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Developing and
enhancing the
genetics of economically-important
cucurbits

Cucurbitaceae 94: Evaluation and Enchancement of Cucurbit Germplasm

South Padre Island, TX November 1-4, 1994

Oral Presentation Abstracts

#1 The combined effects of Whitefly biotype/and virus/host interactions on genetic variability of whitefly variability of whitefly-transmitted geminivirus isolates in cucurbits.

J.K. Brown, Dept. of Plant Sciences, University of Arizona, Tucson AZ 85721.

The concept of whitefly (Bemisia tabaci Genn.) biotypes or host races is emerging as a means of better understanding the interactions between geminivirus/whitefly vector/host plant complexes. The obligate requirement of the whitefly-transmitted (WFT) geminivirus subgroup for Bemisia-mediated transmission imposes certain constraints on the degree of genetic variability that con occur within virus populations. The capsid protein gene product is thought to play an important role in this process, and in this scenario specific features of the capsid gene must be conserved in structure and function. Compatible host plant/geminivirus interactions facilitate completion of a successful virus infection cycle, also mandating compatibility between certain viral genes and the host plant-related factors. Implicit in the biotype concept is the definitive role of a successful interaction between the whitefly vector and the host plant that both accomplishes the dispersal of virions by the ve ctor, and promotes the generation of whitefly progeny. In this light, highly specific and coordinated interactions must occur between the individual components of geminivirus/vector/host plant complexes in order to initiate and continue the cyclic mode of the system. Such interactions may depend on the particular whitefly biotype and the extent of its host plant repertoire, which at the outset limits host range potential of the virus. What effect this selection pressure has on the genetic variability/adaptability and phenotypes of the respective virus isolates/populations is not known. Studies from several laboratories indicate biological and genetic variability among isolates of the squash leaf curl virus (the only known WFT geminivirus of cucurbits in the Americas), leading to the notion of a virus complex, and/or of a helper-dependent situation. The particular factors and specific underlying mechanisms that give rise to this diversity are under investigation.

#2 Whitefly Transmitted Yellowing Viruses of the Cucurbitaceae.

James E. Duffus, USDA-ARS, U.S. Agricultural Research Station, Salinas, CA 93905. Whitefly-transmitted yellowing viruses of cucurbits are causing severe economic losses throughout the world. Three distinct whitefly transmitted cucurbit viruses have been distinguished--beet pseudo yellows (BPYV), lettuce infectious yellows (LIYV), and cucurbit yellow stunting disorder virus (CYSDV). BPYV virus has caused severe losses in greenhouse grown cucurbit crops throughout North America, Europe, and Asia. It has been reported from France, The Netherlands, Japan, Italy, Spain, England, Australia, and Bulgaria. Since 1982, the incidence in melon crops under protected environments and outdoors on the Mediterranean coast of Spain has continually increased inducing considerable economic losses. The virus has a wide host range of important crop, weed and ornamental hosts. BPYV is transmitted by Trialeurodes vaporariorum in a semi-persistent manner and is retained by the insect for 6 days. Purified preparations contained long, flexuous particles 1500 nm long. The virus has been termed cucumber yellows, muskmelon yellows, melon yellows, and cucumber chlorotic spot virus, but these isolates have not been shown to be distinct from BPYV. A distinct whitefly transmitted virus, LIYV, was reported from the desert regions of California and Arizona in 1981. The virus, transmitted specifically by the A biotype of Bemisia tabaci, has a wide host range of important crop hosts. LIYV has long filamentous particles 1800 nm long which are retained by Bemisia for 3 days. The virus has been also found in Texas and Mexico. In the early 1980's a yellowing and stunting disorder of cucurbits was noticed in the Middle East. The disease has been found in Jordan, Israel, UAE and Turkey. The virus, CYSDV, has a narrow host range, mainly in the Cucurbitaceae. CYSDV is transmitted specifically by the B biotype of B. tabaci and is retained by the vector for 10 days. Purified preparations contained long, flexuous particles 1200 nm long.

#3 Genetic Engineering of Virus Resistance in Cucurbits

Rebecca Grumet, Horticulture Department, Michigan State University, East Lansing, MI 48824

Many cucurbit crops are subject to severe losses by an array of viruses including cucumber mosaic virus (CMV) and the potyviruses, zucchini yellow mosaic virus (ZYMV), watermelon mosaic virus (WMV) and the watermelon strain of papaya ringspot virus (PRSV-W). Recent efforts by several groups have been directed toward the genetic engineering of resistance to these viruses. These efforts include: the development of transformation systems for cucurbit species including summer squash, cucumber and muskmelon; the cloning and engineering of potential resistance genes (i.e. coat protein (CP) and/or replicase genes of CMV, ZYMV, WMV, and PRSV-W; and the production, verification and testing of virus-resistant transgenic plants. Our work has included the development of transgenic ZYMV-resistant melons. ZYMV-Ct RNA was isolated and cloned and the CP gene engineered for gene expression and transfer via A. tumefaciens-mediated transformation. Regenerated plants were verified to contain and ex press the ZYMV CP gene by PCR, northern and western analyses. Upon inoculation with ZYMV-Ct, transgenic plants and their progeny showed no symptom development for at least three months and had no detectable virus accumulation. Resistance could be overcome by increasing inoculum concentrations. The transgenic progeny exhibited resistance against several other strains of ZYMV and also showed some protection against infection by WMV, but not PRSV-W. The materials are also being tested in the field for general performance and response to virus infection.

#4 Host Plant Resistance in Melons to Whiteflies.

David G. Riley, Texas Agricultural Experiment Station, 2415 E. Hwy 83, Weslaco, Texas 78596.

Host plant resistance studies involving whiteflies have identified several potential mechanisms that can result in non-preference and antibiosis including density and pattern of leaf trichomes, stickiness of trichomes, thickness of leaf tissue or distance from leaf surface to the phloem, pH of plant sap, nitrogen content and other mechanisms. Quantification of host plant / whitefly interactions is no simple matter because of the complexity both whitefly and plant response to stress that occurs simultaneously in most cases. An additional complication is that whitefly damage can be direct, such as a reduction of plant vigor due to adults and nymphs feeding on the plant phloem and excretion of honey-dew, and indirect, such as the development silver leaf symptoms in squash and irregular ripening of fruit, increase of sooty mold, and the transmission of plant viruses. The b-strain sweetpotato whitefly, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) or the proposed new species nam e of silverleaf whitefly, Bemisia argentifolli (SW), was evaluated on selected melon cultivars in treated and untreated field plots in 1992, 1993 and 1994 at Weslaco, Texas. Whitefly ovipositional preference and plant tolerance were evaluated by monitoring whitefly numbers and plant yield response. Plant characteristics, damage and yield were evaluated and correlated with whitefly numbers. Potential mechanisms in melons for host plant resistance to whiteflies are discussed.

#5 Monosporascus Root Rot/Vine Decline of Melons - A Case Study.

R. D. Martyn1, B. R. Lovic1, and M. E. Miller2. Department of Plant Pathology and Microbiology, Texas A&M University, 1College Station 77843 and 2Weslaco 78596.

Monosporascus cannonballus, causal agent of a severe root rot/vine decline disease of muskmelon and watermelon, is a recently described soilborne ascomycete with a limited known distribution. It has been reported as a pathogen of melons from the southwestern United States (Arizona, Texas, California), Japan, southern Spain, and Tunisia. A similar species, M. eutypoides was described as the causal agent of a vine collapse disease of melons in Israel; however, we believe that M. eutypoides and M. cannonballus are synonymous. M. cannonballus infects young secondary and tertiary roots early in the season, colonizes the cortical tissue, ultimately killing most of the feeder roots. By mid to late season, most of the root system is affected and the vines begin to collapse, typically beginning with the crown leaves and progressing distally. At or near harvest, the entire canopy may collapse exposing the fruit to intense solar radiation. Lateral roots ma y bear numerous black perithecia which contain several hundred jet-black, spherical, ascospores. Ascospores do not germinate under normal laboratory conditions and there is no known asexual conidial stage; therefore, mycelia surviving in infected root tissue is believed to be an important source of inoculum. Under field conditions, M. cannonballus is known only to cause severe disease in watermelon and muskmelon; however, greenhouse inoculation studies have shown that all cucurbits are susceptible. The fungus has an optimum in vitro growth temperature of 30-35 C and will tolerate high salt and high pH, which is consistent with the climatic conditions where it is found. Detection of M. cannonballus early in the season is difficult; therefore, a PCR-mediated detection system based on species-specific primers derived from the internal transcribed spacer regions of the rDNA repeat unit was developed. This protocol is capable of detecting M. cannonballus fro m asymptomatic or deteriorated root tissue and individual ascospores recovered from soil. Degeneration of some isolates of M. cannonballus results in reduced growth and sporulation, increased pigment and phenotypic variability, reduced virulence, and death and has been associated with the presence of dsRNA in those isolates. Several common and unique dsRNA profiles have been identified from isolates originating from Texas and Spain which appear to be stable and, in at least one case, transmissible from one isolate to another. The significance of these extra genetic dsRNA elements in the biology and pathology of M. cannonballus is currently being investigated.

#6 Sudden Wilt of Melons form a Northeastern U.S. Perspective.

T.A. Zitter, Department of Plant Pathology, Cornell University, Ithaca, NY 14853.

"Sudden wilt" is the common name applied to the sudden collapse of cucurbits, and especially melons. Unfortunately, this general term has been used not only to describe the condition on melon, but other cucurbits as well (cucumber and watermelon), and has been applied to similar, but certainly different diseases, as reported from the West, the Midwest and the Northeastern U.S. Any attempt to understand "sudden wilt" of cucurbits, must begin with an examination of the disease characteristics as reported from the different sections of the country where it occurs. However, even within the same section of the country (i.e. the Northeast), the disease has been the basis for much confusion. "Sudden wilt" of melons has been recognized in New York since 1941, and during that time, associated with at least three disorders; Fusarium wilt and root rot, collapse (the sudden wilting of plants near maturity but with no clear causal agent), and decline associated with cucumber mosaic virus. An attempt is made to clarify the confusion associated with the continued collective use of the term "sudden wilt".

#7 Etiology, Epidemiology, and Control of Muskmelon Fruit Rots.

B.D. Bruton, USDA-ARS, P. O. Box 159, Lane, Oklahoma 74555.

The netted cantaloupe typically has a much greater incidence of preharvest and postharvest decay than the non-netted honey dew types. Evidence suggests that epidermal splitting, as the net begins to develop, may provide entry to many of the fungal pathogens. Fusarium spp. are responsible for a majority of the preharvest fruit rots with Macrophomina phaseolina causing significant losses at times. The preharvest environment and production methods have a large impact on postharvest diseases. Incipient infections by Fusarium spp. that go undetected through the packing line cause heavy retail losses. Latent infections, caused by Phomopsis cucurbitae, can incite heavy losses in cantaloupe. Preharvest fungicide application has been somewhat ineffective due to difficulty in obtaining sufficient coverage of the fruit. Fungicides, in combination with hot-water treatment, have generally been successful in controlling most of the postharvest decays. Time of immer sion (1 min) and temperature (57 C) are critical to obtain adequate control.

#8 Breeding Cucurbits for Multiple Disease Resistance.

Molly Kyle, Dept. of Plant Breeding, Cornell University.

The major objective in our applied cucurbit breeding programs involves breeding for broad spectrum disease resistance to reduce losses in quality and yield. For melons and squash, we are combining resistance to four viral diseases with powdery mildew resistance and in melon, Fusarium wilt, in a number of different types of Cucumis melo, Cucurbita pepo and C. moschata using backcross and pedigree breeding methods. Where simply inherited sources of resistance to a major disease have not been identified despite widespread searches, e.g. cucumber mosaic virus in melon and C. pepo, it may be most efficient to invest in transgenic approaches if the regulatory and market issues do not remain prohibitive. For these two species, we are developing resistance derived from a truncated clone of the cucumber mosaic virus replicase gene that has conferred a high level of resistance to the virus when expressed in tobacco. We also are working with plant genes for resista nce to this disease and have recently identified monogenic resistance to CMV in C. moschata Nigerian Local. Much of our work in Cucurbita involves inter-specific transfer through which the inheritance of a characteristic can change. Where adequate sources of resistance are not available, we have begun systematic evaluation of germplasm, e.g. for gummy stem blight/black rot resistance in C. melo, C. moschata, and C. pepo. Promising accessions have been intercrossed and backcrossed with leading commercial types with multiple disease resistance. Our in melon program typifies the situation where molecular markers are theorized to be most profitably applied, i.e. a valuable crop where wild sources of resistance that may be difficult to select are prominent and must be combined with horticultural characteristics. In collaboration with Dr. R. Perl-Treves coordinated with several groups with similar interests, we are developing a molecular map of C. melo and setting up populations to detect linkage between important resistances and molecular markers. Our emphasis on combining a number of resistances is based on the increasing environmental, economic and consumer concern with chemical control measures, and also reflects the observation that secondary diseases may cause major losses once varieties with limited resistance are released.

#9 Molecular Tagging of Virus Resistance Genes in Cucumber.

M. J. Havey,USDA/ARS, Department of Horticulture, University of Wisconsin, Madison, WI 53706 USA.

Pickling cucumber is susceptible to attack by many viruses, the most important being Cucumber Mosaic Virus (CMV), the watermelon strain of Papaya Ringspot Virus (PRSV-W), Watermelon Mosaic Virus 2 (WMV), and Zucchini Yellow Mosaic Virus (ZYMV). Sources of resistance to CMV, PRSV-W, WMV, and ZYMV have been identified. However, most sources of viral resistance are poorly adapted oriental types and transfer of the resistances to an acceptable pickling cultivar is required. Breeding cucumber for resistance to CMV, PRSV-W, WMV, and ZYMV can be slow because the symptoms incited by the different viruses are very similar and the resistance genes are recessive, complexly inherited, and/or affected by the environment and modifier genes. In order to more efficiently breed cucumber for resistance to viruses, a project was undertaken to identify a set of DNA markers (restriction fragment length polymorphisms [RFLPs] or random amplification of polymorphic DNA [RAPDs]) linked to the genes conditioning resistance to CMV, PRSV-W, WMV, and ZYMV. Linkage between a DNA marker and a virus-resistance gene allows one to identify the presence of the resistance gene by evaluating for the genotype at the marker locus. We have generated segregating families from Straight 8 x SMR 18 (CMV), Straight 8 x Marketmore 76 (CMV), and Straight 8 x TMG-1 (CMV, PRSV-W, WMV, and ZYMV) and identified 137 RAPD or RFLP markers in cucumber, of which approximately 20 differ between Straight 8 and SMR 18, 11 between Straight 8 and Marketmore 76, and 45 between Straight 8 and TMG-1. We are presently completing segregation analyses.

#10 Osmotin Mediated Host Plant Phytopathogenic Fungal Resistance.

Paul M. Hasegawa and Ray A. Bressan, Center for Plant Environmental Stress Physiology, 1165 Horticulture Building, Purdue University, West Lafayette IN 47907- 1165.

Osmotin is a basic isoform of the family 5 pathogenesis-related(PR) proteins that was isolated originally from cultured cells of tobacco (Nicotiana tabacum L. var. Wisconsin 38). Osmotin has in vitro antifungal activity against members of several classes of fungi including important cucurbit pathogens, e.g. Fusarium spp., Botrytis cinerea and Colletotrichum lagernarium. Recently, we have determined that transgenic potato plants overproducing osmotin exhibit delayed late blight (Phytophthora infestans) symptom development. Further, activation of osmotin gene expression is correlated with nonpathogenic fungal elicitor induction of black shank (P. parasitica) resistance in tobacco seedlings. Elicitor induction of the osmotin promoter is similar to the hyperactivation that occurs in response to a treatment of combined ethylene/jasmonate. Promoter deletion analyses indicate that the responsive element for both elicitor and ethylene/jasmonate in duction are located in the same region of the promoter, -248/-108.

#11 Inducible Genes and Plant Responses to Stress.

John E. Mullet, Erin Bell, and Robert Creelman, Crop Biotechnology Center, Texas A&M University Department of Biochemistry and Biophysics, Texas A&M University.

Plants respond to stress, pathogen and insect attack by inducing specific defensive responses. The activation of abiotic and biotic stress responses often includes the induction of specific genes. Our laboratory has isolated several genes that are induced in response to drought stress. These genes encode a membrane channel forming protein, a thiol protease, an aldehyde reductase, lipoxygenase and acid phosphatases. Interestingly, the genes that encode lipoxygenase (Lox) and the acid phosphatases (Vsp) are also induced by wounding and other biotic stresses. The induction of these genes by wounding is mediated by a plant lipid derivative, jasmonic acid. Jasmonic acid also induces genes encoding proteinase inhibitors and enzymes involved in flavonoid and isoprenoid biosynthesis. This indicates that jasmonic acid plays an important role in plant responses to pathogen or insect attack. We are presently investigating the jasmonic acid signal transduction pathway by isolating insensitive muta nts and characterizing the cis- and trans-factors that mediate gene induction in response to jasmonic acid. Connections between characterization of inducible gene responses and building genotypes with durable biotic and abiotic stress tolerance will be discussed.

#12 The Use of Plant Growth-Promoting Rhizobacteria to Induce Systemic Resistance in Cucumber Against Diseases and Insects.

Joseph W. Kloepper, Gang Wei, Geoffrey W. Zehnder, and Changbin Yao. Departments of Plant Pathology, Biological Control Institute, Alabama Agricultural Experiment Station, Auburn University, Auburn, Alabama 36849.

Over the past 4 years, work in our laboratories has demonstrated that select strains of plant growth-promoting rhizobacteria (PGPR) can induce systemic resistance of cucumber to foliar and vascular pathogens. Applications of PGPR to cucumber seeds or roots resulted in significant reductions in symptoms of anthracnose (Colletotrichum orbiculare), Fusarium wilt (Fusarium oxysporum f. sp. cucumerinum), bacterial wilt (Erwinia tracheiphila), angular leaf spot (Pseudomonas syringae pv. lachrymans) and cucumber mosaic virus, compared to controls without bacterial treatments. In field trials conducted over two years, protection was observed against anthracnose, angular leafspot, and bacterial wilt. In the case of bacterial wilt, PGPR treatments also significantly reduced populations of the vectors, i.e. spotted and striped cucumber beetles. Some PGPR strains decreased significantly plant accumulation of cucurbitacin, a known feeding arrestant and stimulant for cucumber beetles. In caged studies in the greenhouse, cucumber plants treated with PGPR had significantly less damage from cucumber beetles compared to nontreated plants in the same cage. These results suggest that PGPR which induce systemic resistance may have practical use in integrated pest management strategies for minimizing cucumber disease and insect pests.

#13 Plant Growth Promoting Rhizobacteria Induce Systemic Resistance to Fusarium Fruit Rot in Muskmelon.

Cynthia Eayre, USDA-ARS, SARL, Weslaco TX 78596.

Fusarium fruit rot is the major postharvest problem on muskmelons in south Texas and is also a problem in other areas of the country. The fungus forms an incipient infection during fruit development, the infection becomes active and spreads to the flesh during fruit ripening. Kloepper et al. reported that plant growth promoting rhizobacteria can induce systemic resistance to leaf pathogens in cucumber. Since cucumbers and muskmelons are so closely related, similar resistance may be induced in muskmelon. The purpose of this research was to determine if plant growth promoting rhizobacteria can induce systemic resistance to fusarium fruit rot in muskmelons. Seeds of muskmelon cultivar 'Explorer' were treated with sodium alginate suspensions of 108 cells of plant growth promoting rhizobacteria and planted in flats. Booster treatments were applied at transplant and at start of flowering. Flowers were hand pollinated. At 10 days post anthesis, fruit were challenge inoculated with fus arium infested soil for 10 days. Fruit were harvested at full slip, stored at 8C for 10 days, ambient temperature for 3 days and evaluated for incidence of fusarium fruit rot. Significantly less fruit rot occurred among fruit grown on plants treated with plant growth promoting rhizobacterial strain MN14. Experiments were performed in the greenhouse three times and each included at least six replications.

#14 Broadening the Genetic Base of Cucurbita spp.: Strategies for Evaluation and Incorporation of Germplasm.

Linda Wessel-Beaver Department of Agronomy and Soils Agricultural Experiment Station, College of Agricultural Sciences University of Puerto Rico, Mayaguez, PR 00681.

The need to broaden the genetic base of crops is well recognized but often ignored. This seems to be especially true in many horticultural crops, Cucurbita spp. being no exception. Germplasm collections have been traditionally regarded as sources of simply inherited genes, especially disease resistances, that can be backcrossed into locally adapted breeding lines. Collections are seldom used as sources of quantitatively inherited traits that can contribute to overall performance of the crop. Constraints to testing the performance of these materials include the fact that quantitatively inherited traits are strongly influenced by G x E interactions and that breeding potential can be masked when these stocks carry inappropriate phenotypic traits or extreme disease susceptibility. Genetic resource stocks can be more suitably evaluated if these materials have been sufficiently improved per se. Although the University of Puerto Rico Agricultural Experiment Station has carri ed out pumpkin (C. moschata) breeding activities off and on for 50 years, we have only recently dealt with the issue of how to best evaluate and utilize the resources available in the U.S. National Plant Germplasm System and other sources. The presentation will outline how this genetic variability is being exploited in our population improvement program in pumpkin

#15 Yield Improvement in Cucumber.

Todd C. Wehner, Department of Horticultural Science, North Carolina State University, Raleigh NC, 27695-7609.

In order to improve yield in cucumber (Cucumis sativus L.) most rapidly, it is important to use efficient methods. Performance trials are used to determine yield of selections from breeding programs. The most efficient trials in our studies at NCSU were determined using correlated gain with our main trials, which use 2 seasons, 3 replications, 6 harvests (twice weekly) and large plots (1.5 x 6 m). Efficient trials used 2 or 3 replications of small plots harvested once at green stage (10% oversized fruits). The rapid trials provided 2.6 times the gain of the main trials

Plot shape should be long and narrow to span irregularities in field environment. That shape increases within-plot variance and decreases among-plot variance. Replication (block) shape should be square so as to increase among-block variance and decrease within-block variance. Optimum plot size can be calculated using field variability measurements from uniformity trials, and cost estimates from experienced breeders. Costs are estimated per plot for those that do not depend on plot size, and per unit area for those that increase with plot size. In our studies, the optimum was 1.5 x 1.2 m for once-over harvest of pickles or slicers using hand-pulled or paraquat- defoliated plots. The optimum was 1.5 x 6 m for multiple-harvest of pickles or slicers. All experiments should be surrounded by guard rows on the sides and guard plots on the ends. However, borders are not needed on plot sides or ends except when dwarf and tall are tested in adjacent rows. Thus, single-row plots (with different cultigens in adjacent rows) can be used to save space in a field trial, and spaces can be left at the ends of each plot to make it easier to distinguish them. However, dwarf should be tested separately from tall cultigens in trials. Optimum experiment size was estimated using variances from years, seasons, locations and replications. Most efficiency was obtained using seasons and years, rather than locations and replications. Thus, we use 2 seasons of 1 location and 1 replication in each year of testing for rapid trials. Certain traits are more stable to measure than others, as indicated by the coefficient of variability in a trial. For example, yield in once-over harvest is best measured as fruit number rather than as fruit weight or value per plot. Fruit number remains more constant over time than the other traits since weight increases steadily with time, and value decreases as fruits become oversized. Efficiency can be improved further using subjective ratings rather than slower, direct measurements. Using the methods described above, we have produced significant genetic gain in 6 pickle and 6 slicer populations. Cultivar releases over the last century also indicate genetic gain for yield.

#16 Improvement of Tropical Pumpkin Cucurbita moschata (Lam.) Poir.

Donald N. Maynard, University of Florida, Bradenton, FL 34203; Gary W. Elmstrom, University of Florida, Leesburg, FL 34748; Linda Wessel-Beaver, University of Puerto Rico, Mayaguez, PR 00681.

Tropical pumpkin, known as calabaza, zapallo, or auyama in Spanish-speaking areas and pumpkin in English-speaking areas, is used extensively in the Caribbean and Central America as well as by people of Hispanic and West Indian heritage on the United States mainland. In Puerto Rico, pumpkin is an important vegetable consumed in a variety of traditional dishes and is second only to tomatoes in its contribution to the agricultural income derived from horticultural crops. While its center of origin is Central America or northern South America, and it is used to some extent in these areas, it is most widely consumed in the Caribbean. Mainland production of tropical pumpkin is limited to subtropical areas of Florida although this same species, in its temperate form as butternut squash and field pumpkin, is grown much further north. Florida production does not meet the demands of the growing populations of people of Hispanic and other Caribbean heritages on the mainland. Despite its popu larity, tropical pumpkin is viewed as a traditional food crop, and thus has not benefitted from the same level of research effort dedicated to export crops in the Caribbean. Tropical pumpkin genotypes typically flower and set fruit on long trailing vines over many weeks. Harvests are made every 5 to 7 days for 4 to 8 weeks. While the vigorous vining may initially help to control weeds, foliar diseases and natural senescence of older leaves allow weeds to hinder later harvests. Management of the crop would be easier if an early, concentrated fruit set occurred nearer the crown of the plant. We have already developed some bush and short- vined calabaza inbreds from crosses between 'Bush Butternut' and 'La Segunda' and 'Bush Butternut' and 'La Primera' followed by several generations of selfing and selection for the bush/short vine habit. Tropical vining types including 'La Primera', 'La Segunda', and 'Seminole' from Florida and 'Soler' and 'Linea C Pinta' from Puerto Rico are being used to develop hybrids and improved inbreds that combine larger fruit size, disease resistance, and improved internal flesh color. Preliminary evaluation of these lines has convinced us that the introduction of bush and/or short-vined varieties could improve commercial production of pumpkin in Florida, Puerto Rico and the Caribbean. The current inbreds are not without faults: fruit size, internal color and disease resistance all need improvement, furthermore, the nutritional composition of these lines compared to traditional varieties is not known. We also recognize that cultural practices used for traditional varieties may not be appropriate for the bush/short vine varieties. Commercially produced high quality seed of tropical pumpkin is not available. Growers generally maintain their own seed stocks, usually without knowledge of the importance of isolation for seed purity. Seed companies have expressed little interest in marketing open-pollinated or pure lines since these l ack exclusivity. Development of hybrid tropical pumpkins would provide this exclusivity and profit incentive. Farmers would pay a higher cost but would be assured of a steady supply of commercial seed of good quality. We have developed inbred and hybrid bush and/or short-vined varieties which are being evaluated to determine their nutritional quality and appropriate cultural practices for this plant type.

#17 Problems Associated with Map Construction and the Use of Molecular Markers in Plant Improvement.

Jack E. Staub, USDA-ARS, Horticulture Department, University of Wisconsin, 1575 Linden Drive, Madison WI 53706.

The genetic improvement of a species through artificial selection is dependent upon man's ability to capitalize on genetic differences which he can distinguished from environmental factors (effects). Molecular marker technology provides potential for increasing selection efficiency. In terms of truncation selection, gain from selection (R) is theoretically dependent upon trait heritability (h2) and selection differential (S) or R = h2S. This simple expression of R can most effectively be applied if variability in the selected generation is additive and the environmental effects are negligible. The phenotypic variation that marker loci attempt to define is, in many cases, not completely additive and is affected by environment. Thus, predictions of R are difficult. Map position and linkage of markers to economically important traits (quantitative or qualitative) defines their potential relevance as selection tools. When a significant correlation between markers and a quantitative trait is found, the presence of a quantitative trait loci (QTL) is declared. Single marker and interval mapping of QTLs is discussed using cucumber as a model system (i.e., LOD scores, appropriate mapping thresholds, parental selection, and number of progeny). The effective and efficient application of mapped marker loci is dependent upon the maturation rate of tested families, population structure and size, environmental effects, relative numbers of chromosome regions affecting trait expression, and cost. The utility of molecular markers may be increased if they are tightly linked to an economically important qualitative trait whose selection is difficult. The determinate character (qualitative) in cucumber when incorporated in a multilateral branching background (quantitative) is used as an example.

#18 Genetic Resources of the Cucurbitaceae.

R. W. Robinson, Cornell University, New York Agricultural Experiment Station, Geneva NY.

The genetic resources of the Cucurbitaceae were surveyed by Cucurbit Crop Advisory Committee members in 1987-1988. An update of the present situation will be presented at Cucurbitaceae 94. The CAC reports noted serious problems in the USDA programs to conserve cucurbit germplasm then, including improper pollination procedures in some but not all seed increases, omissions of important germplasm and duplications, and some errors in species identification. The improvements made since then in each of these areas, and some remaining problems, will be discussed at Cucurbitaceae 94. Voluntary curators appointed by the Cucurbit Genetics Cooperative are doing a good job of maintaining seed stocks of single gene mutants of cucumber, melon, squash, and pumpkin, but some other genetic and cytogenetic stocks of cucurbits are not being preserved and are endangered. A number of wild species of the Cucurbitaceae are included in the gene banks of the USDA, botanical gardens, and research institute s throughout the world but seed of many cucurbit species, including some of horticultural importance and others related to major cucurbit crops, are not readily available.

#19 Transformation of Squash Resistant to Viruses.

Hector Quemada, Asgrow Seed/Upjohn Company, Kalamazoo MI.

Squash is extremely susceptible to viral diseases. Traditional breeding in squash has resulted in multiply-resistant varieties which are forthcoming. An alternative complementary approach to traditional breeding is the transformation and regeneration of squash containing genes which confer virus resistance. Using Agrobacterium tumefaciens-mediated transformation, virus-resistant squash lines have been obtained. One particular line, resistant to zucchini yellow mosaic virus (ZYMV) and watermelon mosaic virus 2 (WMV2), is close to commercialization. Other resistances are being developed as well. Transfer of this resistance via traditional breeding to a wide range of varieties is enhanced by the ability to track the genes at the molecular level, and because these genes are expressed as dominant loci.

#20 Organization and Expression of Phloem Protein Genes in Cucurbita.

Gary A. Thompson, Dept. of Plant Sciences, University of Arizona, Tucson AZ 85721.

Phloem is an essential tissue for the long distance transport of photoassimilates from the sites of synthesis or remobilization to the sites of use or storage. Although numerous studies have been undertaken to relate phloem structure with function, many aspects of phloem physiology are still poorly understood. Proteinaceous structures observed in differentiating and mature sieve elements are a major component of the cytoplasmic contents of sieve elements in Cucurbita species. These structures are composed of two very abundant phloem (P-) proteins: PP1 (phloem protein 1), a 96 kDa filament protein, and PP2 (phloem protein 2), a 48 kDa dimeric lectin. To further our understanding of the molecular characteristics of P-protein we have isolated and characterized P-protein cDNAs and genomic clones from Cucurbita maxima. PP2 cDNAs encoded a 218 amino acid polypeptide with chitin-binding activity typical of this phloem-specific lectin. Deletions of the protein coding region and a nalysis of recombinant proteins have defined a unique carbohydrate recognition domain that is unlike other plant chitin- binding lectins. The two genes that encode PP2 in C. maxima (cv Big Max) were 99.8% identical over a region of 3.0 kb and were isolated in a single genomic clone. This is indicative of a recent gene duplication event and the high level of conservation of these genes that also extends to other Cucurbita species. The predicted amino acid sequence from a PP1 cDNA contains repeated regions that are consistent with its proposed structural role. P-protein gene expression is temporally and spatially regulated during early phloem development. Phloem-specific gene expression was demonstrated by in situ hybridization of PP1 and PP2 mRNA in pumpkin hypocotyls and by histochemical localization of ,B-glucuronidase expression directed by the PP2 promoter in transgenic plants. PP1 and PP2 mRNAs are coordinately expressed specifically in the companion cells of p hloem tissue. However, pulse-chase protein labeling studies have indicated that PP1 and PP2 are stable proteins, thus, P-protein synthesis could also occur within immature sieve elements. Ongoing developmental studies show that vascular development is highly correlated with P-protein mRNA accumulation.

#21 Enhancing the Ripening of 'Netted' and 'Honeydew' type Muskmelon Fruit.

Krista C. Shellie. USDA-ARS, Crop Quality and Fruit Insects Research Unit, 2301 S. International Blvd. Weslaco TX 78596.

The different inherent ripening characteristics of 'Netted' (Cucumis melo var. reticulatus Naud.) and 'Honeydew' (Cucumis melo var. inodoris Naud) muskmelons provide an opportunity to genetically manipulate ripening for enhancement of postharvest quality. Both muskmelon types are classified as climacteric, implying that a burst in respiration is coincident with endogenous ethylene production during ripening. However, only 'Netted' types abscise when the fruit is physiologically and horticulturally mature. Also, the postharvest shelflife of 'Netted' melons is shorter than 'Honeydew' melons. Incorporating a harvest indicator, such as abscission, into 'Honeydew' types and factors responsible for longer shelf-life into 'Netted' types would be advantageous for growers and shippers. With this objective in rind, we developed a technique to repeatedly sample gases from the cavity of melons as they ripened on the vine of the plant. The pattern of endogenous ethylene and carbon dio xide production during fruit maturation and ripening indicated that muskmelons do not display a respiratory climacteric until after harvest, that the rate of respiration of 'Honeydew' types is lower than 'Netted' types, and that 'Netted' types produce 10 fold more ethylene than 'Honeydew' types. These differences in ethylene production and respiration indicate potential for regulating ripening to enhance postharvest quality.

#22 Auxin Localization and Metabolism During Fruit Ripening in Cantaloupe.

Jerry D. Cohen1, Nebojsa Ilic1,5, Rosannah Taylor2, James R. Dunlap4 and Janet P. Slovin3. USDA-ARS Horticultural Crops Quality Laboratory1, Soybean and Alfalfa Laboratory2 and Climate Stress Laboratory3, Beltsville, MD 20705-2350, Texas A&M University Agricultural Research and Extension Center4, Weslaco, TX 78596 and Department of Botany5, University of Maryland, College Park, MD 20742 USA.

The roles of phytohormones other than ethylene during fruit ripening and postharvest have not been well established. Based on a variety of studies, several theories have been proposed relative to the involvement of auxin in such processes. These include: 1) that increases in auxin might be involved in induction of ethylene biosynthesis; 2) that low levels of auxin are necessary before tissues become ethylene responsive; and 3) that low levels of auxin initiate ripening and/or senescence events. In order to investigate the role of auxins in fruits, we have developed several approaches to address these events. These include: 1) radiolabeling of tissue sections and a rapid radioimaging technique for evaluation of auxin catabolism; 2) the in vitro cultivation of fruit from immature flowers for use with stable isotope labeling; and 3) tissue printing studies to localize proteins modified by auxin. Results of these studies will be presented and discussed in relation to fruit gro wth and ripening. This work was supported by USDA-NRI grant 91-37304-6655 and DOE grant DE-AI02-94ER20153.

#23 Crop Genetics Cooperatives: The Endless Frontier?

Timothy J. Ng, University of Maryland, College Park, MD 20742-5611

The Cucurbit Genetics Cooperative (CGC), established in 1977, represents one of several crop genetics cooperatives which provide services for researchers interested in the genetics and breeding of specific crops. The primary goal of a crop genetics cooperative is to serve as an international network to facilitate the exchange of information, ideas and genetic materials. This is usually achieved through regular meetings of the Cooperative and related groups (both national and international) and through the publication of an annual report or newsletter. Each Cooperative operates according to its own set of By-Laws and is governed by a Coordinating Committee elected by the membership. Most crop genetics cooperatives are headquartered in the USA and were established when (1) agricultural research programs at land grant institutions and the USDA were large and well-funded, and (2) there were strong ties between industry and public researchers. The recent reductions in the size and emphases of public breeding and genetics research programs, emergence of large national and international private breeding efforts, and development of new technologies for genetic research and information exchange, all pose challenges and opportunities to the continuing evolution of these cooperatives.

#24 The Plant Genome Project - Its Challenges - Where it May Lead Us.

J. P. Miksche; Director, USDA, ARS, NPS. Plant Genome Research Program, Beltsville, MD 20705.

The USDA Plant Genome Research Program will facilitate the improvement of plants - agronomic, horticultural, and forest species through locating important genes and markers on chromosomes, determining the structure of those genes, and transferring the genes to improve performance. The end product will be superior plant varieties that will match marketplace needs and niches for both raw commodities and conversion products. This will be done while reducing the environmental impact.

#25 Issues Involving Genetic Distance and Plant Variety Protection.

Jack E. Staub, USDA/ARS, Horticulture Department, University of Wisconsin, 1575 Linden Drive, Madison WI 53706.

There has been increasing interest and debate over ownership of intellectual property (e.g., plant proprietary rights) in both the private and public agricultural sectors. Such interest has arisen because the protection of research products is necessary to provide incentive for investment. Depending on the type of proprietary protection, a "novel variety" must be distinct (novel, nonobvious), useful, uniform and stable. Regardless of the type of protection, the distinctness criterion is always applied. Distinctness can be measured by plant function and/or genetic markers (e.g., morphological, molecular). A description of difference using molecular markers may be more difficult than a description of plant function. Genetic difference, genetic distance and functional genetic distance are characterized using cucumber lines. Distinctness can be most effectively described if a data base exists for commercial and exotic germplasm. Adequately constructed data bases provide for allelic frequen cies upon which the uniqueness of the discriminating criterion can be defined. It is argued that the number of markers required to assign difference for the distinctness criterion is not absolute, and that this assignment is a function of the probability of occurrence of the marker itself.

#26 Designing Vegetables for Improved Nutrition.

Leonard Pike, Director of Vegetable Improvement Center, Texas A&M University, College Station TX.

#27 Rising World Demand For Cucurbits Should Influence Direction of Cucurbit Research.

Merritt J. Taylor, Extension Economist - Management and Marketing, The Texas A&M University System, Weslaco, Texas.

U.S. per capita consumption of all cucurbits increased in the years between 1970 and 1993. Cucumbers, and honeydews experienced a steady increase with their per capita consumption being doubled. Cantaloupe consumption was more erratic but also doubled in the twenty three years examined. Watermelon consumption was maintained above ten pounds per person with a slight increase in the trend. International trade of cucurbits between the U. S. and other countries more than tripled in the years studied indicating an increased need for uniform products that continue to meet quality standards and expectations of the consumer regardless of the varying conditions confronting the growers and shippers throughout the world. These data emphasize the importance of the research being done on cucurbits. As consumers become more conditioned to consuming cucurbits on a year-round basis the need for an uninterrupted flow of standardized and uniform products from all countries becomes critical to maintaini ng the market share. Thus the increased importance of research on timely topics that influence profitable trade of cucurbit products.

#28 Legal, Scientific and Marketing Issues Affecting Cucurbits.

Al Burkett, Senior Plant Breeder, Petoseed, Woodland Research Station, Woodland CA 95695.

Legal issues associated with seed-borne diseases are currently a primary concern of the industry. In addition to the legal aspects of seed-borne diseases, transgenic plant products and entry into new foreign markets continue receiving attention from the business community. Technology and markets are expanding at a rapid pace. The future success of businesses depends on how well they can capitalize on these developments within a climate of increasing regulation.

#29 Opportunities and Issues of Cucurbit Production from the Producers Perspective.

Bob Peterson, Vice President, Starr Produce Co., Rio Grande City TX.

 

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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
Created by T.C. Wehner and T. Ng, 1 June 2005; design by C.T. Glenn;
send questions to T.C. Wehner; last revised on 23 January, 2008