Grafting Tomatoes - August 6, 2008
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
Tomato aficionados relish the subtle qualities of their favorite heirloom tomato varieties such as ‘Cherokee Purple’, ‘Red Brandywine’, and ‘German Red Strawberry’. However, heirloom tomatoes often have lower resistance to soilborne diseases such as bacterial wilt, southern blight, fusarium, and root knot nematodes. Modern tomato breeding programs have increased the tomato plant’s tolerance of these diseases, but have also led to changes in flavor and texture of the fruit. Producers have started grafting heirloom tomatoes onto disease resistant rootstocks not only addressing the disease problem, but also increasing yield.
The most common method of tomato grafting is called “Japanese top-grafting” or “tube grafting”. This technique is very fast and large numbers of grafted seedlings can be easily managed during the healing process. Each seedling is severed just above the cotyledons with a sanitized single-edged razor blade. The above-ground portion (scion) of the heirloom variety is secured to the root system (rootstock) of the disease-resistant seedling. The attachment is done with small silicon clips which are available from Johnny’s Seeds (www.johnnyseeds.com) or Hydro-Gardens (www.hydro-gardens.com).
The grafted transplants are immediately placed in a healing chamber that is kept between 70 and 80 degrees F, at 80 – 95% humidity, and having minimal direct sunlight. A simple healing chamber consists of a frame covered by polyethylene sheeting, which keeps the humidity level high during the healing process. The floor of the chamber should hold water. Humidity can be maintained by using a cool-water vaporizer and air exchanged by opening the chamber to outside air twice per day. During the first days after grafting, an opaque covering is used to keep all light out of the chamber.
After the plants no longer show signs of wilting, the opaque covering can be removed. The plants should still not receive any direct sunlight. Irrigation is supplied by flooding the floor of the chamber. The healing process is critical and requires continued attention to heat, light and humidity. Typically, the grafts will require two days at medium light and humidity levels before they can be moved into a low-humidity and high-light environment: preferably a greenhouse. It is still best to water the plant from the bottom to prevent any physical damage to the graft union. Once the grafted transplants have recovered in the greenhouse for five to seven days, they can be transplanted to a larger pot or planted in the field or greenhouse.
When planting the transplants, make sure the graft union is well above the soil line. Many tomato growers hope to give transplants an advantage by burying transplants deep into the ground with the stem bent. This practice is not recommended with grafted transplants as the scion tissue may be exposed to soilborne pathogens below ground. Be sure to remove any suckers that form below the graft union.
Tomato grafting sounds easy, and, in general, it is. Here are a few additional pointers. Sanitize tools and the work area with isopropyl alcohol to minimize disease introduction. Select the heirloom tomato scion based on your personal preferences. Select the rootstock based on the necessary disease resistance and desired vigor. For outdoor tomato production in Arizona, we do not have high incidences of soilborne bacterial and fungal pathogens, but we do have root knot nematodes in some areas. Keep good records so that you can reproduce successes and troubleshoot failures.
Heirloom tomato seed is available through many sources but rootstock seed is more difficult to locate. For a tomato rootstock with resistance to soilborne disease and complete resistance to nematodes, try RST-04-105 from D. Palmer. Their website is: www.dpalmerseed.com.
Some readers may wonder if tomato grafting could be explored to alleviate our ever-challenging curly top virus (CTV) problems. It will only work if resistant CTV cultivars (Columbia, Row Pac, Roza and Salad Master) were used as scions. Heirloom varieties are not know to be resistant to CTV. Detailed instructions for tomato grafting, including an inexpensive healing chamber are available from the North Carolina State University website at: www4.ncsu.edu/~clrivard/TubeGraftingTechnique.pdf. If you try tomato grafting, please let me know how it works for you.
The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: July 30, 2008
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