by Sally Colby
The concept of grafting tomatoes is almost too simple: start with good rootstock and graft the scion of the desirable cultivar. The process requires some work but pays off in plants which are more disease-resistant than those that start and finish as one variety.
Although grafting tomatoes isn’t new, the practice has increased in popularity over the past 10 years. Frank Louws, director of plant pathology North Carolina State University, says vegetables were first grafted in Asia in the 1920s as a means of managing Fusarium wilt in melons.
Louws explains that grafting takes advantage of a strong root system. “What we look for in a root system is biotic tolerance [disease resistance to soil-borne pathogens] or abiotic tolerance [tolerance to extreme pH or other soil issues],” he said, adding that a good root system means enhanced water and nutrient uptake.
Grafting studies have been conducted on North Carolina research farms and with the help of producer cooperators, with a focus on managing soil-borne diseases including Verticillium wilt, Fusarium wilt, root knot nematodes, southern blight and bacterial wilt.
Dr. Cary Rivard, fruit and vegetable extension specialist at Kansas State University, says there’s no “recipe for success” in grafting because of the variations which occur in any biological system. However, there are some key practices that result in high success rates.
“When you start putting together a grafted transplant production system, rather than getting caught up in the details of slicing and dicing, it’s good to think of general principles of plant propagation that are important to ensure success,” said Rivard. “One is that you have to be able to produce a large number, so production is key — a high-throughput system like the tube-grafting method is helpful.”
Rivard outlines some of the keys to managing a grafting program, including aiming for uniform rootstock and scion on grafting day. “The other thing that’s important is once you sever these plants from their root systems, you have to learn to deal with water stress and managing that healing chamber,” he said. “One of the hardest things to manage is the healing chamber and keeping it from turning into an oven while making it a proper environment for success.”
Another important factor is sanitation throughout the process. “Bear in mind you will be putting plants into a healing chamber where the humidity will be raised and light reduced, and this is an excellent environment for disease,” said Rivard. “Make sure clips, cutting blades and hands are very clean, and use proper sanitation to disinfect everything.”
One of the cost and logistics factors in tomato grafting is centered on the need to grow two crops — the rootstock and the scion. Rivard cautions that seeding dates for the two crops may have to be staggered if the rootstock and scion grow at different rates. Plants are typically grafted when they are about three weeks old, then go into a healing chamber where relative humidity and light is manipulated to offset the need for a fully functional root system. Grafting typically adds about seven to 10 days to the production system. “If it takes five weeks to grow a non-grafted transplant,” said Rivard, “it’s going to take about six and a half to seven weeks to grow a grafted transplant.”
Rivard likes to sow into small seedling flats then transplant to plug trays. “This allows us to select plants that are more uniform,” he said. “If you have a rootstock or scion crop that’s getting away from you and growing too fast, you can use temperature to slow the crop down.”
Although the process isn’t difficult, Rivard compares the grafting procedure to surgery. Timing is a critical aspect of the process. “At three in the afternoon on a sunny day, the plant is transpiring very quickly and is generally under some water stress already,” he said. “One of the first things we do is take the plants out of the greenhouse, put them in a head house or some other shaded area where they can slow down transpiration and relax a little bit. I like to do the grafting procedure at night — that’s another good way to make sure plants aren’t water stressed before you put them through the procedure.”
Rivard says grafting indoors reduces plant stress. “The amount of water stress on the plant at the beginning of the procedure dictates how long it takes before the scion starts wilting,” he said, “so it’s nice to have plants indoors.”
Be sure to have a clean workspace with sanitized tools prepared ahead of time. Alcohol or a 10 percent bleach solution is a good sanitizer for razor blades and other grafting tools. It’s also important to have the healing chamber prepared ahead of time, with high relative humidity established prior to starting the procedure.
The angle of the cut is less important than consistency. “Generally, it’s about a 45 to 60-degree angle,” said Rivard, referring to the cut. “What’s really important is that it’s consistent between the rootstock and the scion. Making sure those two cuts are consistent will make sure those two plant parts come together within the silicon clip. Insert the scion into the clip, and it should slide in snugly.” Silicon clips are available in two sizes: 1.5 or 2 mm diameter. Rivard prefers to use the 2 mm clips because they fit well without damaging the plant. Various trials showed that grafting below the cotyledon results in fewer suckers in mature plants.
After grafting, plants go into the healing chamber during which numerous biological processes occur. Within the chamber, the plant’s physical environment is altered to offset the stress of the grafting procedure. Rivard has experimented with numerous chambers, many of which included cool mist vaporizers. “Make sure you are not using a warm water vaporizer to add humidity,” he said. “The other thing is not to use a hose and mister such as the kind that would be used for watering seedlings. Any kind of physical force will make the scions fall out of the clips. You want to add moisture as a vapor and not a spray.”
Rivard explains that while the plant is in the healing chamber, it’s reforming its plumbing tissue; reconnecting the vascular bundles within the stem. The healing chamber reduces water stress and makes it possible for the scion to survive without its root system for several days as it heals.
Black plastic or shadecloth can be used to exclude light if the healing chamber is constructed within an existing greenhouse structure. Since plants don’t require light the first few days after grafting, they can be stacked. “Make sure to be careful to watch during the healing chamber period,” said Rivard. “I like to check them at least twice a day. If I’m going to remove a piece of shadecloth, I’ll come back 45 minutes later to make sure it hasn’t heated up too much in the chamber. Keep in mind the activity happening in the chamber and be diligent in observing what’s going on.”
Newly grafted plants typically spend seven to ten days in the greenhouse to harden off. During this time, the clip opens up and falls off the plant. Clips can be reused if they are disinfected. Rivard cautions growers that planting death is possible after hardening off. “The upper portion, the scion, may still be susceptible to disease,” he said. “Make sure the graft union is as high above the soil as possible. You may see a few suckers come from the rootstock and this generally isn’t a problem. It’s important to graft below the cotyledon leaf on the rootstock plant. If you’re doing some suckering, catch them when doing other pruning.”
Rivard says he hasn’t had problems with plants failing at the graft union, but is careful to keep new grafts in the greenhouse for a week then in a cold frame for another week prior to planting.
Grafting tomatoes for disease resistance
by Sally Colby