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Wenjing Guan and Xin Zhao

Grafting has been used for controlling certain soilborne diseases and improving abiotic stress tolerance in muskmelon (Cucumis melo) production. Grafting methods may vary considerably among geographic regions and nurseries, while excision of rootstock roots before graft healing may also be practiced, which allows root regeneration of the grafted plants. In this greenhouse study, four grafting methods including hole insertion, one-cotyledon, noncotyledon, and tongue approach methods were examined for their impacts on plant growth and root characteristics of ‘Athena’ muskmelon grafted onto ‘Strong Tosa’ interspecific hybrid squash rootstock (Cucurbita maxima × C. moschata). Nongrafted rootstock and scion plants were included as controls. Both the grafted and nongrafted plants were examined with or without root excision. The practice of root excision was unsuccessful with the tongue approach method, while it did not exhibit significant effects on graft quality and growth of plants grafted with the one-cotyledon and hole insertion methods. Grafted plants with root excision started to show active and rapid root regeneration at 8 days after grafting (DAG) and reached similar root length and surface area as the root-intact plants at 16 DAG. Plants grafted with the noncotyledon method showed a different root growth pattern with decreased root length and surface area at 16 DAG. As a result, this method reduced the quality of grafted plants. No significant differences in plant growth characteristics were observed among the hole insertion, one-cotyledon, and tongue approach grafted plants.

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Wenjing Guan, Xin Zhao and Donald J. Huber

Interspecific hybrid squash (Cucurbita maxima × Cucurbita moschata) is a well-known cucurbit rootstock for controlling soilborne diseases and improving abiotic stress tolerance. However, reduced fruit quality has been reported on certain melon (Cucumis melo) cultivars when grafted with squash rootstocks. In this study, a field experiment was designed to explore fruit development and quality attributes of galia melon ‘Arava’ by grafting with hybrid squash rootstock ‘Strong Tosa’. Grafted plants with ‘Strong Tosa’ showed delayed anthesis of female flowers by ≈8–9 days, but harvest dates were unaffected compared with non- and self-grafted ‘Arava’ plants. Early and total yields were not significantly different between grafted and nongrafted plants. Grafted plants with ‘Strong Tosa’ rootstock exhibited accelerated fruit development and greater vegetative growth. During the harvest period, ≈27% of grafted plants with ‘Strong Tosa’ wilted, which was determined as nonpathogenic. Grafting with ‘Strong Tosa’ rootstock resulted in reduced fruit total soluble solids (TSS) and consumer rated sensory properties.

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Sanjun Gu, Wenjing Guan and John E. Beck

High-tunnel strawberry (Fragaria ×ananassa) production for extended seasons has a great economic potential for small farmers. However, information on cultivars that are suitable for high tunnels is rather limited. In this study conducted in the 2014–15 season, strawberry plugs of eight June-bearing cultivars (Florida Radiance, Benicia, Camarosa, Camino Real, Chandler, Strawberry Festival, Sweet Charlie, and Winterstar) and two day-neutral cultivars (San Andreas and Albion) were evaluated for yield performance, fruit quality, and vegetative growth in organically managed high tunnels at two locations in North Carolina. Significant cultivar differences in whole-season yield were observed at Greensboro, NC; but not at Goldsboro, NC. The cultivar Florida Radiance had the highest marketable and total yields, followed by Winterstar and Chandler at Greensboro, whereas Benicia, Winterstar, and Chandler were the top producing cultivars at Goldsboro. Harvest of day-neutral cultivars San Andreas and Albion started in November. For June-bearing cultivars, Florida Radiance began to produce harvestable berries in late December, followed by Winterstar in early January. Peak harvest occurred in April for all cultivars. At the end of the season, ‘Albion’ had smaller canopy size than other cultivars. It also developed the fewest number of branch crowns and least aboveground biomass. Total soluble solid (TSS) content in April was lower than that observed early and late in the season for all cultivars, although Strawberry Festival exhibited a relatively stable TSS throughout the season. ‘Benicia’ produced the largest strawberries in the early season, but its fruit weight was remarkably reduced as the season progressed. Severe frost events occurred on 18 and 20 Feb. that caused an average of 61.5% and 32.2% open blossom damage at Greensboro and Goldsboro, respectively. The recommended cultivars based on this 1-year study are Florida Radiance, Benicia, and Camino Real for June-bearing cultivars, and Albion and San Andreas for day-neutral cultivars.

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Wenjing Guan, Dean Haseman and Dennis Nowaskie

Grafting technology is increasingly being accepted in the United States, particularly for tomato (Solanum lycopersicum) production under protected structures. There is a great potential to expand this technology to other high tunnel crops. Using grafting technology in cucumber (Cucumis sativus) production is widely adopted in Asia to enhance cucumbers’ tolerance to low temperatures. But 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. Figleaf gourd (Cucurbita ficifolia), Cucurbita moschata, and squash interspecific hybrid (Cucurbita maxima × C. moschata) are the most used cucumber rootstocks worldwide. But their comparative performance was largely unknown for cucumber production in high tunnels in the Midwest United States. This study 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 area. Nongrafted ‘Socrates’ and ‘Socrates’ grafted with Cucurbita moschata, squash interspecific hybrid, and figleaf gourd rootstocks were evaluated in high tunnels from March to June or July in 2016–19 at the Southwest Purdue Agricultural Center in Vincennes, IN. Transplant establishment, vine growth, and yield in early- and main-crop seasons were investigated. Grafted plants regardless of rootstocks ensured transplant survival even when the soil temperatures were dropped below 10 °C. Suboptimal soil conditions were encountered in the first month after transplanting. Grafted cucumbers with squash interspecific hybrid rootstock significantly increased vine growth from March to April and increased early-season yields (yield before 15 May) by 1.8 to 18.2 times compared with the early-season yields of the nongrafted cucumbers. The benefits provided by using grafting technology dismissed around middle May. Only squash interspecific hybrid rootstock improved cucumber yields in the entire production seasons. Cucumbers grafted with figleaf gourd rootstock had the lowest yield and the least plant growth after mid-May, indicating figleaf gourd rootstock may not be suitable for cucumber production under the current production system. Overall, squash interspecific hybrid was the most promising rootstock for early-season high tunnel cucumber production in the Midwest United States.

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Wenjing Guan, Xin Zhao, Richard Hassell and Judy Thies

Grafting with resistant rootstocks is an effective strategy to manage a variety of soilborne diseases and root-knot nematodes in solanaceous and cucurbitaceous vegetables. In addition, improved resistance to some foliar diseases and viruses has also been reported in grafted plants. Hence, grafting technology is considered an important and innovative practice of integrated pest management and a promising alternative for soil fumigants in vegetable production. Inherent resistance within rootstocks and improved plant nutrient uptake are generally suggested as the main reasons for improved disease control in grafted vegetables. However, increasing evidence indicated that systemic defense mechanisms may also play an important role in plant defense as a result of grafting. This review analyzes current literature on the use of grafting techniques for disease management in vegetable crops, discusses potential mechanisms associated with grafting-conferred plant defense, and identifies needs for future research to promote more effective and efficient use of grafting technology to support sustainable vegetable production.

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Wenjing Guan, Xin Zhao, Danielle D. Treadwell, Michael R. Alligood, Donald J. Huber and Nicholas S. Dufault

Interest in producing specialty melons (Cucumis melo) is increasing in Florida, but information on yield performance, fruit quality, and disease resistance of specialty melon cultivars grown in Florida conditions is limited. In this study conducted at Citra, FL, during the 2011 Spring season, 10 specialty melon cultivars were evaluated, in both certified organic and conventionally managed fields, including: Creme de la Creme and San Juan ananas melon (C. melo var. reticulatus), Brilliant and Camposol canary melon (C. melo var. inodorus), Ginkaku and Sun Jewel asian melon (C. melo var. makuwa), Arava and Diplomat galia melon (C. melo var. reticulatus), and Honey Pearl and Honey Yellow honeydew melon (C. melo var. inodorus). ‘Athena’ cantaloupe (C. melo var. reticulatus) was included as a control. ‘Sun Jewel’, ‘Diplomat’, ‘Honey Yellow’, and ‘Honey Pearl’ were early maturing cultivars that were harvested 10 days earlier than ‘Athena’. ‘Athena’ had the highest marketable yield in the conventional field (10.7 kg/plant), but the yield of ‘Camposol’, ‘Ginkaku’, ‘Honey Yellow’, and ‘Honey Pearl’ did not differ significantly from ‘Athena’. Under organic production, ‘Camposol’ showed a significantly higher marketable yield (8.3 kg/plant) than ‘Athena’ (6.8 kg/plant). ‘Ginkaku’ produced the largest fruit number per plant in both organic (10 fruit/plant) and conventional fields (12 fruit/plant) with smaller fruit size compared with other melon cultivars. Overall, the specialty melon cultivars, except for asian melon, did not differ significantly from ‘Athena’ in terms of marketable fruit number per plant. ‘Sun Jewel’, ‘Diplomat’, and ‘San Juan’ showed relatively high percentages of cull fruit. ‘Honey Yellow’, ‘Honey Pearl’, and ‘Sun Jewel’ exhibited higher soluble solids concentration (SSC) than ‘Athena’ in both organic and conventional fields, while ‘Brilliant’, ‘San Juan’, and ‘Ginkaku’ also had higher SSC than ‘Athena’ under organic production. ‘Honey Yellow’, ‘Sun Jewel’, ‘Brilliant’, and ‘Camposol’ were less affected by powdery mildew (caused by Podosphaera xanthii) and downy mildew (caused by Pseudoperonospora cubensis) in the conventional field. ‘Honey Yellow’ and ‘Camposol’ also had significantly lower aboveground disease severity ratings in the organic field compared with ‘Athena’, although the root-knot nematode (RKN) (Meloidogyne sp.) gall rating was higher in ‘Honey Yellow’ than ‘Athena’.

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Wenjing Guan, Xin Zhao, Donald W. Dickson, Maria L. Mendes and Judy Thies

Interest in specialty melons (Cucumis melo) with distinctive fruit characteristics has grown in the United States in recent years. However, disease management remains a major challenge in specialty melon production. In this study, grafting experiments were conducted to determine the effectiveness of using Cucumis metulifer, a species known for its genetic resistance to root-knot nematodes (RKNs; Meloidogyne spp.), as a potential rootstock for managing RKNs in susceptible specialty melon cultivars. In the greenhouse experiment, honeydew melon ‘Honey Yellow’ was grafted onto C. metulifer and inoculated with M. incognita race 1. The grafted plants exhibited significantly lower gall and egg mass indices and fewer eggs compared with non- and self-grafted ‘Honey Yellow’. Cucumis metulifer was further tested as a rootstock in conventional and organic field trials using honeydew melon ‘Honey Yellow’ and galia melon ‘Arava’ as scions. ‘Honey Yellow’ and ‘Arava’ grafted onto C. metulifer exhibited significantly lower galling and reduced RKN population densities in the organic field; however, total and marketable fruit yields were not significantly different from non- and self-grafted plants. Although the improvement of RKN resistance did not translate into yield enhancements, incorporating grafted specialty melons with C. metulifer rootstock into double-cropping systems with RKN-susceptible vegetables may benefit the overall crop production by reducing RKN population densities in the soil. At the conventional field site, which was not infested with RKNs, ‘Honey Yellow’ grafted onto C. metulifer rootstock had a significantly lower total fruit yield than non-grafted ‘Honey Yellow’ plants; however, fruit yields were similar for ‘Arava’ grafted onto C. metulifer rootstock and non-grafted ‘Arava’ plants. Although no significant impacts on the fruit quality attributes of ‘Honey Yellow’ were observed, grafting onto C. metulifer decreased the flesh firmness of ‘Arava’ in both field trials and resulted in a reduction in total soluble solids content under conventional production. In summary, grafting RKN-susceptible melons onto C. metulifer rootstock offers promise for growing these specialty melons; however, more studies are needed to elucidate the scion–rootstock interaction effect on fruit yield and quality.

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Wenjing Guan, Daniel S. Egel, Larry D. Sutterer and Alexander D. Plummer

Seedless cucumber (Cucumis sativus) is a popular and high-value crop found in many local food markets. Worldwide, it is the third most important high tunnel crop after tomato (Solanum lycopersicum) and pepper (Capsicum annuum). One challenge of growing seedless cucumbers in high tunnels is low soil temperatures in the early season that suppress plant growth even when air temperatures would be adequate. Grafting cucumbers to enhance crop tolerance to suboptimal temperature stresses has been widely used in Asian countries. However, little information is available in the United States about graft compatibility, cold hardiness, and seasonal extension potential of growing grafted seedless cucumbers in high tunnels. In this study, we tested the effects of grafting with two winter squash (Cucurbita moschata) rootstocks (‘Titan’ and ‘Marvel’) on vegetative growth and yield of three seedless cucumbers (‘Excelsior’ pickling cucumber, ‘Socrates’ Beit Alpha cucumber, and ‘Taurus’ long-type cucumber) in the spring seasons of 2016 and 2017 in high tunnels located in U.S. Department of Agriculture (USDA) hardiness zone 6. Nongrafted plants were included as controls. All grafted plants survived the suboptimal temperature stress during transplant period, whereas 59% of nongrafted plants died in the 2016 season. Irrespective of rootstock and cucumber cultivar, vine growth rates of nongrafted cucumbers in April of both years were lower than those of the grafted crops. Cucumber cultivars Excelsior and Taurus grafted onto Marvel winter squash rootstock had higher yields in May 2016 compared with the yields of the nongrafted plants in the same month. The enhanced early-season yields of grafted plants were observed on cucumber cultivars Excelsior and Socrates in 2017 regardless of rootstocks. Grafting also increased the entire season’s yields of the three cucumber cultivars in 2017, but not in 2016. More comprehensive evaluations about cold tolerances of newly released cucumber rootstocks are needed. Further studies are also warranted to improve our understanding of effects of rootstock and scion interactions on cucumber growth and yield in high tunnel production.

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Wenjing Guan, Elizabeth T. Maynard, Bronwyn Aly, Julie Zakes, Daniel S. Egel and Laura L. Ingwell

Fresh-consumed parthenocarpic cucumbers (Cucumis sativus) are a popular and high-value crop sold in local food markets. The parthenocarpic plant characteristics and climbing growth habit make cucumbers an ideal crop for high-tunnel production. Major types of parthenocarpic cucumbers include Beit alpha and mini, Dutch greenhouse, American slicer, and Japanese. Information regarding yield performance, plant growth, and disease resistance of the four types grown in high-tunnel conditions is limited. In this study, 16 parthenocarpic cucumber cultivars from the four major types were evaluated in high tunnels at three locations in Indiana and Illinois during Spring 2018. Plants were pruned to a single stem that was supported on a string. At all locations, the cultivars that had the most total yields were Beit alpha and mini, although their total yields were not always significantly higher than that of all the others. However, Beit alpha and mini cucumbers had high percentages of unmarketable fruit, mainly because of insect feeding damage and mechanical injuries on the skins that led to scarred fruit. Dutch greenhouse cultivars had relatively lower marketable yields at two of the three locations where there was a high percentage of misshaped fruit. ‘Tasty Green’ Japanese cucumber consistently had the lowest yields at all three locations. This cultivar also produced the most side shoot growth and, therefore, more pruning waste. The Japanese types ‘Tasty Jade’ and ‘Taurus’ had yields comparable to those of other cultivars, and they were more tolerant to two-spotted spider mites (Tetranychus urticae). However, ‘Tasty Jade’ was the cultivar most susceptible to powdery mildew (Podosphaera xanthii and Golovinomyces cichoracearum). ‘Corinto’ American slicer cucumber had relatively high yields at two of the three locations. This cultivar also had the highest percentage of marketable fruit. Information provided in the study is readily useful for growers using high tunnels when selecting parthenocarpic cucumber cultivars. It is also valuable for seed companies wishing to breed new cultivars adaptive for high-tunnel production.