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James D. McCreight and Michael D. Coffey

A new strain of cucurbit powdery mildew incited by Podosphaera xanthii (Castagne) Braun & Shishkoff (formerly Sphaerotheca fuliginea), designated race S, is virulent on all the commonly used melon (Cucumis melo L.) powdery mildew race differentials and first appeared on melon in Yuma, AZ, and Imperial Valley, CA, in 2003. Melon PI 313970 (C. melo var. acidulus) was resistant to P. xanthii race S in 2003 and subsequent years. Inheritance of resistance to P. xanthii race S was studied in two naturally infected, replicated field tests in Imperial Valley in 2005 and 2009 using the parents and F1, F2, and reciprocal backcross generations from crosses of PI 313970 with powdery mildew-susceptible ‘Top Mark’. Resistance to race S was recessive: all F1 and BCTM individuals were susceptible. One recessive gene, designated pm-S, conditioned resistance to race S in the F2 and BCPI. The relationship of pm-S to the previously reported recessive and codominant genes in PI 313970 for resistance to P. xanthii races 1, 2, 2U.S., 3, 3.5, 4.5, and 5 remains to be determined.

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Patti Fashing-Burdette and James D. McCreight

In 1963, melon (Cucumis melo L.) plant introductions (PI) 124112 and PI 180280 were reported variable in response to inoculation with the T-1 or Freitag's isolates of watermelon mosaic virus. Most plants were symptomless, but some had small pinpoint necrotic lesions on cotyledons and leaves. The T-1 and Freitag's isolates of watermelon mosaic were later designated watermelon mosaic virus 1, and more recently renamed papaya ringspot virus watermelon strain (PRSV-W). When inoculated with California or Florida isolates of PRSV-W in 1993, WMR 29 a western U.S. shipping type melon derived from PI 180280 was symptomless (incompatible reaction) and SDS-immunodiffusion assays were negative. In contrast when inoculated with the same PRSV-W isolates, PI 124112 had incompatible reactions characterized by wilting, local lesions, systemic necrotic spots and necrosis and negative SDS-immunodiffusion assays.

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James D. McCreight and Yong-Biao Liu

The lettuce aphid, Nasonovia ribisnigri Mosley (Hemiptera: Aphididae), is a major insect pest of lettuce, Lactuca sativa L, in many commercial lettuce production areas around the world. Resistance to lettuce aphid biotype 0 (Nr:0) was first reported in Lactuca virosa L. accession IVT 280 and characterized as complete, i.e., virtually no aphids survived, and genetically dominant to partial resistance in L. virosa accession IVT 273. Complete and partial resistances to Nr:0 were conditioned by two alleles, Nr (complete resistance) and nr (partial resistance), but the genetic relationship to susceptibility was not reported. We previously reported two new potential sources of unique genes for resistance to Nr:0 in Lactuca serriola L. accession PI 491093 and L. virosa PI 274378. We report on the genetic and phenotypic nature of resistance to Nr:0 in these two wild lettuce accessions. Resistance to Nr:0 in PI 274378 is complete and allelic to complete resistance in IVT 280. Resistance to Nr:0 in PI 491093 was partial, recessive to complete resistance in ‘Barcelona’ that was derived from IVT 280, but dominant to susceptibility in ‘Salinas’. We propose the revised gene symbols for resistance to Nr:0: Nr:0C for complete resistance and Nr:0P for partial resistance, which was originally designated as nr but may now be regarded as the symbol for susceptibility to all strains of lettuce aphid. The dominance relationships among these three alleles are Nr:0C (in IVT 280, ‘Barcelona’) > Nr:0P (in PI 491093) > nr (in susceptible genotypes). Expression of partial resistance in PI 491093 was variable in controlled infestation tests, but in a naturally infested field test provided a potentially useful level of resistance to Nr:0. Partial resistance, where complete resistance has not been widely deployed, may either alone or as a component of integrated pest management delay or prevent emergence of genotypes that overcome complete resistance controlled by Nr:0C.

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James D. McCreight and Albert N. Kishaba

Squash leaf curl (SLC) is a virus disease of squash transmitted by the sweetpotato whitefly [Bernisia tabaci (Germ.)]. 'Cucurbita maxima Duch. ex Lam., C. mixta Pang, and C. pepo L. cultivars and the wild taxon. C. texana Gray exhibited severe symptoms in response to SLC in greenhouse and field tests. Symptoms on C. moschata (Duch.) Duch. ex Poir. cultivars were much more severe in greenhouse tests than in field tests. Three wild species, C. ecuadorensis Cutler and Whitaker, C. lundelliana Bailey, and C. martinezii Bailey, were virtually immune in greenhouse tests, but were infected in field tests. Cucurbita foetidissima HBK expressed moderate symptoms in a field test. Benincasa hispida (Thunb.) Cogn., C. ficifolia Bouche, Lagenaria siceraria (Mol.) Standl., Luffa acutangula (L.) Roxb., Luffs aegyptiaca Mill., and Luffs graveolens Roxb. were resistant to SLC in greenhouse and field tests.

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James D. McCreight and William M. Wintermantel

Melon (Cucumis melo L.) is a fresh vegetable and dessert fruit that may also be cooked or dried, processed for juice and flavoring, and the seeds of which are a source of high-quality cooking oil and high protein seed meal. Melon production throughout many parts of the world is now threatened by the crinivirus Cucurbit yellow stunting disorder virus (CYSDV) in tropical and subtropical areas favorable to its whitefly vector. CYSDV is transmitted by the sweetpotato whitefly, Bemisia tabaci Gennadius, biotypes A, B, and Q. CYSDV first appeared on melon in the 1980s in the United Arab Emirates and emerged on melon in the Yuma, AZ, and Imperial Valley, CA, regions and western Mexico during the Fall season of 2006 followed by Florida in 2007. PI 313970, C. melo var. acidulus Naudin, a salad-type melon from India, expressed high-level resistance to CYSDV in Yuma and Imperial Valley in Fall 2006, but it was not immune; the virus was detected in asymptomatic plants. Inheritance of resistance to CYSDV in PI 313970 was studied in three naturally infected, replicated field tests in Imperial Valley during the Fall seasons of 2007 and 2008 and the Spring season of 2009. Resistance in PI 313970 was recessive: all F1 PI 313970 (PI) × susceptible ‘Top Mark’ (TM) and BCTM individuals were susceptible, and the F2 and BCPI segregated 3:1 and 1:1 susceptible to resistance, respectively. Frequency distributions of CYSDV symptom severity ratings suggested a single recessive gene in PI 313970 for resistance to CYSDV. PI 313970 was, however, observed to be variable for resistance; a few plants in each test expressed distinct symptoms of CYSDV infection and its frequency distributions overlapped those of ‘Top Mark’. This variation may represent genetic variation selectable for uniform reaction to infection by CYSDV or phenotypic variation in the resistant reaction. The genetic relationship between the genes for resistance to CYSDV in PI 313970 (recessive) and TGR-1551 (dominant) is not known.

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

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James D. McCreight, Hsing-Yeh Liu, and Thomas A. Turini

Cucurbit leaf crumple geminivirus (CuLCrV) is transmitted by sweet-potato whitefly (Bemisia tabaci) biotype B (SPWF-B) and occurs on cucurbits in Arizona, California, Texas, and Mexico. This virus is identical to Cucurbit leaf curl virus, and their symptoms are similar to Squash leaf curl virus on squash (Cucurbita sp.) and Melonleaf curl virus on melon (Cucumis melo L.). Melon has been reported to be either susceptible to CuLCrV, or to have the ability to recover from infection. Twenty-three melon cultigens were inoculated with CuLCrV in greenhouse tests using SPWF-B. Eighteen of the cultigens tested were highly susceptible to CuLCrV (≥60% infected plants) and generally exhibited pronounced CuLCrV symptoms: `Amarillo', `Edisto 47', `Esteem', `Fuyu 3', `Impac', `Moscatel Grande', `Negro', `Perlita', PI 234607, PI 236355, PI 414723, `PMR 5', `Seminole', `Sol Dorado', `Sol Real', `Top Mark', `Vedrantais', and WMR 29. Five cultigens were resistant to CuLCrV (<40% infected plants that exhibited restricted, mild symptoms): MR-1, PI 124111, PI 124112, PI 179901, and PI 313970. Symptoms abated with time in both groups although infected plants remained positive for the virus. Ten of the cultigens (`Edisto 47', `Fuyu 3', `Impac', MR-1, PI 124112, PI 313970, PI 414723, `PMR 5', `Top Mark', and WMR 29) were included in field tests in 2003 and 2004 that were naturally infected with CuLCrV. With the exception of PI 414723, the greenhouse and field data were consistent for reaction to CuLCrV.

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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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Jack E. Staub, James D. McCreight, and Juan E. Zalapa

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James D. McCreight, Hsing-Yeh Liu, and Thomas A. Turini

Cucurbit leaf crumple virus (CuLCrV) is a geminivirus transmitted by Bemisia tabaci (Gennadius) biotype B (SPW-B) and common in melons (Cucumis melo L.) planted from July through September in the desert southwestern United States. Symptoms include chlorotic leaf spots, leaf curling and crumpling, and interveinal yellowing, and plants may be stunted. Melon breeding line MR-1, and six plant introductions (PIs; PI 124111, PI 124112, PI 179901, PI 234607, PI 313970, and PI 414723) exhibited partial resistance to CuLCrV in naturally infected field tests and controlled inoculation greenhouse tests. PI 236355 was completely resistant in two greenhouse tests. Partially resistant plants exhibited chlorotic spots, or mild expression of other typical CuLCrV symptoms; all such plants were positive for presence of virus using polymerase chain reaction analysis with a CuLCrV-specific primer pair from the BC1 region. Genetic resistance to CuLCrV in melon was recessive. Field and greenhouse data from F1, F2, and backcrosses of the F1 to ‘Top Mark’ and PI 313970 demonstrated a single, recessive gene for resistance to CuLCrV. Progenies from crosses of four partially resistant cultigens with ‘Top Mark’ were susceptible. Resistance in PI 313970 appeared to be allelic, with resistance in the other six cultigens based on F1 data. The name cucurbit leaf crumple virus and symbol culcrv are proposed for this gene.