Grafting with resistant rootstocks offers one of the best methods to avoid soilborne diseases. Grafting involves combining a desirable scion (which is the fruit bearing portion of a grafted plant) with a rootstock which provides resistance to various soilborne pathogens. The scion is generally from a plant that produces highly desirable fruit. As well as managing soilborne diseases, grafting can influence vegetative growth and flowering; affect fruit ripening and fruit quality; enhance abiotic stress resistance; and enhance yield especially under low temperature conditions. At present, most research is being conducted on grafting tomato and watermelon.
The primary motive for grafting tomato and watermelon (and other cucurbits) is to manage soilborne pests and pathogens when genetic or chemical approaches for management of these diseases are not available. Grafting a susceptible scion onto a resistant rootstock can provide a resistant cultivar without the need to breed a resistant cultivar. Additionally, grafting allows a rapid response to new pathogens races, and, in the short-term, provides a less expensive and more flexible solution for controlling soilborne diseases than by breeding new, resistant cultivars.
Grafted transplants are more expensive than non-grafted transplants due to labor, material costs (grafting supplies, seed costs of rootstock and scion), and specialized facilities or structures. These specialized facilities can include healing chambers and trained personnel both to produce the grafted transplant and to care for it. Potential changes in fruit quality, which occur with some rootstocks, must also be considered. Some commercial transplant producers offer grafting services and with improved grafting techniques and mechanization, costs will begin to decrease. Another potential added cost with grafted plants can be from transplanting into areas with high-wind conditions. Grafted transplant can be more easily damaged by wind decreasing transplant survival rates by up to 60%. This resultant loss will greatly increase costs.
From its research beginnings in the 1920's, Cucurbit grafting in Asia has now become the predominantly practiced growing method; currently 95% of watermelons and Oriental melons are grafted in Japan, Korea, and Taiwan. Grafting has only recently been considered as a practice for Cucurbits in the United States for reasons described above. Grafting presents an option for soil-borne pathogen management for diseases such as Fusarium wilt, Monosporascus Vine Decline, Phytophthora blight, and Root-knot nematode. Additionally, grafting can enhance tolerance to abiotic stress; increase water and nutrient use efficiency; extend harvest periods; and improve fruit yield and quality in certain Cucurbits. There is a wide array of potential rootstocks: Lagenaria spp. (Bottled Gourd), interspecific squash hybrids, wild watermelon or melon. The rootstocks can produce vigorous plants with resistances to many soil-borne diseases.
Grafting Watermelons: Hole Insertion Grafting Method. Sow rootstock seeds into a square, 1.2 to 1.6 in. cell, 2-in deep. Sow scion seeds into a smaller cell size (0.4 to 0.8 in. 2-in deep). Trays sown with the rootstock(s) and triploid scions should be well watered but carefully monitored as overwatering can lead to poor germination. Maintain trays at 86°F. When both cotyledons and first true leaves start to develop, the rootstock plant is ready to graft (~7 to 10 day after sowing) depending on greenhouse conditions. Remove the growing point with a sharp probe, and then open a hole on the upper portion of the rootstock hypocotyl at an angle to penetrate the side of the hypocotyl. A bamboo needle, tooth pick, or 1.4-mm drill bit works best. Cut the scion at a 35° to 45° angle, both sides, on the hypocotyls. The scion is then inserted into the hole made into the rootstock. The cut surfaces are matched together and held with or without a grafting clip.
Transfer grafted plants into a humidity chamber or healing room. After the healing process is complete (~7 days), transfer the grafted plants into the greenhouse at 72 to 79°F until the scion is well connected with the rootstock. Transplants should not be older than 33 days before transplanting. For further details and step-by-step instructions, go to http://graftvegetables.org/.
There are 3 primary techniques used for grafting tomatoes, Tongue Approach Grafting, Cleft Grafting and Tube (or Clip) Grafting. Cleft grafting and tube grafting are similar in that the shoot of the fruit producing scion is completely cut off from its own roots and attached to the severed stem of the rootstock. The name 'Tube Grafting' originated because when the technique was first developed; a tube was used to attach the shoot to the root. Clips are now used to make this graft. Tube grafting is quicker and less complicated to do than cleft grafting because it only requires a single, straight cut on both the rootstock and scion. Also, because fewer intricate cuts are involved, this technique can be used on very small seedlings.
Grafting can be performed at various stages of seedling growth. Grafting at the 2-3 true leaf stage is common. With both cleft and tube grafting, the newly grafted plants must be protected from drying out until the graft union is healed. This usually involves covering the plants with a plastic cover or protecting them in a healing chamber where temperature and humidity can be regulated. Additionally, some method should be employed to reduce light intensity to the grafted plants for several days after the procedure.
It is critical to increase the humidity in the chamber to near 100% for the first two days. Humidity must then be reduced incrementally over the next 5 days to prohibit the formation of adventitious roots from the scion and to properly heal the graft. Tomato grafts heal quickly and the seedlings can begin to be acclimated back into the greenhouse after 4-5 days. With both cleft and tube grafting, it is important that the diameter of the cut ends (of the scion and the rootstock) match up perfectly. If the diameter does not match, the graft may not heal properly, if at all. Rootstock cultivars tend to have different growth habits than scion cultivars so it is important to grow a small amount of rootstock and scion seed at first to determine their growth rate relative to each other. Rootstock cultivars tend to be more vigorous than scion cultivars. Another critical factor is to cut rootstock seedlings below the cotyledons. If the cotyledons are left they will generate suckers that will compete with the scion requiring pruning. For further detail and step by step instructions, go to http://graftvegetables.org/.
Extension Specialist, ACES
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