Jan 25, 2016
Project sharpens interest in benefits of vegetable grafting

A sharp focus on the future of vegetable plant grafting is enabling growers, researchers and industry affiliates to map strategy for an expansion of the promising technology in the United States.

Those efforts were on full display in December as the fourth National Vegetable Grafting Symposium was held in Grand Rapids, Michigan as a prelude to the Great Lakes Fruit, Vegetable and Farm Market EXPO.

grafted watermelon plant
A grafted watermelon plant.

It is part of an ongoing project supported by USDA’s Specialty Crops Research Initiative (SCRI).

“The goal is to bring people together from the industry,” said USDA-SCRI project director Frank Louws, a professor and director of the NSF-Center for Integrated Pest Management in the Department of Plant Pathology at North Carolina State University.

“We have what we call the chicken and the egg,” Louws said. “There are many growers who wanted to use grafted plants but didn’t have the availability of plants. And then, there were people who were interested in grafting plants but they weren’t sure if there was a market for those plants.

“A group of people across the United States came together and developed a proposal to USDA-SCRI,” he said. “What that allowed us to do is we hooked up with 54 partners that represent the spectrum of the grafting industry. We were able to come up with a plan of how we might address the opportunities that grafted plants present to the U.S.”

USDA is funding the project for four years, Louws said. “What was really nice about that is a lot of our private partners brought a lot of expertise and money to the table also to address the opportunities and challenges with grafting.”

Louws said the Grand Rapids’ gathering is “one of those symposiums that culminates the information that has been gained. What we wanted to do is make sure we didn’t reinvent the wheel so the symposium brings in international speakers. We have speakers from Israel, Mexico, Guatemala here, and we’ve had others from Asia, so it’s really productive to learn about those experiences.

“We also have researchers who talk about their experiences and the research-based information and then we have growers who talk about how grafting has helped them on their farm for disease management, increasing production or getting a value-added component to their fruiting vegetables,” Louws said.

“We really try to bring that research – Extension, education. Really, the symposium is about building the backbone of the grafting industry – forming relationships (with) seed companies, robotics companies, grafting nurseries and farmers that produce the grafted plants for fruiting vegetable production.”

Chieri Kubota, professor at the School of Plant Sciences, Agricultural and Biosystems Engineering at the University of Arizona, and a USDA SCRI project team member, presented an optimistic overview of vegetable grafting in the U.S.

“The industry status in the use and production of grafted plants in the U.S.
is very much dynamically changing right now, particularly the increase of grafted plants and production in use in open field,” she said. “Many years ago it was mainly for greenhouse growers using grafted plants, but now we can see many more open-field growers testing grafted plants – tomato, watermelon, melon and other crops, including eggplant.”

She said use of grafted watermelon plants in the spring season in Arizona has produced “very high yields – as much as 50 tons per acre with 1,800 plants per acre.”

Price point concerns

“It’s a very interesting time, and we all are thinking that researchers need to collaborate with industry and work together to introduce this environmentally friendly technology to the industry to support that,” Kubota said.

Kubota said some of the advantages of grafting include disease and pest resistance, “and also increasing vigor of the whole plant, and by doing so you can expect yield increase. And the grafting effect on fruit quality has brought increased fruit firmness compared to non-grafted plants.”

She said “one unique application in the processing tomato industry in California is grafting to increase vigor and resistance to low temperature in drought because they have to use marginal land for production in vegetables.”

Rapid escalation in the use of grafted plants has thus far been limited somewhat by the relatively steep cost of production for such systems, according to studies Kubota cited that indicated 20 percent to 30 percent increases in crop yield are required to cover the cost of grafted plants.

“The price point of grafted plants is relatively high at this point because traditionally it was used for high end production for greenhouse growers,” Kubota said. “However, we are expecting to lower the cost as more markets increase along with the effort by nurseries and the demand from growers.

“Right now, about 50 cents to $1 per plant is sort of the average cost range of grafted tomato plants and also watermelon,” she said. “We want to see a little bit
lower cost, and probably introduction of mechanization in the nursery, and also better management of labor can contribute to reducing the cost further down.”

Richard Hassell, associate professor at Clemson University in Charleston,
South Carolina, is a member of the SCRI project team. He presented an overview
of propagation technology developments, including the use of a fatty alcohol rootstock treatment seen as a new approach to cucurbit grafting.

He said grafting has been used to manage serious soil-borne pathogens, including fusarium, verticillium, pseudomonas, monosporascus root rot and vine decline, macrophomina, other fungi, oomycete and bacterial pathogens, nematodes and viruses, Hassell said.

“Fusarium wilt is one of the most serious watermelon diseases, causing a great deal of damage and loss in the United States and worldwide; however, watermelon grafting has not been incorporated in United States production because of the increased cost of grafting,” Hassell said.

“As a contributor to the cost of grafting, rootstock re-growth is a major concern,” he said.

Occasionally termed “suckers,” the rootstock re-growth can decrease yield by robbing the scion of water and nutrients, and could result in scion abortion and graft failure.

“Grafting is very important for the future of cucurbits, especially watermelons in the United States,” Hassell said. “Fusarium is a soil-borne disease amongst other soil-borne diseases that we have no answer for except for the use of grafted plants. So it’s going to be the key in the future as far as things go right now.

Warding off re-growth

“We talk about the ways of reducing the re-growth and the problems we have with grafted plants and keeping those from forming any type of re-growth,” Hassell said. “We developed a method using a fatty alcohol that actually destroys the meristem tissue and eliminates any chance of re- growth. That’s important for anybody that produces grafted plants to keep them from shipping any plants that by chance would have re-growth or growing back to the original rootstock material because they are very expensive. Grafted transplants for cucurbits are going to cost about a $1.30 a plant so that when you ship that and it ends up having re-growth then you have a useless squash out there that just cost you $1.30 to transplant out there. So you want to make sure you are producing a watermelon transplant when you’re shipping it to the producer, so we’ve also developed that method.

“Fatty alcohol applications can successfully control rootstock meristem growth, and by decreasing the labor required, would alleviate the high cost of grafted transplant production,” Hassell said.

This new graft method could increase graft efficiency by allowing for the use of a smaller cell size in production, and could decrease disease incidences by removing the large rootstock cotyledons, he said.

“Using fatty alcohol as a rootstock treatment can improve the efficiency of grafting by decreasing the labor required in producing grafted watermelon, as well as increasing graft survivability and making a new grafting method available.”

Hassell pointed to research investigating the use of a healing chamber, a small covered structure that maximizes humidity and reduces light to allow grafted plants to heal. The primary purpose of the healing chamber is to minimize transpiration (water loss) from the scion. The size and design of the healing chamber depends on the scale of production of grafted plants.

“We’re also looking at healing chambers and the proper use of a healing chamber,” he said. “Do we need light? Do we need heat? How much light do we need? Do we need humidity, and how to control that humidity? We’re using a dry fog system that keeps the humidity up but doesn’t wet the disease surface, which causes disease in the healing chamber.

“We’re also looking at breeding material because right now we’re using interspecific hybrid squash as the main rootstock and that rootstock happens to be very susceptible to root knot nematode, and that’s also a major problem in the U.S.,” Hassell said. “So we’re also looking at rootstocks that are resistant to root knot nematode as well.

“We’ve been able to identify some of those with the help of USDA in Charleston, where I work, and we’ve been able to come up with a rootstock that we’re going to be able to test this year that is actually a cross of two citroides – wild watermelon – that has both fusarium resistance and root knot nematode resistance,” Hassell said.

For more information on the USDA-SCRI vegetable grafting project visit www.vegetablegrafting.org.

Gary Pullano, Associate Editor


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