HORTSCIENCE 34(4):715–717. 1999. Materials and Methods Experiments were conducted for 4 years Parthenocarpy in Summer Squash on a Lima silt loam (fine-loamy, mixed, mesic, Glossoboric Hapludalf) at Geneva, N.Y. Four- 1 2 R.W. Robinson and Stephen Reiners week-old transplants were set in the field and Department of Horticultural Sciences, New York State Agricultural Experiment spaced at 0.9 m in rows on 1.5-m centers. Station, Geneva, NY 14456-0462 Weeds were controlled using recommended herbicides, cultivation, and hand weeding, Additional index words. Cucurbita pepo, fruit set, seedless fruit, row covers, pollination while insect and disease pressure was moni- tored and protective treatments applied when Abstract. Summer squash (Cucurbita pepo L.) cultivars were compared for ability to set warranted (Becker, 1992). parthenocarpic fruit. Some cultivars set no parthenocarpic fruit and others varied in the Over 95% of the first pistillate flowers to amount of fruit set when not pollinated. The degree of parthenocarpy varied with season, develop were closed before anthesis by plac- but the relative ranking of cultivars for parthenocarpy was generally similar. Cultivars ing plastic-covered wire around the unopened with the best parthenocarpic fruit set were of the dark green, zucchini type, but some petals to prevent insect pollination. Any open cultivars of other fruit types also set parthenocarpic fruit. A summer squash cultivar was flowers were removed within 2 d of anthesis to developed that combines a high rate of natural parthenocarpy with multiple disease prevent development of open-pollinated fruit. resistance. Yield of summer squash plants grown under row covers that excluded Fruit from closed flowers were harvested im- pollinating insects was as much as 83% of that of insect-pollinated plants in the open. mature, at the marketable stage for summer squash. Data were taken on percent partheno- Parthenocarpic fruit development has long tion were unnecessary, row covers could be carpy, i.e., the proportion of flowers that de- been recognized as an important characteristic left intact longer to improve growth and early veloped into mature, marketable fruit. Fruit for greenhouse cucumbers (Cucumis sativus yield of squash and reduce losses from insects produced from closed pistillate flowers of L.) (Sturtevant, 1890). European greenhouse and insect-transmitted viral diseases. additional plants were allowed to develop to cultivars were selected in the 19th century for Squash cultivars differ in sex expression, maturity to determine if they were seedless. high yield, often without realizing that the and fruit set may be poor early in the season In 1992, a total of 307 pistillate flowers of basis for this productivity was their ability to when prices are often highest because the low 33 cultivars and breeding lines of summer and set parthenocarpic fruit when bees and other temperature and high light intensity promote winter squash were closed to exclude pollinat- pollinating insects were absent. Most green- female sex expression (Wien, 1997). Partheno- ing insects. Twenty-six of the same cultivars house cultivars of slicing cucumbers grown carpy could permit the use of highly female and lines were grown in 1993, and a total of today can set parthenocarpic fruit, and par- cultivars early in the season without concern 672 pistillate flowers were tested for par- thenocarpic pickling cultivars are of major for pollination. thenocarpic fruit set. The cultivars and lines importance in Europe (Tatlioglu, 1992). Early studies indicated that parthenocarpy were classified according to fruit shape, color, Parthenocarpy of squash has received less of Cucurbita pepo did not occur, or was mani- and usage (summer or winter squash) to deter- attention. Parthenocarpy can be induced in fested too late in the season to be of value for mine if there was any relationship between Cucurbita by growth regulators (Gustafson early production. Durham (1925) closed 301 parthenocarpy and fruit type. 1941; Mori, 1947; Takashima and Hatta, 1955) pistillate flowers of Cucurbita pepo plants to Nine cultivars of summer squash that dif- or pollen extracts (Gustafson, 1937), but this is prevent insect pollination, but none set fruit. fered in parthenocarpic tendency in the 1992– not a commercial practice in the United States. Nitsch (1952) determined that ‘Acorn’ winter 93 trials were selected for further testing in The most practical means of increasing fruit squash would produce parthenocarpic fruit, 1994, as well as another cultivar, ´Zucchini set of squash when pollination is inadequate but only after producing many pistillate flow- Elite’. A total of 308 pistillate flowers were would be the use of cultivars with an innate ers that did not set fruit. ‘Royal Acorn’ did not closed to determine if they would set fruit ability to set parthenocarpic fruit. Partheno- set fruit in our tests, which were terminated without pollination. carpy may enable squash to be grown in green- before the parthenocarpic stage of develop- The same 10 commercial cultivars were houses and in the field out of season, when ment described by Nitsch (1952). grown under row covers in 1994. Floating staminate flowers or pollinating insects may Early set of parthenocarpic fruit in zuc- spunbonded polypropylene row covers (20 be absent. Populations of bees have been chini ranged from 0% to 42%, depending on g.m–2; Kimberly Farms, Roswell, Ga.) were sharply reduced in many areas of the United variety (Nijs and Zanten, 1982). Rylski (1974a) suspended over the plants 10 d after trans- States recently because of mite infestation found that ‘Zucchini Elite’ set more partheno- planting and supported by wire hoops spaced (Buchman and Nabhan, 1996), and this has carpic fruit than ‘Bushy White’, and Nijs and over every other plant. The row covers ex- adversely affected pollination of cucurbits and Balder (1983) reported differences among cluded bees and other pollinating insects. The other crops. In addition, the use of plastic cultivars ranging from 17% set for ‘Elite’ to cultivars were also grown in the open to com- tunnels or other row coverings for summer 87% for ‘DG 4’. Om and Hong (1989) re- pare parthenocarpic fruit yield under row cov- squash is increasing, but these exclude bees ported that ‘Zucchini’ and ‘Caserta’ were ers with insect pollinated fruit production. and must be removed early to allow for insect among the best of the 64 cultivars they tested Fruit at the marketable stage were harvested pollination (Wells and Loy, 1985). If pollina- for fruit set in an insect-proof greenhouse. every 3 to 4 d for 3 weeks. Our preliminary research (Robinson, 1993) In 1996, five summer squash cultivars re- indicated that considerable differences existed sistant to zucchini yellow mosaic virus among summer squash cultivars for partheno- (ZYMV) were compared with three suscep- carpic fruit development. The purpose of this tible cultivars by closing a total of 314 pistil- research was to: 1) further evaluate cultivars late flowers to exclude pollinators. Three and breeding lines of summer squash for par- sources of ZYMV resistance were included: Received for publication 5 Jan. 1998. Accepted for thenocarpy and determine if any disease-resis- the genetically engineered ‘Freedom II’ and publication 22 Oct. 1998. The cost of publishing this tant cultivar had good parthenocarpic fruit set; ‘Prelude II’ cultivars; ‘Tigress’ and ‘Jaguar’, paper was defrayed in part by the payment of page 2) evaluate seasonal variation in partheno- which have ZYMV resistance derived from charges. Under postal regulations, this paper there- fore must be hereby marked advertisement solely to carpy; 3) investigate any association between Cucurbita moschata (Duch. ex Lam.) ex Poir.; indicate this fact. parthenocarpy and fruit type; and 4) determine and ‘Whitaker’, with ZYMV resistance from 1Professor, to whom reprint requests should be ad- if the ability to set fruit without pollination Cucurbita ecuadorensis Cutler and Whitaker. dressed. would be of practical value for summer squash All trials included three replications of six 2Assistant Professor. grown under row covers. plants for each cultivar/treatment. A random- HORTSCIENCE, VOL. 34(4), JULY 1999 715 BREEDING, CULTIVARS, ROOTSTOCKS, & GERMPLASM RESOURCES ized complete-block design was used for all Table 1. Total number female flowers closed and mean of parthenocarpic fruit set of squash cultivars and experiments involving flower closure. The lines, 1992–94. row cover study was a split-plot with cultivar Total no. Mean parthenocarpic as the main plot and row cover as the subplot. female flowers closed fruit set (% ± SE) Plants in all trials were spaced at 0.9 m in-row Cultivar or line Typez 1992 1993 1994 1992 1993 1994 with 1.5 m between rows. Data were tested for Chefini Hybrid Z 11 36 33 82 ± 10 19 ± 3 73 ± 8 significance using analysis of variance and Gold Strike YSN 8 33 50 75 ± 10 0 8 ± 5 means compared using the protected least sig- Black Beauty Z 7 20 27 71 ± 22 75 ± 3 63 ± 10 nificant difference (LSD). Black Magic Z 9 27 29 67 ± 12 44 ± 7 23 ± 12 NY-82-138 Z 2 --- --- 50 ± 33 --- --- ± Results NY-92-728 Z 7 --- --- 43 6 --- --- Green Magic Z 12 28 --- 42 ± 5 15 ± 6 --- NY-82-141 Z 8 --- --- 38 ± 6 --- --- Appearance of fruit from nonpollinated ± ± flowers was similar to that of fruit from polli- Gold Slice YSN 6 28 --- 33 17 4 3 --- Cocozelle SSN 7 27 --- 29 ± 16 15 ± 4 --- nated flowers; there were no significant differ- Goldie Hybrid YCN 17 32 --- 29 ± 5 16 ± 3 --- ences in fruit shape or defects that would affect President Z 14 31 --- 29 ± 15 6 ± 3 --- marketability. Ovaries of <1% of the closed Black Jack Z 16 26 42 25 ± 4 23 ± 2 40 ± 3 flowers enlarged somewhat after anthesis but Gold Rush PYSN 10 31 --- 20 ± 10 13 ± 4 --- aborted before reaching marketable stage.