Hort Series #249

Principal Investigators: Jonathan R. Schultheis, Professor and Extension Specialist, Department of Horticultural Science, NC State University; Stuart W. Michel, Research Technician, Department of Horticultural Science, NC State University; Brandon K. Parker, Research Associate, Department of Horticultural Science, NC State University; Baker E. Stickley, Research Assistant, Department of Horticultural Science, NC State University.

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General Cultural Practices

This zucchini squash study was grown on black plastic mulch and fertigated using drip tube. Pesticides used on all plots were chemicals labeled for that crop.

Acknowledgements

We gratefully acknowledge the assistance of Keith Starke (Superintendent), and Faye Weldon (Horticulture Supervisor), Central Crops Research Station, Clayton, NC, as well as the personnel at the research station including Josh Brady, Andre Schwed, and Michael King, for their help in establishing, maintaining, and harvesting the zucchini squash cultigen evaluation study. We want to acknowledge summer employees Cameron Roberts, Lia Hunt, Daphne Meyer, and Nathaniel Wyschka for their assistance with the study. The cooperation and support of Syngenta, Bejo, Rijk Zwaan, Seminis, and Clifton are also appreciated. We also want to thank Joy Smith for performing the statistical analysis and aiding in the interpretation of the data collected from this study.

Disclaimer

This publication presents data from the zucchini squash cultigen evaluation study conducted during 2024. Information in this report is believed to be reliable but should not be relied upon as a sole source of information. Limited accompanying detail is included, but excludes some pertinent information, which may aid interpretation.

Financial Support

In addition to seed companies, this research was supported by the College of Life and Agricultural Sciences, North Carolina Agriculture Research, the North Carolina Cooperative Extension Services, and the North Carolina Department of Agriculture and Consumer Services.

Introduction

Summer squash production in North Carolina totaled 3,500 acres harvested in 2023, a value of approximately $13 million. Acreage harvested has increased slightly from 3,200 in 2022, but the price per unit (cwt) has decreased from $37.50/cwt in 2022 to $33/cwt in 2023.

In 2024, Dr. Schultheis and the Cultural Management Program in the Horticultural Sciences Department at NC State University conducted an evaluation of 12 zucchini squash cultigens from 5 seed companies at the Central Crops Research Station in Clayton, NC. The zucchini entries were evaluated for yield and quality characteristics such as color, speckling, and shape. The methods and results from the study are presented below.

Materials and Methods

Sowing and Field Preparations

Seeds were sown on 9 May 2024. Hills with seed skips were replanted 7 days after the initial planting to maximize plant stand counts in each plot. Final stand counts were taken 16 May 2024, and hills were thinned to one plant on 30 May 2024.

The field planting site was located at the Central Crops Research Station in Clayton, NC. The zucchini squash were planted in block G4, a Norfolk loamy sand soil type, and the zucchini were grown on black polyethylene plastic mulch (1.25 mil thick, high density plastic film, 54 inches wide; TriEst Ag Group, Inc., Clinton, NC).

Fertilizer and Pest Management

A total of 400 lb/ac of 12-6-24 was applied broadcast (pre-plant) to the entire study area on 22 April 2024. Drip tape (NETAFIM, 12 in spacing, 0.24 gph; NETAFIM, Tel Aviv, Israel) was installed beneath the plastic mulch to irrigate the crop throughout the growing season.

Insecticides (Table 1) and fungicides (Table 2) were applied as needed and as directed by the label for that crop (NC Ag Chem Manual). Different products were rotated to avoid potential development of resistance.

Weather

Temperatures were generally favorable until mid-June. After 20 June, high temperatures were consistently above 90°F for the next two weeks (Figure 1). High temperatures beginning in mid-June may have contributed to the increased number of culls in this study due to limited male flower production and thereby reduced pollination. There were some periods of heavy rainfall accompanied by strong winds that resulted in some plants breaking at the base. Approximately 8 inches of rain occurred at the field planting site in Clayton, NC during the growing season.

Study Design and Data Collection

The study used a randomized complete block design with four replications. Harvests were conducted three times per week, beginning on 17 June 2024 and the final harvest occurring on 15 July 2024 for a total of 13 harvests. Fruits were picked when the blossom was detached from the fruit and the fruit reached a sufficient length (4.5-5 inches). Fruits were then categorized as marketable 1s, marketable 2s, or non-marketable: Marketable 1s are characterized as well-formed, free from decay or damage, fairly young and tender, and have the stem attached (USDA); Marketable 2s are slightly off in shape or color but are still marketable; Fruit that were small or apparently not pollinated, or were misshapen, were categorized as culls (non-marketable). Graded fruit were weighed and counted for each category and plot. In addition to yield, other quality measurements were taken such as the average length and width of the fruit and the spine ratings of the plant.

Results and Discussion

Culls were significantly higher in this year’s study compared to previous years. We surmise that high temperatures inhibited male flowering, resulting in poor pollination and misshapen fruit.

‘Zucchini Elite’, an older zucchini cultigen with no virus resistance, and ‘Respect’, a popular cultigen among North Carolina zucchini squash growers, were entered as check cultigens from HM Clause/Clifton Seeds.

Cumulative Harvests (1-13)

Cumulative Yield: 20 lb boxes/acre and percent by grade

The entry with the highest total marketable yield in 20 lb boxes per acre (boxes/ac) across all 13 harvests was ‘EXP 3640’ from Bejo at 1174 boxes/ac (Table 3). ‘Blade Runner’ from Syngenta and ‘EXP 3595’ from Bejo ranked second and third at 1085 boxes/ac and 1020 boxes/ac, respectively. The average total marketable yield across all 12 entries was 930 boxes/ac. ‘HP-30’ from Bejo, ‘Fortress’ from Syngenta, and ‘Respect’ from HM Clause all yielded above the average. ‘EXP 3640’ was statistically different from ‘HP-28’ from Bejo and ‘Zucchini Elite’ from HM Clause for total marketable yield in 20 lb boxes/ac (Table 3).

Culls greatly impacted the results of the study. Across all 12 entries, the average percentage of harvested fruit in the cull category was 44% (Table 3). ‘Zucchini Elite’, an older check cultigen, had 58% of harvested fruit fall into the cull category. The cultigen with the least percentage of culls was ‘EXP 3640’ at 32% (Table 3). 34% of ‘SVYG6198’ fruit harvested were in the marketable 1 category, the highest percentage of marketable 1s out of all 12 entries in the study, despite ranking 10^th for total marketable yield due to an equally high percentage of culls.

Cumulative Yield: Number of fruit per plant by grade

The cultigen with the highest number of marketable fruit per plant across all 13 harvests was ‘EXP 3640’ from Bejo at 9.0 marketable fruit/plant (Table 4). ‘EXP 3595’ and ‘HP-30’, also from Bejo, ranked second and third at 8.8 fruit/plant and 8.3 fruit/plant, respectively. The average number of marketable fruit per plant across all 12 entries was 7.4 fruit/plant. ‘Blade Runner’ from Syngenta and ‘Respect’ from HM Clause also yielded above the average. Culls were significant, with four cultigens yielding greater than 10 culls per plant and the average number of culls per plant across all entries at 9.2 culls/plant (Table 4).

Early Harvests (1-4): June 17, 19, 21, and 24

Early Yield: 20 lb boxes/acre

For the early harvests (1-4), ‘EXP 3595’ from Bejo had the greatest total marketable yield in 20 lb boxes per acre at 535 boxes/ac (Table 5). ‘HP-30’ and ‘EXP 3640’, also from Bejo, ranked second and third at 518 boxes/ac and 488 boxes/ac, respectively. The average number of marketable boxes per acre across all 12 entries in the early harvests was 429 boxes/ac. ‘Kefren’ from Rijk Zwaan, ‘Fortress’ and ‘Blade Runner’ from Syngenta, and ‘Respect’ from HM Clause all yielded above the average for the early harvests. The average number of 20 lb boxes per acre in the cull category for the early harvests was 302 boxes/ac.

Early Yield: Number of fruit per plant

The entry with the highest number of marketable fruit per plant in the early harvests (1-4) was ‘EXP 3595’ from Bejo at 4.2 fruit/plant (Table 6). ‘EXP 3640’ and ‘HP-30’, also from Bejo, both ranked second at 3.7 fruit/plant. The average number of marketable fruit per plant in the early harvests was 3.2 fruit/plant. ‘Fortress’ from Syngenta and ‘Respect’ from HM Clause both yielded above the average for marketable fruit per plant in the early harvests. Even when temperatures were more favorable over some of the first four harvests, on average across cultigens, cull number was greater (3.7) versus marketable (3.2) number per plant (Table 6).

Mid-season Harvests (5-9): June 26 and 28; July 1, 3, and 5

Mid-season Yield: 20 lb boxes/acre

For the mid-season harvests (5-9), ‘HP-29’ from Bejo had the greatest total marketable yield in 20 lb boxes per acre at 484 boxes/ac (Table 7). ‘EXP 3640’ from Bejo and ‘Blade Runner’ from Syngenta ranked second and third at 458 boxes/ac and 418 boxes/ac, respectively. The average number of marketable 20 lb boxes per acre across all 12 entries in the mid-season harvests was 350 boxes/ac. ‘HP-30’, ‘EXP 3641’, and ‘EXP 3595’ from Bejo all yielded above the average for the mid-season harvests. Marketable yields for the mid-season harvests were 79 boxes/ac less than the early season harvests (Tables 5 and 7). The average number of 20 lb boxes per acre in the cull category for the mid-season harvests was 191 boxes/ac (Table 7).

Mid-season Yield: Number of fruit per plant

The entry with the greatest number of marketable fruit per plant in the mid-season harvests (5-9) was ‘HP-29’ from Bejo at 4.0 fruit/plant (Table 8). ‘HP-30’ and ‘EXP 3640’, also from Bejo, ranked second and third at 3.8 fruit/plant and 3.6 fruit/plant, respectively. The average number of marketable fruit per plant in the mid-season harvests was 3.1 fruit/plant. ‘Blade Runner’ from Syngenta and ‘EXP 3595’ and ‘EXP 3641’ from Bejo all yielded above the average for marketable fruit per plant in the mid-season harvests.

Late Harvests (10-13): 8, 10, 12, and 15 July

Late Yield: 20 lb boxes/acre

For the late harvests (10-13), ‘EXP 3640’ from Bejo had the greatest total marketable yield in 20 lb boxes per acre at 228 boxes/ac (Table 9). ‘Blade Runner’ and ‘Fortress’ from Syngenta ranked second and third at 223 boxes/ac and 212 boxes/ac, respectively. The average number of marketable 20 lb boxes per acre across all 12 entries in the late harvests was 152 boxes/ac. ‘Respect’ and ‘Zucchini Elite’ from HM Clause and ‘HP-28’ from Bejo all yielded above the average for the late harvests. The average number of 20 lb boxes per acre in the cull category for the late harvests was 241 boxes/ac. Quality fruits (marketable no. 1) and overall marketable yields were low due to limited male flower production from very high temperatures.

Late Yield: Number of fruit per plant

The entry with the greatest number of marketable fruit per plant in the late harvests (10-13) was ‘EXP 3640’ from Bejo at 1.7 fruit/plant (Table 10). ‘Blade Runner’ from Syngenta and ‘Respect’ from HM Clause both ranked second at 1.5 fruit/plant. The average number of marketable fruit per plant in the late harvests was 1.2 fruit/plant. ‘EXP 3595’ and ‘HP-28’ from Bejo also yielded greater than the average for marketable fruit per plant in the late harvests.

Percent by Grade and Harvest

On average, greatest marketable yields were achieved in the mid-season harvests (5-9), as it is the only harvest period in which there are a greater average percentage of marketable fruit (58%) compared to culls (42%) (Table 11). Normally, other quality data would be collected as well as photographs of each entry. However, numbers of high quality fruit were limited, precluding collection of these data.