Vegetable Grafting for Improved Disease Resistance/Tolerance
Grafting has been used for thousands of years to propagate and improve tree crops. For vegetable crops this technique is relatively new. Use of grafting for disease management in vegetable production was first introduced about 90 years ago in watermelon to help manage fusarium wilt, said Dr. Xin Zhao, an Associate Professor at the Horticultural Sciences Department of the University of Florida. Vegetable grafting has been used extensively in Asian countries such as China, Japan, and South Korea; primarily employed to help manage soilborne diseases in intensive cultivation systems.
Interest in vegetable grafting is growing in the U.S. in recent years as more farmers are now looking for management alternatives to soil fumigation and integrated approaches for soil-borne disease management. There is also interest from organic growers who have limited disease management tools and from growers using heirloom or specialty cultivars that lack a good disease resistance package. Dr. Zhao has carried out research on vegetable grafting since 2008; specifically on tomatoes, melons, and watermelons. Grafting can be used for other vegetable species, but due to its cost it has mainly been used in high-value solanaceous and cucurbit crops.
The main benefit sought by grafting is to assist with managing soil-borne diseases. For example, fusarium wilt, fusarium crown and root rot, verticillium wilt, southern blight, bacterial wilt, and root-knot nematodes can be effectively controlled by using grafted plants with resistant/tolerant rootstocks in tomato production. In her research, Dr. Zhao has seen significant yield differences between grafted and non-grafted plants where disease pressure is high. In watermelons for example, grafting has been shown to successfully control fusarium wilt. In other experiments, grafting helped heirloom tomatoes overcome root-knot nematode infestation and reduced yield loss to fusarium wilt in production of specialty tomato cultivars.
In addition to helping with soil-borne disease management, many rootstocks have shown advantages for improving plan growth and enhancing nutrient and water uptake. Grafted plants with selected rootstocks have been demonstrated to better handle abiotic stress conditions such as cold and flooding. Research has shown that grafted tomatoes can produce over 25% higher yield than non-grafted tomatoes in field production even under low disease pressure. Dr. Zhao’s research also showed improved nitrogen fertilizer use efficiency and irrigation water use efficiency by using grafted plants with selected rootstocks in tomato production.
Grafting Is Considered a Management Tool
“Don’t see grafting as a silver bullet; it needs to be integrated into your production system.” said Dr. Zhao. For example, using resistant cultivars, crop rotation, cover crops, organic amendment, and other cultural practices and physical and biological approaches can help manage diseases in addition to chemical means. Grafting is not perfect, there’s not a “super-rootstock,” she said. The onsite disease problem need to be clearly identified in order to choose the appropriate rootstocks with the targeted disease resistance package for effective management of the disease. In general, grafting is used for controlling soil-borne diseases and has little influence on foliar diseases, and therefore scion cultivars with good foliar disease resistance package need to be combined with resistant rootstocks to optimize the performance of grafted plants. Dr. Zhao pointed out the importance of using grafting as an integrative tool for solving site-specific production issues, “You need to know what your objective is when using grafting to make it beneficial to your production operation.”
The Process of Grafting
We focus a lot on the physical grafting process, but the healing process is just as critical, said Dr. Zhao. Grafting means that the plants have just had undergone wounding and need the appropriate conditions to recover. In the period of 7-10 days following grafting, you need to carefully provide the proper conditions for healing of seedlings, i.e., making vascular collections between scion and rootstock plants. Relatively high humidity and low light are generally needed for the first 3-4 days after grafting to minimize water loss from plants. The environmental conditions can then be adjusted during graft healing to gradually expose grafted seedlings to normal growing conditions, Dr. Zhao said.
The Cost of Grafting
“It all comes down to whether or not this makes economic sense for the growers.” If you have a small farm that produces transplants onsite you are in the best position to do grafting on your own, and that would reduce your costs, said Dr. Zhao. The price of grafted transplants can be 3-4 times higher than the price of non-grafted transplants. This price can go up depending on the rootstock you are using, the size of the transplants, and other variables. Several seed companies in the U.S. now carry rootstock seeds and some of them also sell grafted transplants. Some nurseries in Canada have been supplying large quantities of grafted vegetable transplants to growers in the U.S., while more nurseries in the U.S. are recently getting into the grafting business including the newly established Tri-Hishtil in NC. Currently, Dr. Zhao is exploring the feasibility of increasing plant spacing to reduce grafted transplant costs while maintaining and improving fruit yield. She is also working with economists to seek solutions to provide assistance to growers with decision making for optimizing economic returns in grafted vegetable production.
Interested? Learn More
Once you identify the diseases that you want to target, then you have to look for rootstock that will match what you are looking for. Research-based information on the preparation, use, evaluation, and purchase of grafted vegetable plants can be found at: http://vegetablegrafting.org. This vegetable grafting website came out of a multi-institutional project on vegetable grafting funded by the USDA Specialty Crop Research Initiative that Dr. Zhao is involved in.
Dr. Zhao is going to provide a hands-on demonstration on how to graft tomato plants at the Panhandle Fruit & Vegetable Conference on October 11 in Marianna, Florida. You will be able to do your own transplant grafting under her guidance. “You can do this on your own and also get good at it but it requires practice,” she said.
To register and find out more information about the conference visit: https://pfvc.eventbrite.com. Early bird registration is $40 and ends September 5.
Dr. Zhao’s vegetable grafting research has been possible thanks to the support of University of Florida IFAS Research Innovation Grant, USDA- Specialty Crop Research Initiative and Southern Region Sustainable Agriculture Research and Education (SARE), and Florida Department of Agriculture and Consumer Services – Specialty Crop Block Grant Programs.
Photo Credit: Xin Zhao