause a 50% infection varied from five to 20 on susceptible controls and from 60 to possibly more than 400 on a range of melon aphid-resistant populations. An F family from a cross of the melon aphid-resistant AR Topmark (AR TM) with the susceptible `PMR 45' had significantly less resistance to virus transmission than AR TM. Breeding line AR 5 (an aphid-resistant population with `PMR 5' as the recurrent parent) had significantly greater resistance to transmission than other aphid-resistant populations.

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A.N. Kishaba, S. Castle, J.D. McCreight, and P.R. Desjardins

Confined-leaf tests in a greenhouse showed Lagenaria siceraria (Molina) Standley plant introduction (PI) 442369 was as susceptible to sweetpotato whitefly, Bemisia tabaci Gennadius, oviposition as Cucumis melo L., Cucurbita ecuadorensis Cutler and Whitaker, and Cucurbita lundelliana Bailey, whereas L. siceraria accessions PI 419090, PI 419215, PI 432341, and PI 432342 were resistant. Resistance rankings of L. siceraria accessions based on adult counts in greenhouse and field tests were similar. Adult entrapment among trichomes was highest on adaxial leaf surfaces of L. siceraria PI 419090. Abaxial leaf trichome density was 48.7/mm on sweetpotato whitefly-resistant L. siceraria PI 432342, 42.1/mm2 on Cucurbita lundelliana PI 540895, and ranged from 51.0 to 85.5/mm2 on Cucurbita ecuadorensis PI 540896. Leaf trichome densities of selected plants of four L. siceraria accessions ranged from 33.0 to 52/mm2 on the abaxial and from 6.3 to 20.8/mm2 on the adaxial surface. Scanning electron micrographs of the abaxial leaf surface, the preferred surface for oviposition, suggest that trichome configuration (density and arrangement of different lengths) could be a factor in reduction of whitefly oviposition on L. siceraria.

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Todd C. Wehner and Beiquan Mou