Hort Series #245

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; Andrew P. Pfefferkorn, Research Technician Support, Department of Horticultural Science, NC State University.

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General Cultural Practices

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

Acknowledgments

We gratefully acknowledge the assistance of Keith Starke (Superintendent), and Eric Linder (Horticulture Supervisor), Central Crops Research Station, Clayton, NC, as well as the personnel at the research station including Faye Weldon, 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 Lia Hunt and Daphne Meyer for their assistance with the study. The cooperation and support of Bejo, Enza Zaden, Rijk Zwaan, Seedway, Seminis, Syngenta, and US Agriseeds 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 2023. 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.

Introduction

Summer squash production in North Carolina totaled 3,200 acres harvested in 2022, a value of approximately $15 million. Acreage has decreased from 3,500 acres in 2021 and 3,600 acres in 2020. However, the price per unit (cwt) has increased from $21.50/cwt in 2021 to $37.50/cwt in 2022 (NC Ag Stats).

Late season summer squash plantings in North Carolina typically experience higher incidence of disease and insect pressure since they are harvested in the fall, when environmental factors are more favorable for pests and pathogens. Virus pressure was an important factor in this study and increased throughout the season, having a large impact on some zucchini cultigens.

In 2023, Dr. Schultheis and the Cultural Management Program in the Horticultural Sciences Department at NC State University conducted an evaluation of 19 zucchini squash cultigens from 8 seed companies at the Central Crops Research Station in Clayton, NC. The zucchini entries were evaluated for yield, virus incidence, and quality characteristics such as color, speckling, and shape. The results from the study, methods, and representative photos are present below. All cultigen photographs were taken on 15 September 2023 at the sixth harvest (Figures 2-20).

Materials and Methods

Sowing and Field Preparations

Seeds were sown on 17 July 2023. 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 15 September 2023.

The field planting site was located at the Central Crops Research Station in Clayton, NC. The zucchini squash were planted in block G5; a Norfolk loamy sand soil type. Black polyethylene plastic (1.25 mil thick, high density plastic film, 54 inches wide; TriEast Ag Group, Inc., Clinton, NC) was laid out in the field on 30 March 2023. The black plastic was painted white on 12 July 2023 to reduce the temperature beneath the plastic and allow for better germination and growth.

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 24 March 2023. Drip tape (NETAFIM, 12 in spacing, 0.24 gph; NETAFIM, Tel Aviv, Israel) was installed beneath the plastic mulch to fertigate the crop throughout the growing season. Liquid fertilizer with 4-0-8 analysis was applied through drip tape fertigation 23 times on the following dates: 2, 9, 14, 16, 18, 21, 23, 25, 28, 30 August; 1, 6, 13, 18, 20, 22, 25, 27, 29 September; 2, 4 October. A total of 100.5 gal of 4-0-8 liquid fertilizer was applied throughout the growing season; 165 units of N and 330 units of K. The zucchini plants were not responding to the fertigation initially. We theorized that the pre-plant fertilizer leached out of the soil due to heavy rains (Figure 1a) in the time between applying the fertilizer and planting the zucchini and that the liquid fertilizer applied after planting was insufficient. Thus, we increased the fertilizer rate starting 14 August and the plants responded with expected growth, reaching their appropriate size before harvests began.

Fungicides (Table 1) and insecticides (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

Weather was generally favorable for the duration of the study (Figure 1a and 1b). Temperatures at planting and during growth periods were hot, averaging around 80°F, and decreased steadily through harvests in September. 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 (Figure 1b).

Study Design and Data Collection

The study used a randomized complete block design with 4 replications. Harvests were conducted three times per week, beginning on 5 September 2023 and the final harvest occurring on 6 October 2023 for a total of 15 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 well-formed, free from decay or damage, fairly young and tender, and has stem attached (USDA); Marketable 2s are slightly off in shape or color but still marketable; Fruit that were small or apparently not pollinated, or were misshapen, were categorized as culls (non-marketable). Virus-symptomatic fruit were categorized separately to assess the progression of virus pressure among the 19 cultigens throughout the growing season. Leaves with virus symptoms were collected by Syngenta and Seminis. Both companies found papaya ring spot to be the only virus present. Graded fruit were weighed and counted for each category and plot. In addition to yield, other quality measurements were taken such as average length and width, plant canopy, fruit color and speckling, and spine ratings of the plant. After the final harvest, the zucchini plants were rated for powdery mildew susceptibility.

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. This work was in part supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2020-51181-32139 (CucCAP).

Results and Discussion

Cumulative Harvests (1-15)

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

The entry with the highest total marketable yield in 20 lb boxes per acre across all 15 harvests was ‘MG0477’ from Syngenta at 1,669 boxes/ac (Table 3). ‘EXP 3595’ from Bejo and ‘Blade Runner’ from Syngenta ranked second and third at 1,467 boxes/ac and 1,283 boxes/ac, respectively. The average yield in 20 lb boxes per acre across all 19 entries was 826 boxes/ac. ‘SV0782’ and ‘SV0813’ from Seminis and ‘EXP 3641’ from Bejo all yielded well above the average. Virus symptoms on zucchini leaves were readily apparent prior to the first harvest on certain cultigens. Large amounts of virus-symptomatic, and therefore unmarketable, fruit in some cultivars reduced the overall average number of marketable 20 lb boxes per acre. For example, the cultivar ‘Zucchini Elite’ was entered as a check and had no virus resistance. It yielded 894 boxes/ac of virus-symptomatic fruit and only 51 boxes/ac of marketable fruit.

86% of ‘MG0477’ fruit harvested were in the marketable 1 category (Table 3), the highest percentage of marketable 1s out of all 19 entries. Some entries such as ‘Renegade’ from Enza Zaden and ‘SV6198’ from Seminis had high percentages of marketable fruit and low virus incidence, (fourth and fifth for percentage of marketable 1s, respectively), but had lower yields overall (eighth and tenth for total marketable yield). ‘EXP 3595’ had high yields and the lowest virus incidence across all entries and harvests, with 2% of fruit having virus symptoms. ‘SV5432’ from Seminis had the highest incidence of virus, with 99% of fruit across all harvests having virus symptoms.

Cumulative Yield: Number of fruit per plant by grade

The entry with the greatest number of marketable fruit per plant across all 15 harvests was ‘MG0477’ from Syngenta with 15.1 no/plant (Table 4). ‘EXP 3595’ from Bejo and ‘Blade Runner’ from Syngenta ranked second and third for total marketable fruit per plant with 14.4 no/plant and 13.4 no/plant, respectively. The average number of marketable fruit per plant across all 19 cultigens was 8.9 per plant. More than half of the 19 cultigens yielded above this average, which was greatly reduced due to the large number of virus-symptomatic fruit in some cultigens.

Quality: Fruit quality

Zucchini fruit were evaluated for quality characteristics such as color, speckling, and average length and width. The average color rating across all 19 cultigens was 3.5 on a 1-5 scale, with 1 = pale green and 5 = dark green (Table 5). The greatest difference from this average was ‘SV6198’ from Seminis, which had a color rating of 4.5. The average speckling rating across all 19 cultigens was 2.0 on a scale of 1-5, with 1 = no speckling and 5 = heavy speckling. The most speckled cultigens were ‘Leopard’ from Clifton (HM Clause) at 3.6 and ‘EXP 3595’ from Bejo at 3.3. The average LD ratio across all 19 cultigens was 3.8.

Quality: Plant quality

Zucchini plants were evaluated for quality characteristics such as spininess, canopy, and powdery mildew severity. The average spininess rating across all 19 cultigens was 4.3 on a 1-10 scale, with 1 = no spines and 10 = many coarse spines (Table 6). The entry with the highest spininess rating was ‘Zucchini Elite’ from Clifton (HM Clause) at 7.8, and the entry with the lowest spininess rating was ‘Legend’ from US AgriSeeds at 1.0. The average canopy rating across all 19 entries was 4.2 on a 1-10 scale, with 1 = open and 10 = dense. The entries with the most open canopies were ‘SV6198’ from Seminis at 1.0 and ‘Renegade’ from Enza Zaden at 2.1.

Plants were rated for powdery mildew severity on 11 October 2023, following the last harvests. The average powdery mildew rating across all 19 cultigens was 5.2 on a scale of 1-9, with 1 = none and 9 = very high. The entries with the lowest powdery mildew ratings were ‘Outlaw’ from Enza Zaden and ‘SV5432’ from Seminis at 2.5 for both entries.

Early Harvests (1-5): September 5, 6, 8, 11, 13

Yield: 20 lb boxes/acre for the early harvests

For the early harvests (1-5), ‘SV0782’ from Seminis had the highest total marketable yield in 20 lb boxes per ac at 489 boxes/ac (Table 7). ‘MG0477’ from Syngenta and ‘EXP 3595’ from Bejo ranked second and third at 480 boxes/ac and 428 boxes/ac, respectively, for the early harvests. The average total marketable yield for the early harvests was 296 boxes/ac.

Ten cultigens yielded virus-symptomatic fruit for the early harvests. The average yield of symptomatic fruit across those 10 cultigens was 72 boxes/ac (Table 7). Four cultigens yielded above the average number of boxes of virus-symptomatic fruit per acre: ‘Zucchini Elite’ from Clifton (HM Clause), ‘SV5432’ and ‘SV5434’ from Seminis, and ‘Legend’ from US AgriSeeds.

Yield: Number of fruit per plant for the early harvests

The entries with the greatest number of marketable fruit per plant for the early harvests (1-5) were ‘MG0477’ from Syngenta and ‘SV0782’ from Seminis at 5.5 no/plant for both entries (Table 8). ‘EXP 3595’ from Bejo and ‘Kefren’ from Rijk Zwaan ranked second and third for total marketable yield per plant in the early harvests at 5.2 no/plant and 5.1 no/plant, respectively. The average number of marketable fruit per plant for the early harvests was 3.7 no/plant.

While half of the entries had some incidence of virus-symptomatic fruit for the early harvests, it had little impact on the number of marketable fruit per plant for most cultigens. Only four cultigens had greater than 1.0 virus-symptomatic fruit per plant for the early harvests (Table 8).

Mid-season Harvests (6-10): September 15, 18, 20, 22, 25

Yield: 20 lb boxes/acre for the mid-season harvests

For the mid-season harvests (6-10), ‘MG0477’ from Syngenta had the highest total marketable yield in 20 lb boxes per acre at 715 boxes/ac (Table 9). ‘EXP 3595’ from Bejo and ‘Blade Runner’ from Syngenta ranked second and third at 608 boxes/ac and 537 boxes/ac, respectively, for the mid-season harvests. The average total marketable yield for the mid-season harvests was 330 boxes/ac.

Virus pressure increased with higher temperatures and more favorable conditions for insect vectors for the mid-season harvests. Five additional cultigens yielded virus-symptomatic fruit for the mid-season harvests, but not for the early harvests: ‘Respect’ from Clifton (HM Clause), ‘SV6198’ from Seminis, ‘EXP 3641’ from Bejo, ‘Outlaw’ from Enza Zaden, and ‘MG0477’ from Syngenta. A total of 15 cultigens out of 19 total yielded some virus-symptomatic fruit for the mid-season harvests, averaging 200 boxes of symptomatic fruit per acre (Table 9).

Yield: Number of fruit per plant for the mid-season harvests

For the mid-season harvests (6-10), ‘MG0477’ and ‘Blade Runner’ from Syngenta had the greatest numbers of marketable fruit per plant at 6.0 and 5.9, respectively (Table 10). ‘EXP 3595’ from Bejo and ‘SV0813’ from Seminis ranked third and fourth at 5.4 and 4.8, respectively. These four cultigens also had no virus-symptomatic fruit for the early or mid-season harvests, along with ‘Renegade’ from Enza Zaden. The average number of marketable fruit per plant for the mid-season harvests was 3.3.

Virus incidence and severity began to have a large impact on the number of marketable fruit for several cultigens for the mid-season harvests. Six cultigens yielded more virus-symptomatic fruit than marketable fruit per plant, and the check entry ‘Zucchini Elite’ yielded the greatest amount of any grade of fruit per plant at 6.7 no/plant of virus-symptomatic fruit for the mid-season harvests (Table 10).

Late Harvests (11-15): September 27, 29; October 2, 4, 6

Yield: 20 lb boxes/acre for the late harvests

For the late harvests (11-15), ‘MG0477’ from Syngenta had the highest total marketable yield in 20 lb boxes per acre at 475 boxes/ac (Table 11). ‘EXP 3595’ from Bejo and ‘Blade Runner’ from Syngenta ranked second and third at 431 boxes/ac and 373 boxes/ac, respectively, for the late harvests. The average total marketable yield for the late harvests was 222 boxes/ac.

All 19 entries yielded some virus-symptomatic fruit for the late harvests. Four cultigens yielded virus-symptomatic fruit only in the late harvests: ‘Blade Runner’ from Syngenta, ‘EXP 3595’ from Bejo, ‘Renegade’ from Enza Zaden, and ‘SV0813’ from Seminis.

Yield: Number of fruit per plant for the late harvests

The entry with the greatest number of marketable fruit per plant for the late harvests was ‘EXP 3641’ from Bejo at 3.9 no/plant (Table 12). ‘EXP 3595’ from Bejo and ‘MG0477’ from Syngenta ranked second at 3.7 no/plant for both entries. The average number of marketable fruit per plant for the late harvests was 2.1 no/plant. This average decreased as the season went on primarily due to increased virus pressure that therefore resulted in large numbers of virus-symptomatic fruit per plant in later harvests.

All cultigens had some degree of virus incidence by the late harvests, which took place in late September and early October. ‘EXP 3641’, which ranked first for marketable fruit per plant, also yielded 1.1 no/plant of virus symptomatic fruit – more than any other cultigen ranked in the top five for total marketable yield (Table 12).

Percentages by grade and harvest

Virus pressure was minimal for the early harvests and increased throughout the growing season as environmental factors became more hospitable for insect vectors and disease progression. For example, in the cultivar ‘Respect’, entered as a check because it is commonly grown in North Carolina, 88% of fruit harvested for the early harvests were in the marketable 1s category (Table 13). For the mid-season harvests, only 48% of fruit were marketable 1s, and by the late harvests, only 12% of fruit were in the marketable 1s category.