By Sarah Dyer
Introduction
Solanum aethiopicum subsp. gilo, commonly known as gilo (jiló), African eggplant, Ethiopian eggplant, or garden egg, is an underutilized crop species native to West Africa (National Research Council, 2006). Despite its potential for adaptation to diverse agroecological conditions and its high nutritional value, gilo remains unknown in many parts of the world (Yang & Ojiewo, 2013).
In recent years, there has been growing interest in gilo as a crop for small-scale farmers and local food systems due to its ability to thrive in poor soil conditions and its resistance to many common pests and diseases (Hill, 2001; Marchese et al, 2014). However, the limited availability of gilo germplasm and the lack of standardized breeding and testing protocols have hindered the development of new varieties adapted to specific regional conditions.
To address this gap, student researchers in collaboration with the Evergreen State College Organic Farm, have designed a replicated variety trial to evaluate the performance of different gilo accessions under maritime Pacific Northwest conditions. The trial aims to provide valuable information on the genetic diversity and adaptability of gilo and identify promising varieties that can be further developed and used by local farmers and breeders (Han et al., 2021).
This paper describes the design of the replicated block variety trial, including the selection of trial site, design, and management methods. The trial is a crucial step towards increasing the availability and utilization of gilo in the Pacific Northwest region and will contribute to our understanding of the potential of this underutilized crop species for sustainable agriculture and food systems (Han et al, 2021; Yang & Ojiewo, 2013).
By evaluating the performance of four varieties of gilo and a control variety of S. melongena, the trial aims to provide valuable data on the benefits and limitations of gilo as a potential crop for the Pacific Northwest, informing future breeding and selection decisions for Solanum aethiopicum (Colley et al., 2018). Gilo is an unknown variety in this region, and its potential to thrive in the PNW climate is still uncertain.
This trial aims to provide a comprehensive evaluation of gilo’s performance in terms of germination, early vigor, yield, disease resistance, adaptability to local conditions, and flavor, which will inform its potential for commercial production in the region. The success of this trial will contribute to the development of a more diverse and resilient crop portfolio in the Pacific Northwest and help to ensure a sustainable and food-secure future for the region.
Site and Soil
The Evergreen State College Organic Farm, located in Olympia, Washington, was chosen as the site for the replicated variety trial of Solanum aethiopicum subsp. gilo. The trial was conducted in a field on the southwest side of the farm, characterized by Giles silt loam soil (NRCS, 2004). The Giles soil series consists of deep, well drained soils formed from volcanic ash and glacial outwash, with slow to medium runoff and moderate permeability (NRCS, 2004).
The marine climate is characterized by temperate, dry summers, and mild, wet winters. The mean annual precipitation accompanying the Giles series is 35 to 60 inches, the mean annual soil temperature ranges from 49 to 52 degrees Fahrenheit, and the frost-free season is 170 to 200-days (NRCS, 2004).
Trial Design and Known Sources of Field Variation
The trial was laid out in a Randomized Complete Block Design (RCBD) with three replications, in five rows 43 feet long and 32 inches wide running north to south. Each row comprised three plots measuring 13 feet by 32 inches; blocks ran east to west across the rows.
Blocks consisted of 5 plots, each containing eight plants of a single variety. Each of the five varieties was assigned a code consisting of numbers and letters to prevent bias in the field. Each number in the diagram in Figure 1 represents a plot of that numbered variety; varieties were assigned to each replication (block) utilizing a random number generator.
| BLOCK 1 | OE | SW | MR | Z | CVC |
| BLOCK 2 | CVC | Z | OE | MR | SW |
| BLOCK 3 | Z | MR | SW | CVC | OE |
| ROW 1 | ROW 2 | ROW 3 | ROW 4 | ROW 5 |
Figure 1: Field Diagram of the Trial
The weather and prevailing winds usually come from the west or southwest (NRCS, 2021). Trees bordering the farm’s south side shade some of the southern side of the field in the spring and fall. The trial was established in a field previously managed as a perennial lavender bed. The area was treated with woodchip mulch and has been under organic management since its establishment; lavender plants were removed at the end of 2022, and the field has since remained fallow, allowing weeds such as field bindweed (Convolvulus arvensis) and Canada thistle (Cirsium arvense) to establish themselves, further complicating the soil’s conditions.
The plot is along the field’s edge, and no border rows were planted due to space constraints of the farm. The five rows for the trial will be placed in a flat area of the field where the soil type is consistent and away from barriers like trees to avoid shading part of the plot, ensuring that all plots in the field will be similarly exposed to the sun, heat, and weather. The five rows are in an area with consistent soil conditions, and no significant differences are expected within or between rows.
Cultivars
The variety trial evaluates the performance of four different varieties of gilo, including, ‘Morro Redondo’, ‘Comprido Verde Claro’, ‘Simeon’s White’, and ‘Zebra’, all of which are unique in their characteristics and were chosen for their cold tolerance, which is a major limiting factor in growing Solanaceous plants in the Pacific Northwest. Additionally, the trial includes the control variety ‘Orient Express,’ which the farm grows annually.
The ‘Morro Redondo’ and ‘Comprido Verde Claro’ varieties, both offered by Thresh Seed Co., were selected because they were trialed in the Northeastern United States, because they are each distinct in coloration and morphology, and because they are the two predominant types of gilo found in Brazilian cuisine. Seed for both “Brazilian” varieties were provided by the USDA National Genetic Resources Program (Thresh Seed Co., 2024).
‘Simeon’s White’ and ‘Zebra’, offered by North Circle Seeds, were chosen because they are bred in the northern Midwest, as well as for their unique colors and patterns. The seeds for this variety originate from Simeon Bakunda, an immigrant from Congo, and have been selected for the past 7 years in Fargo, North Dakota where he grows them with the Growing Together project (North Circle Seeds, 2024).
The single Solanum melongena variety, ‘Orient Express’, offered by Johnny’s Selected Seeds, was included as a control variety to provide a benchmark for comparison with the other varieties. ‘Orient Express’ has a proven history of success at Evergreen Organic Farm, in terms of yield, marketability, and cold tolerance (Johnny’s Selected Seeds, 2019).
| Cultivars | Characteristics |
| ‘Morro Redondo’ | Dark Green, Round, and Large. |
| ‘Comprido Verde Claro’ | Light Green, Oval Shaped, Long and Thin. Fruits 3 to 4 inches in Length. |
| ‘Simeon’s White’ | White, Egg-Shaped, Mango-sized. Young leaves are cut and used in soups. |
| ‘Zebra’ | Green and White Striped, Egg Shaped, Sweet Cultivar. |
| ‘Orient Express’ | S. melongena. Glossy, Dark Purple, and Slender. Fruits 8–10″ long by 1 1/2–2 1/2″ diameter. Cold tolerant. |
Figure 2: Characteristics of Cultivars Grown in the Trial
Culture
Seeds for the trial were sown on March 11th-18th, in 72-slot rigid plastic plug trays filled with Black Gold potting mix and placed in a heated greenhouse maintained at 65-85F. Trays were placed on heated germination mats set to 65 degrees Fahrenheit. On April 22nd, the seedlings were repotted to 2” plastic pots. On May 15th, seedlings were removed to the unheated greenhouse for hardening.
On June 3rd, the seedlings were moved to the field and transplanted into predefined 13’ x 32” plots marked with each variety’s individual code. Seedlings were transplanted 18 inches apart in rows 50 inches apart. Plots were divided by 12-inch borders within rows and 18- inch borders between rows. The planting consisted of three blocks with the five cultivars planted randomly within each block. There were eight plants of each cultivar in each of the five plots, for a total of 40 plants per block.
Pest, Insect, and Disease Control
Mice ate many of the newly germinated seedlings between March 27th and March 29th. 68 plants were effectively destroyed, though thankfully the required number of each variety (24 plants) remained. The twenty-four seedlings selected for each variety had no evidence of damage from the mice, and so were unimpacted by the unexpected rodent interference. Plug trays were thereafter covered with plastic dome lids to protect the seedlings from rodent interference.
From April 24th, it was determined that an aphid infestation established itself on the eggplants in the heated greenhouse; seedlings were sprayed with water to remove insects as an initial preventative treatment. On May 8th and continuing weekly until seedlings were transplanted to the field, Safer Insect Killing Soap was used to treat for aphids five times.
To treat plants, 1 ounce of Safer Soap was mixed with 48 ounces of water in a small, pump-action, multi-purpose farm sprayer, mixing well to insure an even distribution of soap in the solution. Seedlings were sprayed thoroughly, with great care to saturate all sides of the plant. Heavily infested leaves with a large amount of insect damage were removed and placed in a soap bath.
In addition to aphids, other pests were noted when the plants were moved to the unheated greenhouse, including incidences of leafhoppers, flea beetles, and thrips. Damage from non-aphid pests, however, was insignificant. Researchers will continue monitoring insect damage and incidences of disease over the summer.
Fertilization
The soil in the proposed variety trial plot presented a challenging environment for crop growth. Previous management of the field included treatment with woodchips, and careful attention was paid to removing woodchips from the plot before it was tilled (McGrath, 2013). With a pH level of 5.3, the soil was extremely acidic, which can inhibit microbial activity and limit the availability of essential nutrients. Additionally, the elevated levels of calcium in the soil may have exacerbated these issues (Magdoff & Van Es, 2021).
Furthermore, the combined soil test results indicated low levels of nitrogen, potassium, magnesium, sodium, sulfur, zinc, manganese, and boron, making it essential to design a comprehensive soil management plan. It is important to note that the soil test results were not specific to the trial plot itself, but rather were collected from a larger area. This may not provide a truly representative sample of the soil’s conditions within the specific boundaries of the trial.
The test results are likely to be influenced by the varying soil types and management practices that may have occurred in the surrounding areas. Therefore, while the test results provide valuable information about the general soil conditions, they should be considered in conjunction with on-site observations and data collected during the trial to gain a more comprehensive understanding of the soil’s characteristics and potential impacts on crop growth
The soil management plan for the variety trial plot utilized a comprehensive strategy that aimed to address the soil’s pH, nutrient deficiencies, and micronutrient limitations. (See Appendix A: Trial Soil Test)
To combat the low pH, Microna lime was applied at a rate of 120 pounds per 1000 square feet at a depth of 6 inches. This should raise the pH from 5.3 to around 5.9, which is not ideal, but the best solution given the circumstances. Previous studies have found that Solanum aethiopicum thrives with a pH range anywhere between 5.5 – 6.8 (Yang & Ojiewo, 2013).
To address nitrogen deficiency, Perfect Blend fertilizer (4-4-4) was applied at a rate of 51.65 pounds per 1000 square feet. Additionally, 100 pounds of feather meal was applied per 1000 square feet, providing an extra 12 pounds of nitrogen (11 pounds water soluble). This was side dressed into each bed to ensure optimal nitrogen availability for the crop.
Potassium was another concern, and to address this, Sul-Po-Mag (0-0-22) was applied at a rate of 136 pounds per acre (3.13 pounds per 1000 square feet). This will provide an extra 30 pounds of potassium per acre, essential for the crop’s flower set and fruit production, as well as the necessary magnesium and sulfur for healthy photosynthesis and plant growth (Magdoff & Van Es, 2021).
In addition to the above soil amendments, Kelp Meal was added at a rate of ¼ cup per planting hole before the eggplant seedling varieties were transplanted. This provided essential micronutrients such as zinc and boron, which were recommended by the soil test.
Data collected in this variety trial could provide more insight into soil preferences for Solanum aethiopicum subsp. gilo.
Irrigation, Mulch, and Trellising
5/8” drip irrigation was used to deliver 1-inch of water per week to the seedlings. A timer was utilized at the header line to ensure regular watering. Because drip irrigation was used, watering is consistent but can be slightly heavier at the beginning of the row and lighter at the ends. One line of drip irrigation was utilized per bed and placed 2 inches from center to water the single sow planted center-bed. During droughty periods, moisture stress has been shown to cause flowers to abort with little or no fruit set; therefore, at least 1 inch of water each week from rainfall or irrigation during the growing season should promote fruit set (Maynard, 2016; Maynard & Hill, 2001).
Black poly mulch was utilized in the field to inhibit weeds, retain moisture, and raise soil temperature. As temperature is a limiting factor in growing gilo, studies have shown that the increase in soil temperatures provided by black poly mulch can increase yields by 15-30%, as well as promoting an earlier yield. Additionally, black poly mulch leads to improved weed control and nutrient conservation (Hill, 2001).
A Florida weave (basket weave) trellis scheme was utilized to support plants in the trial as they increase in size. Five-foot-tall wooden stakes were hammered-in every 5 ¼ feet, and white poly twine was used to weave the trellis around the plants. New levels of the trellis should be added at intervals of 8-12 inches as plants outgrow the previous levels of twine.
Conclusion
Looking forward to the summer, researchers will undertake fieldwork to observe insect pressure and incidences of disease. Data will be collected on agronomic and yield-related characteristics. Yield and marketable yield will be measured twice weekly, and total dry yield will be calculated after harvest (pounds). The length of harvest window will be recorded for each variety.
Additionally, related work will be pursued to educate consumers about gilo. Information sheets and recipe cards will be produced to introduce gilo to the local consumer. A sensory evaluation of the gilo varieties tested in the trial will be held for Evergreen State College students and community members during the academic quarter of Fall 2024, and feedback on flavor and culinary characteristics for each gilo variety will be collected and analyzed in concert with agronomic data.
References
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Appendix A: Trial Soil Test
