for Higher Yields,
by Dorothy Noble
Steve Groff in Lancaster County, Pa., achieves consistently greater yields by grafting his high tunnel tomatoes.
Bret Fahrmeier harvests over 90 percent No. 1 fruit from his Cherokee Purple heirloom tomatoes at Fahrmeier Farms' high tunnels in Lexington, Mo. What's more, he says, "I've never seen 3 to 4-pound fruit from my regular [nongrafted] tomatoes," but his grafted Mr. Stripey and Great White heirlooms have produced 3.5-pound specimens.
Growers in Asia have been grafting vegetable transplants for well over 50 years. Although common for fruit trees and prevalent in greenhouse vegetables, relatively little grafting is performed for field and high tunnel vegetable production in the U. S.
Kansas State Extension Vegetable and Crop Specialist Dr. Cary L. Rivard began studying how grafting the heirloom tomato German Johnson to disease-resistant rootstocks managed soilborne diseases while a graduate assistant at North Carolina State in mid-2000. Following the favorable results, other studies and educational presentations ensued.
Rivard concludes, "Tomato grafting shows promise for growers who face disease challenges, specifically organic, heirloom and high tunnels."
The concept is simple. A disease-resistant seedling (the rootstock) is grafted onto a seedling with desirable fruiting characteristics (the scion). There are many ways to perform this fusion; Rivard uses the Japanese tube method, while Ohio State researchers prefer the cleft method. USDA has a robot for grafting.
Regardless of the grafting method, researchers agree that the next steps-rootstock choice, sanitation while handling and providing the desirable humidity, light and temperature to the seedling while healing-are critical.
Because hybrids bred specifically for rootstock use typically resist a wide range of diseases but vary in their resistance levels and in the diseases they resist, a grower must choose one that can control his particular disease situation. Rootstocks are currently available with resistance to tomato mosaic virus, corky root rot, fusarium races 1 and 2, verticillium wilt, root-knot nematode, bacterial wilt and Southern blight. In addition, rootstocks vary in vigor-overly rampant growth can be a problem in some environments. Also, a rootstock bred for tomato grafting is generally recommended. Seed companies are developing more rootstocks; many intended for greenhouse tomato grafting could work well in high tunnels.
Sowing the rootstock and scion seeds about two weeks prior to transplanting allows time for healing the union and acclimation. However, the rootstocks and the scion should be the same diameter for grafting, consequently the grower may need to adjust the growth of one or the other by manipulating temperature.
At North Carolina State, a team of three trained workers can graft 700 seedlings an hour, with a high success rate.
Rivard says tube grafting should be done when the seedlings have two to four true leaves and stems are 2 to 2.5 millimeters in diameter. Aim for early in the morning or just after dark to minimize the plant's water stress. Practice strict sanitation by washing hands, using latex gloves and sterilizing tools. Cut the rootstock seedling-razor blades are commonly used-at a 45-degree angle and below the cotyledon (the first leaves of the rootstocks). Slide a grafting clip over the rootstock. Matching the angle and the stem diameter, cut the scion. Insert the scion stem into the clip so that the cut angles fit.
Place the grafted seedling into a healing chamber immediately. Excluding all light for a few days, maintain 80 to 95 percent humidity and a temperature of 70 to 80 degrees Fahrenheit. Allow minimal sunlight for the next several days. After the seedlings show no signs of moisture stress in about a week, gradually acclimate them for transplanting as one would harden a greenhouse-grown seedling. When planting, be sure the graft union remains above the soil line.
Luis Urdaneta, while a visiting scholar to Penn State, displays a dome to be used as a healing chamber for newly grafted tomato seedlings. With proper care, the tomatoes on the right will cease drooping and be ready to transplant within a few weeks.
Working with Dr. Frank J. Louws and Dr. Mary M. Peet of North Carolina State, Rivard found that Beaufort, Big Power, Maxifort and RST-105 rootstocks reduced incidence of bacterial wilt, root-knot nematodes and Southern stem blight.
With continuous tomato production in Groff's high tunnels, Verticillium dahliae
became problematic. In a 1988 study sponsored by the Sustainable Agriculture Research and Education (SARE) program, BHN 589 was grafted onto Maxifort rootstock. The 20 percent marketable yield increase translated into 9.4 additional tons per acre, or 752 more boxes per acre. At $12 a box, this represents a $9,024 increase in gross income, or $1.88 per plant. Even accounting for the higher costs of grafted plants-some reports range from 50 cents to $1 per plant-grafting was profitable.
Groff has continued producing high tunnel tomatoes in the same location. Kaitlin Dye, who monitored the original study and now performs Groff's grafting, reports that the 2011 season's grafted crop, which were all heirloom tomatoes, performed better than the nongrafted plants. She uses 36-inch spacing on the heirlooms and encourages twin leaders, which has resulted in higher production. "The Beaufort rootstock can support two stems. I was very pleased with the yield," she observed, "And, the longevity of the plants was impressive."
After Fahrmeier Farms' yield increases were publicized, more Missouri farmers began grafting a year or two ago. Horticulture specialist Dr. Sanjun Gu, Lincoln University of Missouri, reports that most tomato growers increased their yield and vigor with grafting. He notes, though, "A few people failed because they did not remove suckers from rootstocks." That can be avoided in the future. Recent research by Dr. Michael Bausher, USDA, observed recurring suckering in open field grafted tomatoes when cotyledons had been left intact.
Rivard, Louws and others have noticed benefits of grafting beyond disease control, reporting in the SARE fact sheet, "Scientists have discovered that it can increase stress tolerance and productivity while maintaining high fruit quality. Using the right rootstock can also help overcome abiotic stressors, such as high salinity, excess moisture and soil temperature extremes, even allowing the extension of the growing season. In addition, grafted plants have produced increased yields and have shown increased water and nutrient uptake." This reference, published October 2011, includes rootstock resistances, instructions for building a healing chamber, grafting tips and resources and is accessible at www.sare.org/factsheet/12AGI2011
The University of Arizona's vegetable grafting information site, www.cals.arizona.edu/grafting
, presents comprehensive grafting instructions and resources. Horticulture engineer Dr. Chieri Kubota works with grafting other vegetables as well as tomatoes.
Ohio State's vegetable production systems laboratory has links to videos in English, Spanish and Chinese, resources including websites and publications, plus a complete and a condensed grafting guide. The electronic versions of the guides are free. The site can be accessed at www.hcs.osu.edu/vpslab
Ohio State geneticist Dr. David M. Francis directs a multiyear USDA-sponsored organic transitions study to test how grafting can provide solutions to the stresses that limit field and high tunnel tomato production. Both he and Dr. Matthew D. Kleinhenz, along with Louws and Rivard, continue to conduct workshops, field days and webinars that instruct growers and other researchers to assess tomato grafting.
In October 2011, researchers around the world held a symposium on grafting in Viterbo, Italy. Factors affecting the quality of grafted transplants, rootstock breeding and grafting opportunities to enhance tomato flavor and health-promoting compounds were among the presentations. After peer review, the papers will be available on their site, www.grafting2011.com
By combining the traits of two plants, grafting opens the possibility of enjoying the best characteristics of both. At this time, use of tomato grafted plants is greatest in greenhouse and high tunnel production, but it is becoming more valuable for organic, sustainable and conventional open field production of any size. As techniques become more efficient and refined and more rootstocks, equipment and supplies become more widely available and known, fumigants become more restricted, and more commercial businesses supply grafted transplants, usage should grow.
Grower Bret Fahrmeier offers grafting advice to his fellow farmers: "Be smart about it. Do what you need to do to make your business profitable. Try it by starting small and becoming good at it."
The author is a writer/researcher currently residing in central Pennsylvania. Photos by Bob Ferguson.
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