Organic Broadcaster

Research plots at an organic farm in North Carolina show how various rootstocks are faring in soil infested with a Sclerotium rolfsii. Celebrity was used as the scion variety. The Ohio State University and North Carolina University researchers collaborated in this on-farm research.

Grafting holds promise for vegetable production

By Matthew Kleinhenz, Ohio State University

Grafting woody plants to produce desired traits has become routine. The value for producing fruit is rarely questioned. The same is true of vegetable grafting in many other countries as well as in large portions of the North American greenhouse vegetable industry. A small, but growing number of vegetable farmers and researchers in the U.S. is looking at the practice to determine its best application in soil-based production (including sustainable-organic) of tomato, pepper, watermelon, cucumber, cantaloupe, and other crops.

As a production tool and source of income, grafting is regarded as an emerging, must-test technology capable of significant and timely outcomes for the U.S. vegetable industry. The reason for this is straightforward with a logic hinging on three points: grower need; shortcomings of traditional variety development; and, grafting’s ability to target specific traits.

First, growers will continue to rely heavily on the genetic makeup of their plants to provide abundant, high quality crops with ever-greater reliability and efficiency.

Second, using standard approaches, variety development teams will continue to fall just short of providing farmers with exactly what they need, at least in the short-term. Developing a range of individual farmer-friendly varieties—each containing all the desired traits—has been impossible so far. Variety development teams are exceptionally talented. However, no matter their ability, these teams are typically required to select one trait over another (i.e., to compromise), resulting in most varieties having at least one troublesome weakness.

Third, grafting dramatically alters the variety development and selection process, improving it for farmers. Scion (fruiting) and rootstock varieties are developed and selected separately, each to bring key attributes to the physical hybrid created by their being grafted. Breeders face fewer and less significant compromises since they are free to focus on scion and root system traits separately, not the dilemma of having to combine a full range of traits into one variety. Growers can select from an array of scion and rootstock varieties to create combinations meeting their specific needs.

Taken together, these points reveal why changing the variety development and selection process through grafting is speeding the delivery of a wider array of important, naturally occurring traits to farms, allowing farmers to make faster and more effective use of genetics.

Still, all is not perfect. Although some major benefits made available by using grafted plants have been demonstrated, evidence for others is promising but less convincing, so far.

Limiting damage caused by nematodes and soilborne diseases (e.g., Verticillium and Fusarium wilts) that attack the roots or lower stems of crops drives much of the current interest in grafting among tomato, pepper, watermelon, cucumber, and cantaloupe growers, grower-advisors, and investigators. That ability has been made clear through collaborative on-station and on-farm studies and farmer-led evaluations, with some conducted in organic systems. Findings are also summarized in an increasing number of extension/industry resources calibrated for local conditions. Indeed, farmers are rarely far from a university or industry professional with some grafting expertise. (See updates at www.vegetablegrafting.org/about/regional.) Regardless, for some farmers unable to rotate away from infested soils, change crops, or use another tactic, superior genetics represents an opportunity to maintain an income.

More effective nematode and disease management is responsible for much of the interest in vegetable grafting but it is not the only driver. Rootstock-scion combinations that: a) withstand abiotic stress (e.g., drought/flood, salinity, extreme temperature) more effectively; b) use inputs (e.g., land, water, nutrients) more efficiently; c) produce greater marketable yield under good or alternative (e.g., strip-till or no-till) production conditions; and/or d) improve fruit quality are also possible and a topic of study among farmers and investigators ­—again, with many evaluations completed in organic systems. Findings are shared in scientific articles (see www.vegetablegrafting.org/resources/reference-database) and in publications, field days, workshops, presentations, webinars, etc. (See www.vegetablegrafting.org/resources.)

Naturally, the opportunity to supply grafted plants to eager vegetable farmers (and gardeners!) has caught the attention of seedling/transplant growers and propagators. Demand for grafted plants in the U.S. currently exceeds the domestic supply, and appears to be increasing. To the extent that farmers and gardeners see the benefits of grafted plants, it will be necessary for someone to provide them, including certified-organic versions, in many rootstock-scion combinations and in varying numbers. Important advances in the process of preparing grafted vegetable plants have been made recently thanks to industry-university partnerships, and additional improvements are likely. Online or downloadable how-to guides (e.g., www.vegetablegrafting.org/resources/grafting-manual and u.osu.edu/vegprolab/research-areas/grafting-2), hands-on workshops and trainings, and other resources address many questions about making grafted plants. In the Midwest, experts including Drs. Wenjing Guan (Purdue), Ajay Nair (Iowa State University), and Cary Rivard (KSU) provide excellent research-based information on a range of grafting-related topics.

The overall expectation is that grafting will help more people make more money, especially by farming more sustainably and diversifying their product line. Skeptics claim that most of the money made on vegetable grafting currently goes to seed companies and propagators, given the costs of rootstock seed and grafted plants. Fair point but that is not the whole of it.

Careful, comprehensive analyses of “grafting economics” –the economic feasibility of producing grafted plants as a business and of using them in commercial fruit production – are complete and others are underway. It is clear that producing grafted plants is more costly than producing standard seedlings (transplants) and that increases in production costs depend on the number of plants produced. Also, most agree that the costs of producing grafted plants can be estimated more reliably than the economic benefits of using grafted plants in commercial fruit production, although yield increases exceeding 5-10% are common. Real benefits must be determined one farm, one season, one crop and, maybe, one field at a time. In one experiment focused on organic heirloom tomato production, economic risk of crop loss due to root-knot nematode damage exceeded the 46% higher cost of grafted transplants.

The consensus is that grafting economics are not settled and that they are complicated to evaluate but should always attempt to take all related costs and gains into account. Grafting economics appear to be personal (farm-/farmer-specific) in that they depend on individual valuations of the perceived gains and losses resulting from making and/or using grafted plants.

Anecdotally, over the years, in discussing grafted plants as a product (propagator point of view) and tool (fruit grower point of view) with farmers who have experimented with them well and/or analyzed reports about them carefully, it seems that for every four farmers I spoke with, two were generally positive, one was negative, and the fourth was unsure. This mixed perspective on grafting is consistent with the views of people currently most optimistic about the potential of grafting to benefit U.S. vegetable growers and gardeners, including organic. While use of grafted plants has risen sharply in the last 10-15 years, utilization has not reached its expected peak. Improvements in rootstock varieties, identifying optimal rootstock-scion combinations, and grafting methods and supply streams are required, along with better recommendations for using grafted plants more cost-effectively under more conditions on more farms and in more gardens.

Seed companies, grower and commodity organizations, private organizations and foundations, universities, USDA and SARE, and others have sponsored an impressive amount and range of studies and extension-outreach activities aimed at strengthening the utility of vegetable grafting. Consider completing your own fair and reliable evaluation, perhaps with extension or another trusted collaborator.

The consensus view is that grafted plants tend to offer vegetable growers in the U.S. the greatest return on investment when one or more of the following conditions are true:

1. Their operation is being hurt or is likely to be hurt by a damaging infestation of nematodes or outbreak of soilborne disease that can be tolerated by at least one rootstock variety.
2. Their preferred scion variety is susceptible to that nematode or disease and using another scion variety is unappealing or impossible (e.g., for market reasons).
3. The farm has a small land base and few other nematode/disease management options will bring the needed level of (economic) control.
4. Crop vigor and yield are very important, especially to enhance or maintain the farm’s presence in the market.
5. Single plantings will be picked as many times as conditions allow; in other words, total seasonal yield is important and the grower may be willing to tolerate having a smaller early yield in exchange for a greater total seasonal yield.

Learning more about vegetable grafting may be most relevant in the near-term for farms for which one or more of these conditions are true. However, grower experiences and research-based evidence suggest that learning more about this long-standing technology also would be useful for many others.

Matthew Kleinhenz is a professor in Horticulture and Crop Science at The Ohio State University.

 

 

From the November | December 2018 Issue

Comments are closed.