Integrated Use of Grafting Technology to Improve Disease Resistance and Fruit Yield in Specialty Melon Production

Integrated Use of Grafting Technology to Improve Disease Resistance and Fruit Yield in Specialty Melon Production

Summary

Studies were conducted to identify molecular markers linked to Fusarium wilt resistance, Alternaria leaf blight resistance, and sulfur resistance in melon. Melon rootstocks for root-knot nematode control were evaluated in an on-farm trial. The rootstock fatty alcohol treatments were further assessed for improving grafting efficiency in cucurbits. Greenhouse experiments were also performed to determine the effects of different grafting methods and root excision on growth characteristics of grafted muskmelon plants. Using the grafting combination of galia melon and interspecific hybrid squash rootstock, a field study was carried out to explore the potential factors that may lead to reduced fruit quality.  

Objectives/Performance Targets

Objective 1. Identify effective rootstocks for managing Fusarium wilt and root-knot nematodes in grafted specialty melon production in the Southeastern US.   

Objective 2. Examine new grafting methods to reduce labor and increase production efficiency.

Objective 3. Assess the growth promotion, yield increase, and fruit quality in grafted melon production beyond disease resistance. 

Objective 4. Develop education and outreach programs on integrated use of grafting in sustainable production of specialty melon.

Accomplishments/Milestones

Studies were conducted to identify the quantitative trait loci (QTLs) in Fusarium oxysporum f. sp. melonis Race 1 and Race 2 resistant lines as well as Alternaria leaf blight resistant lines from a recombinant inbred line derived from the multi-resistant melon ‘MR-1’ and the susceptible ‘Ananas Yokneum’. Elemental sulfur is effective for controlling some important foliar diseases in cucurbits, whereas severe phytotoxicity may be developed. QTLs were also further identified in sulfur resistant lines from a recombinant inbred line derived from the sulfur susceptible ‘MR-1’ and the sulfur resistant ‘Ananas Yokneum’ melon lines.
  

A rootstock evaluation study for root-knot nematode control was carried out at the Possum Hollow Farm, Alachua, FL. Galia melon ‘Arava’ was grafted onto two interspecific hybrid squash rootstocks ‘Strong Tosa’ and ‘Tetsukabuto’ and the Cucumis metulifer rootstocks. The root gall index was significantly lower in grafted ‘Arava’ with Cucumis metulifer and ‘Strong Tosa’ rootstocks than that of non-grafted and ‘Tetsukabuto’-grafted ‘Arava’ (P < 0.0001), while the root-knot nematode population in soil was also reduced by 84% on average (P = 0.08). Although the total fruit yield seemed to be numerically higher in ‘Arava’ grafted onto ‘Strong Tosa’ and Cucumis metulifer rootstocks as compared with non-grafted ‘Arava’ and ‘Arava’ grafted onto ‘Tetsukabuto’, the difference was not statistically significant (P = 0.34).

Effects of rootstock age after fatty alcohol treatment on graft survival were examined. Graft survival using two different graft methods was recorded using rootstocks at 1, 7, 14, and 21 days after treatment. Graft survival using the one-cotyledon method, an industry standard, remained at or above 90% for 14 and 21 days after rootstock treatment for ‘Carnivor’ and ‘Emphasis’ rootstocks, respectively. Graft survival using the hypocotyl-only (non-cotyledon) graft method, one that has not yet been successful in industry practices, increased significantly at 7 and 14 days for ‘Carnivor’ and ‘Emphasis’ rootstock, respectively.   

A seed germination study was conducted to examine the root characteristics of different cucurbit rootstocks between germination and full emergence of the cotyledons. Five out of the seven rootstocks evaluated showed significantly higher root volume than that of the muskmelon control ‘Athena’. We also evaluated three grafting methods including hole-insertion (HI), one-cotyledon (OC), and non-cotyledon (NC) methods for their effects on seedling growth and root characteristics of ‘Athena’ grafted onto interspecific hybrid squash rootstock ‘Strong Tosa’. Non-grafted rootstock and scion plants were included as controls. Both grafted and non-grafted plants were examined with or without root excision. Root measurements were conducted every four days after grafting (DAG). At 16 DAG, root length and surface area and average root diameter were similar between re-rooted plants and plants without root excision. NC method resulted in a decrease in root length and surface area as well as shoot fresh and dry weights compared with HI and OC methods and non-grafted controls. Due to the reduced transplant quality, NC method was excluded from the following experiment. In the second part of the experiment, plants were transplanted into 3.8 L pots and grown in a greenhouse until anthesis of melon female flowers for assessment of plant growth characteristics. Flowering time did not differ between HI- and OC-grafted plants and non-grafted scion controls. Shoot fresh and dry weights, leaf area, chlorophyll content, and stomatal conductance were also similar between these treatments. Root excision did not impact aboveground growth characteristics. 

Using galia melon scion ‘Arava’ and the interspecific hybrid squash rootstock ‘Strong Tosa’, we found in the 2013 spring field trial that grafting with ‘Strong Tosa’ delayed the anthesis of female flowers by about 10 days but did not affect the harvest date compared with the non- and self-grafted ‘Arava’ plants. The early and total yields were not significantly different between grafted and non-grafted plants. ‘Arava’ plants grafted onto ‘Strong Tosa’ demonstrated stimulated fruit development with a shorter period between fruit set and fruit harvest at full slip stage in contrast to the non- and self-grafted plants. Vegetative growth was greater in plants grafted with ‘Strong Tosa’ at flowering and fruit expansion stages. During the harvest period, about 27% of ‘Strong Tosa’ grafted plants started to show non-pathogenic wilt symptoms, which eventually died at the end of the season. The collapse of grafted plants observed in this study deserves more in-depth research to understand the scion-rootstock interactions and the environmental impacts. 

We continued to host the interactive vegetable grafting exhibit at the Florida Small Farms and Alternative Enterprises Conference in 2013. 

Impacts and Contributions/Outcomes

Identifying the QTLs in resistant lines will help select and develop melon cultivars with resistance to soilborne and foliar diseases. As the increased cost associated with the increased amount of labor required for grafting still remains a major concern, more research is needed to improve efficiency of grafted melon transplant production with the ultimate goal of reducing cost. Rootstock fatty alcohol treatments can also improve the efficiency of grafting, as a result of the increased carbohydrates accumulated over time in treated rootstocks. Moreover, accumulated carbohydrates increase the window in which rootstocks can successfully be grafted. Increased carbohydrates also provide energy for successfully grafting below the rootstock cotyledons. This new method, which previously has not been a possibility in cucurbits because of the dependence of cucurbits on the energy provided by the cotyledon for growth, can now be used by commercial grafting operations to save space and decrease chances for disease by grafting below the cotyledons. Results from the examination of hole-insertion and one-cotyledon grafting methods together with root excision provided useful information on plant growth characteristics as affected by different grafting practices. Our field experiment elucidated the complex scion-rootstock interactions, suggesting the need for testing interspecific hybrid squash rootstocks in different types of melons with respect to their impacts on fruit development and quality. 

Our vegetable grafting exhibit attracted tremendous interest at the Florida Small Farms and Alternative Enterprises Conference. A diverse group of conference participants consisting of small growers, organic producers, extension agents, home gardeners, nursery managers, school teachers, and college students greatly enjoyed the hands-on demonstration to learn the basics of grafting practices. 

We have presented research results at several professional conferences such as the American Society for Horticultural Science Annual Conference and Vegetable Grafting Symposium at Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. Research manuscripts were also submitted for publication in peer-reviewed journals.

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