Grafting is a cultural practice used on cucurbit and solanaceous crops for controlling soilborne diseases and improving plants’ tolerance to abiotic stresses (Lee et al., 2010; Louws et al., 2010; Schwarz et al., 2010). Although the cost for grafted plants is two to five times more expensive compared with the normal transplants (Rivard et al., 2010), using grafted plants is steadily increasing in the United States (Grieneisen et al., 2018), particularly for tomato production under protected structures (Kubota, 2015; Singh et al., 2017). Studies have shown that regardless of the presence of soilborne diseases, grafting has the potential to increase high tunnel tomato yields (Meyer, 2016), and the high economic returns can generally compensate for the cost of using grafted plants (Rysin et al., 2015). Rapid adoption of high tunnels in the United States contributes to the increased expansion of vegetable grafting technology (Louws et al., 2010). In addition to tomatoes, there is a great potential to expand the use of grafting technology to other high tunnel vegetables in the United States.
Cucumber is a high-value crop commonly grown in high tunnels (Lamont, 2009). As most high tunnels are not equipped with advanced environmental control systems in the United States (Carey et al., 2009), crops often suffer from low temperatures in the spring (Hunter et al., 2012). This is becoming a challenge for high tunnel cucumber production because this crop is highly sensitive to low-temperature stress.
Using grafting technology to strengthen cucumbers’ tolerance to low temperatures is widely adopted in Asia (Davis et al., 2008). Although research for cucumber grafting was initially conducted for the purpose of controlling fusarium wilt (Fusarium oxysporum Schlechtend:Fr f.sp. cucumerinum J.H. Owen) in the 1920s in Japan, cucumber grafting was not widely used until the 1960s, when it was recognized that grafting improves cucumbers’ tolerance to low temperatures (Davis et al., 2008; Sakata et al., 2008).
Multiple species were found to be grafting compatible with cucumbers; they are Cucurbita moschata, Cucurbita maxima, figleaf gourd (Cucurbita ficifolia), squash interspecific hybrid (C. maxima × C. moschata), bottle gourd (Lagenaria siceraria), wax gourd (Benincasa hispida), luffa (Luffa cylindrica), burr cucumber (Sicyos angulatus), and melon (Cucumis melo) (King et al., 2010; Wang et al., 2004).
Cucurbita moschata and squash interspecific hybrid were the early developed cucumber rootstocks, but figleaf gourd was by far the most popular cucumber rootstock used in Asia because of its superior cold tolerance (King et al., 2010; Li et al., 2015). Cucurbita spp. rootstocks also provide grafted plants tolerance to fusarium wilt, phytophthora rot (Phytophthora melonis Katsura), and flooding conditions (Louws et al., 2010; Pavlou et al., 2002; Sakata et al., 2008; Wang et al., 2004). A unique feature for certain Cucurbita moschata-type rootstocks is the capability to produce “bloomingless” cucumber fruit. Such fruit has a distinct appearance and a longer shelf life, traits that are more favorable, especially in the Japanese market (Sakata et al., 2008). Burr cucumber has also been extensively evaluated as a cucumber rootstock because of its resistance to southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] (Gu et al., 2006; Zhang et al., 2006). However, poor and slow seed germination (Davis et al., 2008), as well as susceptibility to damping-off and gummy stem blight [Didymella bryoniae (Auersw.) Rehm] (Sakata et al., 2008) prevented the use of burr cucumber as a major cucumber rootstock.
Using grafted cucumber seedlings accounts for about 75% of the cucumber production in Japan and Korea (Lee et al., 2010) and nearly 100% of cucumbers grown in solar greenhouses in Northern China (Huang et al., 2015). Figleaf gourd, Cucurbita moschata, and squash interspecific hybrid are the most used rootstocks (Huang et al., 2015). In general, figleaf gourd was used as a rootstock for winter cucumber production, and squash interspecific hybrid and C. moschata rootstocks are more commonly used for summer cucumber production in Japan (Sakata et al., 2008).
Although using grafted cucumber plants is a routine practice in Asia, this technique is rarely used in the United States mainly because of the lack of information on the performance of the grafted plants under local production systems. In the Midwest United States, cucumber production primarily happens in the summer months, although high tunnels have extended the production season into spring. This is different from the solar greenhouse system, which allows cucumber production throughout the winter in northern China (Wang et al., 2009). In our previous research, we found grafting cucumbers with C. moschata rootstock can enhance early-season high tunnel cucumber production, but results were inconsistent when cucumber production extended into the summer (Guan et al., 2018). As only one rootstock was evaluated, the previous study may not exhibit the full potential of using grafting technology in high tunnel cucumber production. The current research was therefore designed to compare the major types of cucumber rootstocks with the goal of identifying a rootstock with the maximized benefits for high tunnel cucumber production in the Midwest United States.
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