112 – II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES)
diverse resistance sources, primarily to diseases caused by viruses and fungi. Resistance
to zucchini yellow mosaic virus (ZYMV) and watermelon mosaic virus (WMV), which
C. moschata was reported to display, has been incorporated into cultivars of C. pepo by
crosses with C. moschata (Garzón-Tiznado, Montes-Hernández and Becerra, 1993;
Gilbert-Albertini et al., 1993). Wild species of Cucurbita including C. ecuadorensis and
C. foetidissima have been found to be resistant to a number of viruses (Provvidenti,
1990), and have been used as sources of resistance to these diseases. It is difficult to
hybridise Cucurbita foetidissima with other members of the genus because it is
phylogentically distant from the cultivated species; nevertheless, its virus-resistant alleles
can be introduced into the extended Cucurbita gene pool for use in genetic improvement
of the cultivated species as it is a member of the tertiary gene pool of C. argyrosperma
(see Table 2.6).
In terms of intraspecific crosses being useful in increasing resistance to pathogens and
disease, Lebeda and Widrlechner (2004) published the results of screenings on cultivated
C. pepo, represented by eight groups of morphotypes, for susceptibility or resistance to
the fungi P. cubensis or P. xanthii. The C. pepo morphotypes expressed significant
differences in resistance/susceptibility to P. cubensis or P. xanthii. Generally, there was
an inverse relationship detected in resistance to the two fungi. While zucchini, cocozelle
and vegetable marrow (ssp. pepo) were highly resistant to P. cubensis, they had relatively
high powdery mildew sporulation. Cultivars with the fruit type acorn, straightneck and
ornamental gourd (ssp. ovifera) were quite susceptible to P. cubensis; however, they were
considered resistant to P. xanthii in laboratory and field evaluations (Lebeda and
Křístová, 2000).
Interspecific hybrids have been made to incorporate the gene responsible for the
“bush” phenotype of C. pepo into C. moschata and C. argyrosperma, species that are in
the secondary gene pool of C. pepo, providing these species with the characteristics of a
compact plant (Robinson and Decker-Walters, 1997). Bush plants have a more uniform
growth and better response to high-density planting compared to vine plants (Loy and
Broderick, 1990).
Hybridisation and introgression in the field
The amount and frequency of gene flow between a cultivated plant and its closest
wild relatives are affected by several factors, e.g. the existing mating system, similarities
in flowering phenology, ease in which the gametes can move and overlapping
ecogeographic distribution. Several authors, including Decker (1986) and
Decker-Walters et al. (1990), have presented genetic evidence for introgression in the
field among various Cucurbita.
As noted earlier in this chapter, the Cucurbita with limited exception are monoecious,
plants may produce flowers over much of their maturity, and the species are insect
pollinated. Kirkpatrick and Wilson (1988) examined the potential for gene flow between
cultivated Cucurbita pepo and its wild relative C. pepo var. texana by monitoring flower
patterns and gene flow among experimental populations. While flowering patterns and
pollinator movements tended to maximise self-pollination and local gene exchange,
movement of effective pollen was detected up to a distance of 1 300 metres.
Hybridisation rates of 5% have been reported (see also Montes, 2002). Spencer and Snow
(2001) compared the fitness component of wild Cucurbita pepo from Arkansas
(United States) with C. pepo wild-crop hybrids. Their results suggest that the F1
generation of the wild-crop cross does not present a strong barrier to introgression of crop
