Interspecific Hybrid Squash Research Articles

Effect of Grafting Compatibility on Fruit Yield and Quality of Cantaloupe in a Mediterranean-Type Climate

Grafting is effectively used worldwide to overcome abiotic and biotic factors impacting yield, including soil temperature. Field studies were conducted in 2020 and 2021 in a cool Mediterranean climate (average daily air temperature range of 12.8–17.1 °C) to identify suitable rootstock combinations for grafted cantaloupe (Cucumis melo var. reticulatus) and evaluate fruit yield and quality. Cantaloupe cultivars Sugar Rush (SR), Goddess (G), and Athena (A) were compatible with interspecific hybrid squash (Cucurbita maxima × C. moschata) rootstock cultivars Super Shintosa (SS) and Carnivor (CN) but were incompatible with ‘Carolina Strongback (CS)’ (Citrullus amarus) and ‘Pelop (P)’ (Lagenaria siceraria) rootstocks. Nongrafted cultivars exhibited vine decline at harvest in 2020 but not in 2021, and grafting tended to delay harvest by 15–18 days. Overall, while grafting with interspecific hybrid squash rootstocks may have delayed fruit harvest, fruit quality was not compromised. Further, yield was increased for ‘Goddess’ and ‘Athena’, but not ‘Sugar Rush’. ‘A/CN’ had the highest fruit yield/ha and number per plant. The grafted treatments of each cantaloupe cultivar with interspecific hybrid squash rootstocks met the U.S. fancy grade criteria.

Grafting Melons Increases Yield, Extends the Harvest Season, and Prevents Sudden Wilt in New England

Melons (Cucumis melo var. reticulatis) are potentially a high value crop for New England, but production is limited by cool spring temperatures and sudden wilt. The sudden wilt syndrome in melon, attributed to both biotic and abiotic factors, is characterized by rapid wilting of vines either just preceding or during the harvest season, reducing melon quality and shortening the harvest period. We investigated the effects of grafting melons to rootstocks of interspecific hybrid squash (Cucurbita maxima × C. moschata), which have exhibited tolerance to soilborne diseases and cooler soil temperatures. In 2015, we compared the performance of ‘Halona’ melon grafted to two rootstocks, ‘Carnivor’ and ‘NH1320’, to that of nongrafted (NG) plants at two New Hampshire Agriculture Experiment Research Farms, Woodman (WRF) and Kingman (KRF). Pistillate flowering and melon harvests were 3 to 9 days earlier in NG than grafted (GR) plants. By harvest period, GR plant growth exceeded that of NG plants, and GR plants did not display wilting symptoms observed in NG plants. Total marketable yields were 57% and 90% higher for GR/‘Carnivor’ (47.8 and 45.0 Mg·ha−1) and 44% and 89% higher for GR/‘NH1320’ (43.9 and 44.9 Mg·ha−1) compared with the NG treatment (30.5 and 23.7 Mg·ha−1) at WRF and KRF, respectively. There were no differences in fruit numbers per plot between treatments, but mean fruit weight was between 33% and 71% larger in GR than NG treatments. In 2016, GR (‘NH1320’ rootstock) and NG ‘Halona’ were compared at three transplantation dates, 12 and 21 May and 1 June, and with two irrigation frequencies, drip irrigation every 2 days (2-d Irr) or every 4 days (4-d Irr). NG plants exhibited symptoms of sudden wilt in early August compared with no symptoms on GR plants. Harvests of NG melons were 3 to 8 days earlier than GR plants for all three planting dates. The increase in yields of GR plants compared with NG plants for the 12 and 21 May and 1 June planting dates were, respectively, 131%, 123%, and 149% greater with the 2-day Irr, and 93%, 100%, and 78% greater with 4-d Irr. Irrigation frequency did not significantly affect fruit size or soluble solids content (SSC), whereas grafting increased both fruit number and fruit size but did not significantly affect SSC at all three planting dates.

Rootstock and Plastic Mulch Effect on Watermelon Flowering and Fruit Maturity in a Verticillium dahliae–Infested Field

Separately, grafting and the use of plastic mulch can increase yield, quality, and early harvest of watermelon (Citrullus lanatus), especially when plants are under biotic and/or abiotic stress. A 2-year field study was conducted to evaluate the combination of four different rootstocks and two types of plastic mulch (black and clear) on date of watermelon first flowering, fruit ripening, yield, and fruit quality when plants were exposed to Verticillium dahliae. Seedless watermelon cv. Secretariat was grafted onto rootstocks Lagenaria siceraria cv. Pelop, Benincasa hispida cv. Round, and two interspecific hybrid squash rootstocks Cucurbita maxima × C. moschata cvs. Super Shintosa and Tetsukabuto, with nongrafted ‘Secretariat’ as the control. Fruit were harvested 0, 7, and 14 days after both the leaflet and tendril attached to the fruit pedicel were completely dry (fruit considered to be physiologically mature). The area under the disease progress curve (AUDPC) values for verticillium wilt were not different for mulch type in either year, although the overall AUDPC value was greatly reduced in the four grafted treatments (227) compared with nongrafted (743). There was no difference in days to male or female flowering due to mulch type or year, and rootstock did not affect first flowering of male flowers. Female flowering was 14 and 11 days later in 2018 and 2019, respectively, for ‘Secretariat’ grafted onto bottle gourd ‘Round’ compared with ‘Secretariat’ grafted onto ‘Tetsukabuto’. Female flowering of ‘Secretariat’ on ‘Round’ was also 7 days later compared with nongrafted ‘Secretariat’ both years. However, days to first harvest was not different with mulch or rootstock and was 92 days after transplanting (DAT) in 2018 and 114 DAT in 2019. There was no difference in yield (fruit number and weight) due to year, harvest date, or mulch, but there was a difference due to grafting. ‘Secretariat’ grafted onto ‘Super Shintosa’ had the greatest total number and weight of fruit per plant (3.7 and 14.8 kg, respectively), and nongrafted ‘Secretariat’ had the lowest (0.7 and 3.2 kg, respectively). Fruit quality attributes hollow heart formation (rating 3.2/5 on average), hard seed count (6 on average), total soluble solids (11% on average), and lycopene content were not different among mulch type, rootstock treatment, or harvest date; however, lycopene content did differ due to year (52.44 and 32.51 µg·g−1 in 2018 and 2019, respectively). Flesh firmness was highest for watermelon grafted onto ‘Super Shintosa’ rootstock (6.7 N) and lowest for nongrafted watermelon (4.3 N). Overall, rootstocks reduced verticillium wilt severity and increased fruit yield whereas mulch had no effects, and 5 V. dahliae colony forming units (cfu)/g of soil may be the minimum level for impact on watermelon fruit yield.

Evaluating Cucurbit Rootstocks to Prevent Disease Caused by Pythium aphanidermatum and P. myriotylum on Watermelon.

Pythium species cause root and stem rot in watermelon (Citrullus lanatus), but cucurbit rootstocks used to graft watermelon have not been evaluated for resistance. P. aphanidermatum and P. myriotylum were inoculated onto 15 nongrafted watermelon, citron (Citrullus amarus), bottle gourd (Lagenaria siceraria), and interspecific hybrid squash (Cucurbita maxima × C. moschata) cultivars in a growth chamber. Watermelon was more susceptible than bottle gourd and interspecific hybrid squash at 20 and 30°C. Twenty-one cultivars were inoculated in a field with an equal blend of both Pythium species. Interspecific hybrid squash was less susceptible than bottle gourd and watermelon in 2018 and 2019. Seedless watermelon cultivar Tri-X 313 was grafted to one citron, one bottle gourd, and three interspecific hybrid squash rootstocks. Plants were inoculated in the field as described. Grafting to interspecific hybrid squash rootstocks reduced disease incidence compared with nongrafted controls in 2018 and 2019. Mefenoxam and propamocarb applied at transplanting did not affect disease compared with non-fungicide-treated plots. Grafting to interspecific hybrid squash Camelforce significantly increased total and marketable fruit numbers and total weight in 2019 compared with the nongrafted control. In summary, interspecific hybrid squash was consistently resistant to Pythium, demonstrating resistance and utility in watermelon grafting.

Cucurbit Rootstocks Resistant to Fusarium oxysporum f. sp. niveum Remain Resistant When Coinfected by Meloidogyne incognita in the Field.

Interspecific hybrid squash (Cucurbita maxima × Cucurbita moschata) rootstocks used to graft watermelon (Citrullus lanatus var. lanatus) are resistant to Fusarium oxysporum f. sp. niveum, the fungus that causes Fusarium wilt of watermelon, but they are susceptible to Meloidogyne incognita, the southern root knot nematode. A new citron (Citrullus amarus) rootstock cultivar Carolina Strongback is resistant to F. oxysporum f. sp. niveum and M. incognita. The objective of this study was to determine if an interaction between M. incognita and F. oxysporum f. sp. niveum race 2 occurred on grafted or nongrafted triploid watermelon susceptible to F. oxysporum f. sp. niveum race 2. In 2016 and 2018, plants of nongrafted cultivar Fascination and Fascination grafted onto Carolina Strongback and interspecific hybrid squash cultivar Carnivor were inoculated or not inoculated with M. incognita before transplanting into field plots infested or not infested with F. oxysporum f. sp. niveum race 2. Incidence of Fusarium wilt and area under the disease progress curve did not differ when hosts were inoculated with F. oxysporum f. sp. niveum alone or F. oxysporum f. sp. niveum and M. incognita together. Fusarium wilt was greater on nongrafted watermelon (78% mean incidence) than on both grafted rootstocks and lower on Carnivor (1% incidence) than on Carolina Strongback (12% incidence; P ≤ 0.01). Plants not inoculated with F. oxysporum f. sp. niveum did not wilt. At the end of the season, Carnivor had a greater percentage of the root system galled than the other two hosts, whereas galling did not differ on Fascination and Carolina Strongback. F. oxysporum f. sp. niveum reduced marketable weight of nongrafted Fascination with and without coinoculation with M. incognita. M. incognita reduced marketable weight of Fascination grafted onto Carnivor compared with noninoculated, nongrafted Fascination. In conclusion, cucurbit rootstocks that are susceptible and resistant to M. incognita retain resistance to F. oxysporum f. sp. niveum when they are coinfected with M. incognita.

Interference of Palmer amaranth (Amaranthus palmeri) Density in Grafted and Nongrafted Watermelon

AbstractWatermelon [Citrullus lanatus(Thunb.) Matsum & Nakai] grafting is commonly used for management of diseases caused by soilborne pathogens; however, little research exists describing the effect of grafting on the weed-competitive ability of watermelon. Field experiments determined the response in yield, fruit number, and fruit quality of grafted and nongrafted watermelon exposed to increasing densities of Palmer amaranth (Amaranthus palmeriS. Watson). Grafting treatments included ‘Exclamation’ triploid (seedless) watermelon grafted on two interspecific hybrid squash rootstocks ‘Carnivor’ and ‘Kazako’, with nongrafted Exclamation as the control. Weed treatments includedA. palmeriat densities of 1, 2, 3, and 4A. palmeriplants per watermelon planting hole (0.76-m row) and a weed-free control. IncreasingA. palmeridensities caused significant reductions (P <0.05) in marketable watermelon yield and marketable fruit number. Watermelon yield reduction was described by a rectangular hyperbola model, and 4A. palmeriplants planting hole−1reduced marketable yield 41%, 38%, and 65% for Exclamation, Carnivor, and Kazako, respectively. Neither grafting treatment norA. palmeridensity had a biologically meaningful effect on soluble solids content or on the incidence of hollow heart in watermelon fruit.Amaranthus palmeriseed and biomass production was similar across weed population densities, but seed number per femaleA. palmeridecreased according to a two-parameter exponential decay equation. Thus, increasing weed population densities resulted in increased intraspecific competition amongA. palmeriplants. While grafting may offer benefits for disease resistance, no benefits regarding weed-competitive ability were observed, and a consistent yield penalty was associated with grafting, even in weed-free treatments.

Critical Period for Weed Control in Grafted and Nongrafted Watermelon Grown in Plasticulture

AbstractField experiments determined the critical period for weed control (CPWC) in grafted and nongrafted watermelon [Citrullus lanatus(Thumb.) Matsum. & Nakai] grown in plasticulture. Transplant types included ‘Exclamation’ seedless watermelon as the nongrafted control as well as Exclamation grafted onto two interspecific hybrid squash (ISH) rootstocks, ‘Carnivor’ and ‘Kazako’. To simulate weed emergence throughout the season, establishment treatments (EST) consisted of two seedlings each of common purslane (Portulaca oleraceaL.), large crabgrass [Digitaria sanguinalis(L.) Scop.], and yellow nutsedge (Cyperus esculentusL.) transplanted in a 15 by 15 cm square centered on watermelon plants at 0, 2, 3, 4, and 6 wk after watermelon transplanting (WATr) and remained until the final watermelon harvest at 11 WATr. To simulate weed control at different times in the season, removal treatments (REM) consisted of two seedlings of the same weed species transplanted in a 15 by 15 cm square centered on watermelon plants on the same day of watermelon transplanting and allowed to remain until 2, 3, 4, 6, and 11 WATr, at which time they were removed. Season-long weedy and weed-free controls were included for both EST and REM studies in both years. For all transplant types, aboveground biomass of weeds decreased as weed establishment was delayed and increased as weed removal was delayed. The predicted CPWC for nongrafted Exclamation and Carnivor required only a single weed removal between 2.3 and 2.5 WATr and 1.9 and 2.6 WATr, respectively, while predicted CPWC for Kazako rootstock occurred from 0.3 to 2.6 WATr. Our study results suggest that weed control for this mixed population of weeds would be similar between nongrafted Exclamation and Exclamation grafted onto Carnivor. But the observed CPWC of Exclamation grafted onto Kazako suggests that CPWC may vary with specific rootstock–scion combinations.

Comparison of Root System Morphology of Cucurbit Rootstocks for Use in Watermelon Grafting

Grafting of watermelon (Citrullus lanatus) is an established production practice that provides resistance to soilborne diseases or tolerance to abiotic stresses. Watermelon may be grafted on several cucurbit species (interspecific grafting); however, little research exists to describe root systems of these diverse rootstocks. A greenhouse study was conducted to compare root system morphology of nine commercially available cucurbit rootstocks, representing four species: pumpkin (Cucurbita maxima), squash (Cucurbita pepo), bottle gourd (Lagenaria siceraria), and an interspecific hybrid squash (C. maxima × C. moschata). Rootstocks were grafted with a triploid watermelon scion (‘Exclamation’), and root systems were compared with nongrafted (NG) and self-grafted (SG) ‘Exclamation’. Plants were harvested destructively at 1, 2, and 3 weeks after transplant (WAT), and data were collected on scion dry weight, total root length (TRL), average root diameter, root surface area, root:shoot dry-weight ratio, root diameter class proportions, and specific root length. For all response variables, the main effect of rootstock and rootstock species was significant (P < 0.05). The main effect of harvest was significant (P < 0.05) for all response variables, with the exception of TRL proportion in diameter class 2. ‘Ferro’ rootstock produced the largest TRL and root surface area, with observed values 122% and 120% greater than the smallest root system (‘Exclamation’ SG), respectively. Among rootstock species, pumpkin produced the largest TRL and root surface area, with observed values 100% and 82% greater than those of watermelon, respectively. These results demonstrate that substantial differences exist during the initial 3 WAT in root system morphology of rootstocks and rootstock species available for watermelon grafting and that morphologic differences of root systems can be characterized using image analysis.

Early Season Growth, Yield, and Fruit Quality of Standard and Mini Watermelon Grafted onto Several Commercially Available Cucurbit Rootstocks

Grafting watermelon (Citrullus lanatus) is a common practice in many parts of the world and has recently received increased interest in the United States. The present study was designed to evaluate early season growth, yield, and fruit quality of watermelon in response to grafting and in the absence of known disease pressure in a fumigated system. Field experiments were conducted using standard and mini watermelons (cv. Exclamation and Extazy, respectively) grafted onto 20 commercially available cucurbit rootstocks representing four species: giant pumpkin (Cucurbita maxima), summer squash (Cucurbita pepo), bottle gourd (Lagenaria siceraria), and interspecific hybrid squash [ISH (C. maxima × Cucurbita moschata)]. Nongrafted ‘Exclamation’ and ‘Extazy’ were included as controls. To determine early season growth, leaf area was measured at 1, 2, and 3 weeks after transplant (WAT). At 1 WAT, nongrafted ‘Exclamation’ produced the smallest leaf area; however, at 3 WAT, nongrafted ‘Exclamation’ produced the largest leaf area in 2015, and no differences were observed in 2016. Leaf area was very similar among rootstocks in the ‘Extazy’ study, with minimal differences observed. Marketable yield included fruit weighing ≥9 and ≥3 lb for ‘Exclamation’ and ‘Extazy’, respectively. In the ‘Exclamation’ study, highest marketable yields were observed in nongrafted ‘Exclamation’, and ‘Exclamation’ grafted to ‘Pelops’, ‘TZ148’, and ‘Coloso’, and lowest marketable yields were observed when using ‘Marvel’ and ‘Kazako’ rootstocks, which produced 47% and 32% of nongrafted ‘Exclamation’ yield, respectively. In the ‘Extazy’ study, the highest marketable yield was observed in nongrafted ‘Extazy’, and ‘Kazako’ produced the lowest yields (48% of nongrafted ‘Extazy’). Fruit quality was determined by measuring fruit acidity (pH), soluble solids concentration (SSC), lycopene content, and flesh firmness from a sample of two fruit from each plot from the initial two harvests of each year. Across both studies, rootstock had no effect on SSC or lycopene content. As reported in previous studies, flesh firmness was increased as a result of grafting, and nongrafted ‘Exclamation’ and ‘Extazy’ had the lowest flesh firmness among standard and mini watermelons, respectively. The present study evaluated two scions with a selection of 20 cucurbit rootstocks and observed no benefits in early season growth, yield, or phytonutrient content. Only three of 20 rootstocks in each study produced marketable yields similar to the nongrafted treatments, and no grafted treatment produced higher yields than nongrafted ‘Exclamation’ or ‘Extazy’. Because grafted seedlings have an associated increase in cost and do not produce increased yields, grafting in these optimized farming systems and using fumigated soils does not offer an advantage in the absence of soilborne pathogens or other stressors that interfere with watermelon production.

Retention of Resistance to Fusarium oxysporum f. sp. niveum in Cucurbit Rootstocks Infected by Meloidogyne incognita.

Interspecific hybrid squash (Cucurbita maxima × C. moschata 'Strong Tosa') and bottle gourd (Lagenaria siceraria 'Macis') rootstocks are resistant to Fusarium oxysporum f. sp. niveum but susceptible to Meloidogyne incognita (Southern root-knot nematode). Coinfection of Early Prolific Straightneck summer squash (C. pepo) with root-knot nematode and F. oxysporum f. sp. niveum has been reported to increase susceptibility to Fusarium wilt. The objectives of this study were to determine whether such an interaction occurred between M. incognita and F. oxysporum f. sp. niveum races 1 and 2 on Strong Tosa, Macis, and watermelon cultivars Fascination (resistant to race 1) and Tri-X 313 (susceptible to both races). Hosts were inoculated in a greenhouse with one of four pathogen treatments: F. oxysporum f. sp. niveum, M. incognita, both pathogens, or neither pathogen. Galling was present on ≥10% of the root systems of 90% of the plants inoculated with M. incognita. Bottle gourd had less galling than interspecific hybrid squash. Plants not inoculated with F. oxysporum f. sp. niveum did not wilt. Four weeks after inoculation, incidence and severity of Fusarium wilt and recovery of F. oxysporum did not differ for any hosts inoculated with F. oxysporum f. sp. niveum alone and F. oxysporum f. sp. niveum plus M. incognita (host-treatment interactions not significant). In general, Early Prolific Straightneck grouped with the F. oxysporum f. sp. niveum-resistant rootstocks when inoculated with F. oxysporum f. sp. niveum race 2 and with the susceptible watermelon when inoculated with race 1, regardless of inoculation with M. incognita. Recovery of F. oxysporum from stems of inoculated watermelon was greater than recovery from the other three hosts, regardless of nematode inoculation. In conclusion, our experiments do not support the hypothesis that resistance to F. oxysporum f. sp. niveum in cucurbit rootstocks or resistant watermelon cultivars would be compromised when M. incognita infects the roots.

Disinfectant Treatments That Reduce Transmission of Stagonosporopsis citrulli During Cucurbit Grafting.

Gummy stem blight can develop in greenhouses on cucurbit seedlings grown as scions and rootstocks for grafting. When diseased seedlings are cut during grafting, Stagonosporopsis spp., the fungal pathogens causing gummy stem blight, may be transferred to healthy seedlings. The objective of this study was to evaluate efficacy of disinfectant treatments to prevent or reduce transmission during cutting and grafting. A blade contaminated with mycelium and spores from a culture of S. citrulli transferred the pathogen to 72%, 73%, and 55% of the second, third, and fourth seedlings, respectively, cut in sequence when 100% of the first seedlings cut were infected. Kleengro (didecyl dimethyl ammonium chloride), Physan 20 (dimethyl benzyl ammonium chloride), and Virkon S (potassium peroxymonosulfate) were ineffective when sprayed onto watermelon seedlings before or after cutting hypocotyls with a contaminated blade. Dipping a contaminated blade in 0.3% sodium hypochlorite, 0.4% Physan 20, or 70% or 95% ethanol before cutting watermelon hypocotyls significantly (P = 0.01) reduced incidence of gummy stem blight compared with water but did not prevent transmission. Soaking a contaminated blade for 3 s in 95% ethanol, 30 s in 0.8% sodium hypochlorite, or flaming the blade after dipping in 95% ethanol, prevented transmission. In a grafting experiment, both the watermelon scion and the interspecific hybrid squash rootstock were cut with contaminated or contaminated and treated blades before grafting. Sterilizing a contaminated blade by flaming significantly (P = 0.01) reduced incidence of gummy stem when compared with a 1-s dip in ethanol, a 1-s dip in Physan 20, or water. Disease incidences in these treatments were 11%, 45%, 100%, and 100%, respectively. Using heat, ethanol, or sodium hypochlorite to disinfest cutting tools may reduce transmission of S. citrulli during cucurbit grafting.

Grafting of Romanian Melons and Watermelons for Culture from South Area of Romania

The vegetable grafting is useful in Romania; it is more difficult in watermelons and melons and it is continuously developing. The research was aimed the establishing of the technological stages for seedling producing of scions (Romanian melons and watermelons) and rootstocks (F1 hybrids of Lagenaria siceraria and Cucurbita maxima x C. moschata) for obtaining of grafted plant seedlings. The experience was realized out on a collection consisting from two Romanian scions, melon (‘Fondant’ variety) and watermelon (‘Dochiţa’ variety) obtained at Research and Development Station for Vegetable Growing Buzău and two rootstocks, bottle gourd - L. siceraria (‘Emphasis’ F1) and interspecific hybrid squash - C. maxima x C. moschata (‘Cobalt’ F1). The obtaining of scion and rootstock plants was made according to the ecological requirements of the species. The grafting was made by annexation (splice grafting). The plants had optimal diameters for splice grafting. Between scions (‘Fondant’ and ‘Dochiţa’) are no diference, statistical analysis could not be performed. Technological stages for producing grafted seedlings of Romanian melon and watermelon were established. The grafting was performed successfully for cucurbit symbiotes (scions and rootstocks). These technological stages for grafting by annexation of Romanian melons and watermelons are recommended for cultures in the south area of Romania.

Grafting with an Interspecific Hybrid Squash Rootstock Accelerated Fruit Development and Impaired Fruit Quality of Galia Melon

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.

Effects of Grafting Methods and Root Excision on Growth Characteristics of Grafted Muskmelon Plants

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.

Configuration of watermelon fruit quality inresponse to rootstock-mediated harvest maturity and postharvest storage.

The configuration of watermelon fruit quality was analysed in a multi-factorial approach accounting for the effects of grafting, harvest maturity and postharvest storage. Diploid, seeded, hybrid cv. Pegasus, cultivated as scion on interspecific hybrid squash rootstock TZ148 and as non-grafted control, was stored at 25 °C following sequential harvests from the onset of ripening to over-maturity. Delayed rootstock-mediated climax in pulp lycopene and chroma was observed, while both were heightened by postharvest storage when harvest preceded full maturity. Pulp firmness was increased by 46.5% on TZ148, while postharvest decrease in firmness was non-significant. Non-grafted fruits attained their peak in pulp carbohydrate content earlier during ripening. Monosaccharide content declined and sucrose content increased both preharvest and postharvest; overall sugar content declined by 4.3% during storage. Pulp acidity decreased steadily with ripening but was moderately increased by grafting. Citrulline content increased by 12.5% on TZ148; moreover, it climaxed with ripening and declined with storage only in grafted fruit. Grafting enhances pulp texture and bioactive composition. Potential suppression of sugar content as a result of grafting is minimized at full commercial maturity. Brief postharvest ambient storage enhances pulp lycopene and chroma, especially in early-picked fruit, notwithstanding the depletion of monosaccharides and citrulline and a limited deterioration of texture. © 2015 Society of Chemical Industry.

Suppression of Fusarium Wilt Caused by Fusarium oxysporum f. sp. niveum Race 2 on Grafted Triploid Watermelon.

Fusarium wilt of watermelon, caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. niveum race 2, is a serious, widespread disease present in major watermelon-growing regions of the United States and other countries. 'Fascination,' a high yielding triploid resistant to race 1, is grown in southeastern states in fields that contain a mixture of races 1 and 2. There is some benefit to using cultivars with race 1 resistance in such fields, even though Fascination is susceptible to Fusarium wilt caused by race 2. Experiments in 2012 and 2013 were done in fields infested primarily with race 2 and a mixture of races 1 and 2, respectively. Fascination was grafted onto four rootstock cultivars: bottle gourd (Lagenaria siceraria) 'Macis' and 'Emphasis' and interspecific hybrid squash (Cucurbita maxima× C. moschata) 'Strong Tosa' and 'Carnivor.' Nongrafted and self-grafted Fascination were used as susceptible control treatments. In both experiments, mean incidence of plants with symptoms of Fusarium wilt was ≥52% in the susceptible control treatments and ≤6% on the grafted rootstocks. Disease incidence did not differ between rootstock species or cultivars. In both years, Fascination grafted onto Strong Tosa and Macis produced more marketable-sized fruit than the susceptible control treatments. Grafted Emphasis and Carnivor also produced more fruit than the control treatments in 2012. The cucurbit rootstocks suppressed Fusarium wilt caused by race 2 and increased marketable yield of triploid watermelon grown in infested soil.

Improvement of Grafted Watermelon Transplant Survival as a Result of Size and Starch Increases Over Time Caused by Rootstock Fatty Alcohol Treatment: Part I

Application of fatty alcohol to rootstocks used for vegetable grafting has been shown to increase the efficiency of producing grafted transplants by controlling cucurbit (Cucurbitaceae) rootstock meristematic regrowth and by allowing the rootstocks to accumulate carbohydrates, especially starch, over time in the hypocotyl and cotyledon. A grafting experiment was conducted to determine the effect of increased carbohydrates on survival of watermelon (Citrullus lanatus) grafts using standard grafting procedures. ‘Carnivor’ interspecific hybrid squash (Cucurbita maxima × C. moschata) and ‘Macis’ bottle gourd (Lagenaria sicereria) rootstocks at 1, 7, 14, and 21 days after fatty alcohol treatment were grafted with ‘Tri-X 313’ seedless watermelon using the one-cotyledon method. Graft survival on ‘Macis’ rootstock was acceptable or significantly increased up to day 14, with a slight decrease at day 21. Graft survival on ‘Carnivor rootstock was also acceptable up to day 21, with a significant increase between days 1 and 7. The second experiment was conducted to determine whether the increased carbohydrates provide sufficient energy to successfully graft without the rootstock cotyledon, a method that has previously shown inconsistent results. Graft survival was improved by 90% using treated ‘Carnivor’ rootstock 7 days after fatty alcohol treatment and ‘Macis’ rootstock 14 days after fatty alcohol treatment. Adoption of the hypocotyl-only graft method in commercial production may increase efficiency by better using greenhouse space and could decrease disease probability by removing the cotyledons before grafting.

Improvement of Grafted Watermelon Transplant Survival as a Result of Size and Starch Increases Over Time Caused by Rootstock Fatty Alcohol Treatment: Part II

Fatty alcohol treatments can be used to eliminate the meristem of cucurbit (Cucurbitaceae) rootstocks, which prevents regrowth when grafting, but the effects of the treatment on the rootstock have not been documented. Two rootstock types, ‘Emphasis’ bottle gourd (Lagenaria siceraria) and ‘Carnivor’ interspecific hybrid squash (Cucurbita maxima × C. moschata) commonly used in watermelon (Citrullus lanatus) grafting significantly increased in cotyledon and hypocotyl size over 21 days after treatment (DAT) with a 6.25% fatty alcohol emulsion. There was a significant increase in total soluble sugar (glucose, sucrose, and fructose) content for each rootstock hypocotyl and cotyledon. Starch concentrations of hypocotyls and cotyledons also increased significantly in both rootstocks. This increase in stored energy could greatly increase the success rate of the grafting process. Increased rootstock energy reserves could overcome the need for keeping the rootstock cotyledon intact when grafting.

Fatty Alcohol Application to Control Meristematic Regrowth in Bottle Gourd and Interspecific Hybrid Squash Rootstocks Used for Grafting Watermelon

Application of fatty alcohol compounds to rootstock meristems can control rootstock meristematic regrowth, thus decreasing the cost of producing grafted watermelon transplants by reducing the labor. Eight rates of Fair 85® and Off-Shoot T®, two commercially available fatty alcohol compounds, were applied to the meristem region of bottle gourd (Lagenaria sicereria cv. Emphasis) and interspecific hybrid squash (Cucurbita maxima × Cucurbita moschata cv. Carnivor) rootstocks to determine the optimal application rate to control regrowth without damaging the remaining plant parts. A water-only control treatment was also included. Rootstock seedlings were rated for damage and regrowth on Days 1, 7, 14, and 21 after treatment. Damage increased and regrowth decreased with increasing rates of fatty alcohol compound. In addition, a significant compound-by-rate interaction indicated that inert ingredients in the fatty alcohol formulation have an effect on both damage and regrowth. The optimal treatment rate, e.g., providing at least 95% control of regrowth with less than 10% damage, was found to be between a 5% (Off-Shoot T®) and 6.25% (Fair 85®) fatty alcohol application. At the optimal treatment rate, no adverse effects to grafting success were observed in the grafting procedure.

Control of Fusarium Wilt of Watermelon by Grafting onto Bottlegourd or Interspecific Hybrid Squash Despite Colonization of Rootstocks by Fusarium

Grafting watermelon (Citrullus lanatus var. lanatus) onto rootstocks of interspecific hybrid squash (Cucurbita moschata × C. maxima), bottle gourd (Lagenaria siceraria), or citron (Citrullus lanatus var. citroides) has been used in Asia and Israel to mange Fusarium wilt of watermelon caused by Fusarium oxysporum f. sp. niveum. The objectives of this study were to determine the frequency of infection of six rootstocks by F. oxysporum f. sp. niveum races 1 and 2 and the field performance of grafted rootstocks in Charleston, SC. Grafted and nongrafted watermelon and rootstock plants were inoculated in the greenhouse with race 1, race 2, or water (the control treatment). With both races, the frequency of recovery of F. oxysporum from scion and rootstock portions of inoculated watermelon plants grafted onto 'Ojakkyo' citron was greater than from watermelon plants grafted onto 'Shintosa Camel' and 'Strong Tosa' interspecific hybrid squash, and from plants grafted onto 'Emphasis', 'Macis', and 'WMXP 3945' bottlegourd. For nongrafted plants inoculated with race 1, percent recovery also was greater from Ojakkyo than from interspecific hybrid squash and bottlegourd. For nongrafted plants inoculated with race 2, F. oxysporum was recovered from the base of ≥79% of all inoculated plants. More than two-thirds (15) of 21 isolates recovered from the tops or scions of inoculated plants were pathogenic on watermelon. In spring 2010 and 2011, the six rootstocks were grafted with seedless watermelon 'Tri-X 313', which is susceptible to both races, and transplanted in a field infested with races 1 and 2 of F. oxysporum f. sp. niveum. Disease incidence for nongrafted and self-grafted Tri-X 313 (the control treatments) and Tri-X 313 grafted onto Ojakkyo citron did not differ significantly. Grafted watermelon plants produced greater weights and numbers of fruit than plants of the two control treatments. Nonpathogenic isolates of F. oxysporum and isolates of F. oxysporum f. sp. niveum colonized interspecific hybrid squash, bottlegourd, and grafted watermelon. The rootstocks evaluated, however, restricted movement of F. oxysporum f. sp. niveum into the watermelon scion, suppressed wilt symptoms, and increased fruit yields in an infested field.