Improving Seedless Cucumber Production to Diversify High Tunnel Crops in the North Central Region

While high tunnels have become increasingly popular over the last 20 years as a valuable season extension tool, the considerable potential of this agricultural tool has not, to date, been fully realized. In this regard, the primary use of high tunnels to date has been to extend the growing season for tomatoes and winter-grown leafy greens, with lesser popularity of other vegetables ^[1]. As tomatoes have proven to be a crop with the capacity to provide excellent returns ^[2], growers are usually compelled to grow this single crop year-after-year. Continuously growing the single crop is frequently highly detrimental to soil health and increases risk of disease and pest problems. A classic example is tomato leaf mold which has become an important disease significantly contribute to tomato yield loss and quality reduction. 

While primary considerations for decisions regarding which vegetables to grow in high tunnels involve efficient space utilization in order to optimize profits, the establishment of optimal cultural practices is also an important consideration. High tunnel systems provide crops with intermediate environmental control. Such systems increase heat utilization and prevent challenges associated with rain, snow and strong wind. High humidity, no free water, and the wide range of day-and-night temperature differences serve to characterize high tunnel microclimate conditions. Because such conditions are significantly different than those in open-fields, suitable varieties and production practices commonly used in field production do not always apply to high tunnel production.

Unfortunately, except for tomatoes, relatively little information is available regarding variety selection and production practices for growing other vegetables in high tunnel systems. As a consequence, there is an increasing critical need to identify alternative crops and establish conditions that allow crops other than tomatoes to be adapted and grown profitably in high tunnel production. Not addressing the critical need would likely be problematic in advancing the sustainability of the high tunnel systems, as well as the roles they play in providing the local food community. Our central hypothesis in this application is that seedless cucumbers can be successfully and profitably grown in high tunnels and will generate adequate profit for both conventional and organic systems.

Seedless cucumbers, also called greenhouse cucumbers, are a popular and high-value crop in many local food markets. The plants are parthenocarpic with the capacity to set fruit without need for pollination. There are different types of seedless cucumbers. The cumbers with extended length are often referred to as European, Japanese or English cucumbers. They have thin skins with longitudinal ridges. They are often individually wrapped to prevent postharvest fresh weight loss. Beit Alpha cucumbers are smaller, with a range of sizes. Large Beit Alpha cucumber is similar to slicing cucumbers. Pickling cucumbers with the parthenocarpic character are also available in the market. Seedless cucumbers sell at premium price in early market as do tomatoes ^[3]. In addition, the characteristic climbing habit allows crop for trellising to maximize the use of vertical spaces, making seedless cucumber a potentially valuable crop for high tunnel production. Worldwide, seedless cucumber ranks third in the list of most important high tunnel crops following tomatoes and peppers ^[4]. However, because peppers and tomatoes are both in the Solanaceous family and share some of the same diseases and pests, cucumber would likely be a preferred crop to be rotated with tomatoes in high tunnel systems for the reasons summarized above.

At the present time, however, several challenges have prevented many growers from growing seedless cucumber in high tunnels. One particular challenge is the relatively low soil temperatures in early season that suppress plant growth even when the air temperature would be adequate for cucumber production. Soil temperatures lower than 63 °F greatly inhibit absorption of water and mineral nutrients of the crop ^[5]. In USDA plant hardiness zone 6, we found soil temperatures inside a high tunnel in April to frequently be below 55 °F, which prevents crop establishment and delays early cucumber production. In this regard, it may take eight to nine weeks for early spring-planted cucumbers to reach the harvest-stage, compared to approximately six weeks for the later-planted vines. As a result, it is usually difficult to achieve the early-season advantages for growing cucumbers in high tunnels. To overcome this obstacle, a system for heating the soil within the high tunnels is essential for early production of seedless cucumbers in Europe. However, this approach requires additional energy input and is, therefore, less sustainable for high tunnel growers in the North Central Region. Disease and pest problems are additional challenges that prevent growers from growing cucumbers in high tunnels. Among them, powdery mildew, two-spotted spider mites and bacterial wilt are the most common and destructive pest problems. Since many high tunnel growers are using organic approaches, they are even more vulnerable to these disease and pest-related damages.

Vegetable grafting is a cultural practice commonly used for controlling biotic and abiotic stresses. By physically conjoining a plant with desirable fruit characteristics (called a scion) onto another plant with specific disease resistance or stress tolerance (called a rootstock), grafted plants acquire the capacity to combine the beneficial characteristics of both the rootstock and scion cultivars ^[6]. Most commercial rootstocks (Cucurbita maxima or C. mochata) developed to date for cucumber grafting have demonstrated cold tolerance. Grafting cucumbers onto these rootstocks could potentially help to enhance early season cucumber production. Results from our preliminary study have provided some evidence that grafted cucumber seedlings can be successfully grown in high tunnels even when soil temperatures were in lower 50 °F. The grafted plants that were transplanted in high tunnels at the end of March started to produce cucumbers in early May (~6 weeks), and the seasonal yield was significantly higher than that of non-grafted plants. 

Vegetable grafting was introduced in the U.S. in the 1990s. Over the years, farmers have become increasingly familiar with the technique. In addition, Sustainable Agricultural Research and Education (SARE) has funded 13 projects related to tomato grafting. These projects have achieved great success by introducing this technique to small farmers in the U.S. A number of tomato farmers, especially those growing tomatoes in high tunnels, have successfully adopted this technology. These funded projects have primarily focused on control of soilborne diseases. In discussions with farmers who have adopted the technology, we found that they actually know relatively little about grafting cucurbit crops, nor the use of this technology to enhance crops’ tolerance to suboptimal temperatures. In the proposed project, we will build on the previous SARE funded projects on vegetable grafting, and expand the scope of the research from tomatoes to cucumbers, and from disease control to suboptimal temperature tolerance.

Bacterial wilt is one of the more destructive diseases in cucumber production. The pathogen (Erwinia tracheiphila) are vectored by cucumber beetles. Infected plants wilt and die in only a few days. Cucumbers and muskmelons are generally more susceptible to bacterial wilt than watermelons, pumpkins and squashes. Our preliminary field trials conducted by Laura Ingwell have found that grafting cucumbers onto bacterial wilt-resistant watermelon reduced the number of cucumber beetles per plant and the incidence of bacterial wilt. One mechanism that has been identified to provide resistance against bacterial wilt is plant growth-promoting rhizobacteria (PGPR) induced systemic resistance. It reduces concentrations of beetle feeding stimulant and activates plant defense mechanisms after the pathogen has been introduced ^[7]. The systemic resistance may also exist in grafted plants. One evidence is the expression of plant defense responses-related proteins was significantly higher in C. moschata grafted cucumbers compared with self-rooted plants ^[8]. Although the underlying mechanisms are still largely unknown, the applicability of grafting to control bacterial wilt in cucumber production warrants further investigation.

A limitation of using grafting as an approach to improve crop characteristic is the potential negative impact on fruit quality. Currently, relatively little information is available in this respect with regard to cucumbers. Considering that cucumbers are routinely harvested as immature fruit, the rootstock effects on sugar contents might be negligible. But other vegetable quality attributes including firmness, fruit appearance and sensory property need to be evaluated.

Powdery mildew and two-spotted spider mites are known to cause significant damage to cucumbers grown in protected agricultural environments. Both pests prefer dry-atmosphere and higher temperatures, which are the predominant environmental conditions in high tunnels in the summer. Cucumbers have been shown to exhibit differential levels of relative resistance or tolerance to the two pests. For example, our primary data indicate that the pickling cucumber, ‘Excelsior’, is more susceptible to two-spotted spider mites than other varieties. But such types of information are not always presented or not adequately described in the seed packages. Comprehensive variety evaluation, with a focus on powdery mildew and two-spotted spider mites resistance/tolerance, would be of considerable importance in identifying those varieties that would be most suitable for high tunnel production.

Canopy modification through pruning and trellising can play a significant role in disease management. Dense plant canopies, for example, result in high relative humidity and low light intensity that are favorable conditions for powdery mildew development ^[9]. In addition, older leaves can serve as a source of inoculum, and they are often more susceptible to diseases and pests than younger leaves. Farmers who grow cucumbers in high tunnels primarily use two pruning/trellising systems: a mesh trellis with no or relatively limited pruning, or trellis to a single leader and prunes suckers. In our preliminary study, we used an intensive leaf pruning system in which we trellised the plant to a single leader and pruned not only suckers, but also older leaves. We pruned the leaves weekly as the plant was actively growing, leaving only 10-15 leaves for each plant during the entire production season. Using this system, we successfully suppressed powdery mildew and two-spotted spider mites without the need to spray pesticide. Although our results are promising, since we removed a large number of leaves that were green and still actively contributing photosynthesis, the intensive leaf pruning system could potentially negatively affect yield. Moreover, this approach required an extensive commitment of labor. A thorough evaluation of the intensive leaf pruning system, taking into considerations of pest control, yield and labor requirement is needed.

Our long-term goal in this multi-disciplinary project is to establish reproducible conditions that allow increased crop diversification and enhanced sustainability of high tunnel production systems. Our overall objective in this application is to identify alternative crops that can be successfully and profitably grown in high tunnels, and the production practices required to achieve these goals, with a primary focus on seedless cucumbers. The project is timely and important because it addresses one of the more important issues in high tunnel production systems, namely the need for crop diversification. In the past years, SARE has funded multiple projects related to the use of high tunnels. Among the projects focusing on vegetables, 12 were for tomatoes, eight for winter greens, one for peppers and one for melons. It is of considerable importance to expand the list of crops to encompass cucumbers, one of the most promising high tunnel crops.