Cucurbitaceae 94: Evaluation and Enchancement of Cucurbit
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
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
#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
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
#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,
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.)
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
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
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
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
#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
Bob Peterson, Vice President, Starr Produce Co., Rio Grande City