Many small-acreage and/or organic vegetable growers in the United States are implementing high tunnels as a way to reduce foliar disease and extend the growing season (Carey et al., 2009; O’Connell et al., 2012). In particular, tomato is a popular crop for high tunnels and production in the central United States is increasing (Carey et al., 2009). Not only do high tunnels extend the growing season (Hunter et al., 2010; Wells and Loy, 1993), they also increase yield, economic return, and marketing opportunities (O’Connell et al., 2012; Wells, 1991). High producer interest, general affordability, as well as availability of federally funded cost-share programs (U.S. Department of Agriculture, 2013) have resulted in a dramatic increase in the amount of growers using high tunnels for vegetable crops and tomato in particular (Carey et al., 2009).
One challenge of managing high tunnel production systems is limiting soilborne diseases such as root-knot nematode (RKN; Meloidogyne sp.), southern blight (Sclerotium rolfsii), and fusarium wilt (Fusarium oxysporum f. sp. lycopersici), and grafting with resistant rootstock has been proposed as a way to mitigate these issues. In the southeastern United States, where these pests are very common, using resistant rootstocks has been an effective Integrated Pest Management strategy to reduce RKNs (Rivard et al., 2010a), southern blight (Rivard et al., 2010a), fusarium wilt (Rivard and Louws, 2008), and verticillum wilt [Verticillum dahliae (Louws et al., 2010)]; however, there is little information focused on grafting in production settings where less disease pressure exists, particularly in the central United States.
Rootstock selection is a key step in successful deployment of this technology as certain rootstocks may target specific diseases, abiotic stress, or overall yield benefits (Louws et al., 2010; Rivero et al., 2003). There are a number of rootstock options available to high tunnel growers in the United States, including interspecific hybrid rootstocks that may provide added vigor to the plant (Kubota et al., 2008; Louws et al., 2010). The crop performance and/or conferred disease-resistance characteristics of ‘Maxifort’ rootstock (De Ruiter Seeds, Bergschenhoek, The Netherlands) has been reported in North Carolina (Louws et al., 2010; Rivard and Louws, 2008), and this rootstock is very popular among growers using grafted plants throughout the United States. ‘Trooper Lite’ is an interspecific hybrid rootstock that was released into the U.S. market in 2010 and little has been reported about its overall vigor. Unfortunately, ‘Trooper Lite’ was pulled off the U.S. seed market in 2013 as it had inconsistent germination characteristics (Seedway, unpublished data).
Despite potential advantages and increasing grower interest, market availability of grafted tomato in the United States is currently limited. More than 40 million grafted tomato plants were imported annually from specialty nurseries in Canada and Mexico (Kubota et al., 2008). Recently, herbaceous grafting nurseries have begun to appear in the United States, but long-distance shipping and nurseries of this size have difficulty catering to the specialty requirements of small to midsize growers (Kubota et al., 2008). Growers can perform their own grafting (Rivard and Louws, 2011); however, management of grafted plants can be difficult.
We surveyed fruit and vegetable growers at a regional growers’ conference (St. Joseph, MO) on their interest and usage of grafted plants. Of the 265 participants surveyed (65% of which were growing in high tunnels), 19% were using grafted plants, 56% were interested in learning more, and 24% were not using grafted plants, but would like to. Interestingly, 47% of respondents indicated they would prefer to grow their own grafted plants, whereas 25% indicated they would prefer to purchase grafted plants (C.L. Rivard, unpublished data). These data highlight the potential impact that development of accessible propagation systems could have at overcoming barriers related to grafting in the central United States.
Finding a way to reduce the high humidity requirements of grafted seedlings immediately postgrafting would be extremely beneficial to facilitate on-farm grafting with limited greenhouse facilities. Grafted plants are placed inside “healing chambers” covered with polyethylene film and shadecloth to maintain high humidity and reduce light intensity directly after being grafted (Masterson et al., 2016; Rivard and Louws, 2011). However, healing chambers built inside of greenhouses that do not have cooling equipment can become excessively hot, leading to plant wilting and death. Healing chambers also add to the cost of producing a grafted transplant, as they require additional materials and labor for management (Rivard et al., 2010b). Reducing leaf area could subsequently reduce transpiration and therefore reduce the need for intensive management of relative humidity during graft union healing. Leaf removal is recommended for the cleft and splice grafting method in an extension bulletin for tomato (Bumgarner and Kleinhenz, 2014). Furthermore, recent studies with leaf removal have shown that it can increase grafting success (Masterson et al., 2016).
In addition to reducing water stress, removing both the leaf and apical meristem (SR) could result in a plant that has two “leaders,” each of which grows from the two cotyledon nodes. In the study by Masterson et al. (2016), using the SR method did not increase or penalize grafting success in various healing chamber environments. However, in an economics report (Rivard et al., 2010b), a propagation model was presented in which grafted plants were pinched to form two leaders 10 d postgrafting, and this was done at the request of the tomato grower who purchased the plants (Rivard et al., 2010b). Pruning grafted plants to two leaders is common with grafted plants, particularly when European string trellis systems are used in protected culture systems (Besri, 2003; Kubota et al., 2008). By removing the meristem after the grafted plant has been healed, additional regrowth time is required, and this can delay planting in the field/greenhouse 10 to 14 d and/or slow early growth. If the meristem is removed during grafting, it could potentially advance the timeline toward a finished transplant by 10 to 14 d.
By reducing leaf area via scion SR, reliance on the healing chamber for additional microclimate modification beyond greenhouse conditions could potentially be reduced or eliminated altogether. Additionally, this could be a valuable technique for propagators looking to grow plants that are pruned to two leaders. However, there is little information available as to whether scion SR will affect tomato yield and fruit marketability in a production setting. Therefore, this research had two primary objectives to 1) determine the efficacy of two rootstock cultivars at increasing tomato fruit yield in high tunnels in the central region of the United States and 2) test the effect of scion SR on tomato plant yield and biomass in a commercial production setting.
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