In recent decades, F1 hybrids in the genus Cucurbita have increasingly gained prominence over open-pollinated varieties, particularly in summer squash, ornamental pumpkin and acorn cultivars of C. pepo L. and kabocha cultivars of C. maxima (Loy, 2012; Robinson, 2000). By contrast, open-pollinated cultivars such as GD (C. maxima) and strains of DF (C. moschata) still dominate the production of squash or pumpkin used for pie processing (Loy, 2004). Interspecific C. maxima x C. moschata F1 hybrids may offer unique combinations of traits attractive to growers and processors. Castetter (1930) and Erwin and Haber (1929) reported that certain parental combinations of C. maxima x C. moschata set fruit and produced viable F1 seed, and moreover, F1 plants often produced normal looking fruit with the aid of a pollen donor strain. Interspecific hybrid plants have been described as vigorous and highly gynoecious, with leaf shape intermediate to parental lines, vines resembling the C. moschata parent, and fruit with the general appearance of the C. maxima parent (Castetter, 1930; Erwin and Haber, 1929; Robinson et al., 1978; Whitaker and Bohn, 1950). Interspecific hybrids are largely seedless and therefore may exhibit high pericarp yields because assimilates do not have to be allocated to seed development. Because the root systems of interspecific hybrids are highly resistant to abiotic and biotic stress, they have been used widely as rootstocks for melon and watermelon production (Davis et al., 2008). They express insect tolerance of the C. moschata parents, showing resistance to squash vine borers [Melittia cucurbitae (Harris)], and are less attractive to squash bugs [Anasa tristis (DeGeer)] than cultigens of C. maxima. In addition, we have observed that the interspecific hybrids exhibit intermediate resistance to powdery mildew disease [Podosphaera xanthii (Castagna) U. Braun and N. Shirshkoff].
At the University of New Hampshire, bush breeding lines of C. maxima squash with large, orange fruit, similar to GD, have been developed for use in hybrids for pie or baby food processing. We considered these breeding lines to be good candidate parents for producing C. maxima x C. moschata interspecific hybrids exhibiting a favorable semibush growth habit for rapid leaf canopy cover and high yield potential (Loy, 2004; Loy and Broderick, 1990). In terms of fruit set, seed yield, seed fill and germinability, initial testing indicated that two of four bush C. maxima lines, NH245 and NH65, exhibited good compatibility when crossed to C. moschata cultigens (Uretsky, 2012). For C. moschata parents we relied on two established processing cultivars LIC and DF. We also evaluated the open-pollinated cultivar Waltham Butternut (WBN) as a male parent, but in contrast to LIC and DF, interspecific hybrids from this parent exhibited precocious appearance of pistillate flowers (Loy and Uretsky, 2010), resulting in production of numerous fruit near the base of plants.
To more thoroughly evaluate the yield potential of interspecific hybrids, we conducted greenhouse and field research to compare flowering patterns and traits affecting leaf canopy development in two interspecific hybrids, NH65xLIC and NH65xDF, and three open-pollinated processing cultivars DF and LIC (C. moschata), and GD (C. maxima).
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