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 in Florida and South Carolina to continue the work on selecting potential rootstocks for controlling root-knot nematodes in melon production. The influence of nematode resistant Cucumis metulifer rootstock on fruit yield and quality of grafted specialty melons was also evaluated using different scion cultivars under both organic and conventional production. The scion-rootstock interaction effect was revealed. A new method of minimizing rootstock re-growth in melon grafting was developed to improve the grafted melon transplant production. 

Objectives/Performance Targets

Objective 1. Identify effective rootstocks for managing root-knot nematodes in grafted specialty melon production in the Southeastern U.S.   

Objective 2. Assess the growth promotion, yield increase, and fruit quality in grafted melon production beyond disease resistance. 

Objective 3. Examine grafting methods for specialty melon to improve survival rate and quality of grafted melon transplants. 

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

Accomplishments/Milestones

Ten selected C. metulifer (African horned cucumber) breeding lines were evaluated as rootstocks for grafted melon in a root-knot nematode-infested field at the U.S. Vegetable Laboratory in Charleston, SC.  All ten rootstocks were highly compatible with melon and nine of ten C. metulifer rootstocks were moderately to highly resistant to root-knot nematodes. 

Forty-one accessions of C. metulifer were evaluated for reaction to root-knot nematodes in a greenhouse test at the U.S. Vegetable Laboratory in Charleston, SC.  The most resistant accessions were selected for further evaluation as rootstocks for grafted melon. 

Resistance to Alternaria leaf spot (caused by Alternaria cucumerina) in melon was also studied. We have screened 275 random amplified polymorphic DNA (RAPD) primers, 117 High Frequency Sequence-Watermelon (HFSW) primers, and 256 High frequency Oligonucleotide-targeting active gene (HFO-TAG) primers using agarose gel fingerprinting techniques. Additionally we screened 392 modified HFO-TAG primers using an automated acrylamide gel fragment analysis system. All primers screened were tested against parental DNA from melon MR-1 and Ananas Yokneum. In total, we have identified 833 genetic polymorphisms between the two parental lines. These polymorphisms are being mapped to the melon genome using a recombinant inbred line of 94 individuals. Associations of the polymorphisms and resistance to Alternaria leaf spot are being identified. Additionally, an improved screening and phenotyping system is being developed for use in identifying resistance to A. cucumerina in melon. 

Field experiments were conducted at the University of Florida Plant Science Research and Education Unit in Citra, FL to further assess the root-knot nematode resistance, yield, and fruit quality of specialty melons grafted with C. metulifer. Honeydew melon ‘Honey Yellow’ (Cucumis melo var. inodorus) and galia melon ‘Arava’ (C. melo var. reticulatus) both susceptible to root-knot nematodes were grafted onto C. metulifer and grown in organic and non-fumigated conventional fields during March-June 2012. The organic plot was naturally infested by Meloidogyne javanica. Compared with non- and self-grafted plants, ‘Honey Yellow’ and ‘Arava’ grafted onto C. metulifer exhibited significantly lower gall ratings and reduced root-knot nematode population densities in the soil. However, total and marketable fruit yields were not significantly different from those of non- and self-grafted plants. There was a lack of root-knot nematode infestation in the conventional field plot where ‘Honey Yellow’ grafted onto C. metulifer showed a significantly lower total yield compared to non-grafted plants, whereas the fruit yield of ‘Arava’ was not affected by grafting with C. metulifer. Grafting with C. metulifer decreased the flesh firmness of ‘Arava’ in both organic and conventional fields and resulted in a reduction in total soluble solids content under conventional production. In contrast, C. metulifer did not exhibit any significant impacts on the fruit quality attributes of ‘Honey Yellow’. 

Consumer sensory analysis was also performed to compare melon fruit from grafted vs. non-grafted plants using a 1-9 hedonic scale (1 = dislike extremely, 9 = like extremely). Regardless of the production systems, ‘Arava’ grafted onto the interspecific hybrid squash rootstock ‘Strong Tosa’ (Cucurbita maxima × C. moschata) received significantly lower scores in consumer overall acceptability, flavor, and firmness liking compared to non-grafted ‘Arava’. Grafting with C. metulifer significantly decreased consumer overall acceptability and flavor liking for organically grown ‘Arava’ fruit, but the difference between grafted and non-grafted treatments was not detected in melons produced from the non-fumigated conventional filed. Different from ‘Arava’, grafting did not exhibit any significant effect on consumer perceived sensory attributes of ‘Honey Yellow’ melons.

Rootstock re-growth is a major problem in melon grafting, and the cost of re-growth control is a major reason for the lack of grafted transplants in U.S. melon production. Two experiments were conducted on a chemical method of re-growth control to 1) determine the optimal application rate and 2) determine the effect of time after application on rootstock size and carbohydrate content. In the first experiment, two fatty alcohol products (Fair 85^® and Off-Shoot T^®) at six concentrations (3.75, 5.0, 6.25, 7.5, 8.75, and 10% Fatty Alcohol) were applied to interspecific hybrid squash ‘Carnivor’ rootstock as the cotyledons unfolded. On days 1, 7, 14, and 21 after application, rootstocks were individually rated for both damage and re-growth responses. Results showed a significant decrease in re-growth as concentration increased up to 7.5% fatty alcohol, while damage increased significantly at fatty alcohol concentrations of 6.25% and above. Based on this data, we conclude that the best control of re-growth with a level of acceptable damage is achieved using an application rate between 6.25% and 7.5%, depending on environmental conditions within the greenhouse. In the second experiment, hypocotyl and cotyledons of both rootstocks were analyzed for size and carbohydrate content on days 1, 7, 14, and 21 after fatty alcohol treatment. Results showed significant increases over time for hypocotyl and cotyledon widths and dry and fresh weights, as well as cotyledon length and leaf area. No change was observed in cotyledon thickness or hypocotyl length. Total carbohydrates per rootstock increased significantly, with starch increasing most significantly. 

We hosted an interactive vegetable grafting exhibit at the Florida Small Farms and Alternative Enterprises Conference in 2012.  Over 50 participants visited our exhibit and many of them took this hands-on opportunity to learn specifics of the vegetable grafting technique by working with scion and rootstock plants we provided. 

Impacts and Contributions/Outcomes

Results from both greenhouse and field studies demonstrated the potential of using C. metulifer for grafting specialty melons for root-knot nematode management. Grafting of speciality melon on root-knot nematode-resistant C. metulifer rootstocks will be useful for managing root-knot nematodes in melons in the Southeastern U.S. Although the improvement of root-knot nematode resistance did not translate into yield enhancement, the reduction in soil root-knot nematode population densities could make grafting a viable rotational tool for organic specialty melon growers. More research is needed to better understand the scion-rootstock interaction effect on fruit quality. The influence of scion-rootstock interactions on fruit quality in relation to consumer perceived sensory properties of grafted melons deserves more comprehensive studies.

Fatty alcohol treatment can help decrease the cost of grafted melon transplant production by controlling rootstock re-growth and can further increase grafting efficiency by increasing the grafting window of rootstocks from 2 days to 3 weeks. The carbohydrate increase over time after treatment could also provide energy to allow for new, more efficient grafting methods.

Our vegetable grafting exhibit at the Florida Small Farms and Alternative Enterprises Conference was well received. Research results were presented at several professional conferences such as the American Society for Horticultural Science (ASHS) Annual Conference, Southern Region ASHS Annual Conference, and Vegetable Grafting Symposium at the Methyl Bromide Alternatives Conference. 

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