Grafting Transplants

Grafting for Disease Resistance in Heirloom Tomatoes

Grafting is a simple technique that growers can use to increase soilborne disease resistance in tomatoes without chemical fumigants or pesticides.

Heirloom cultivars lack genetic Grafting has been used in the horticultural disease resistance and are particularly sus- industry for woody species, such as apples ceptible to epidemics in the field. Growers and grapes, for centuries. The first use of interested in this niche market, however, vegetable grafting to reduce soilborne dis- are not willing to give up the high-qual- ease occurred in the early 1900s to dimin-

ity fruit that they deliver. Grafting can be ish fusarium wilt on . More used to unite the soilborne disease resis- recently, this technique has been used in tance and enhanced vigor of hybrid to- plant production systems when the ge- mato cultivars with the high fruit quality netic trait for disease resistance in a crop of heirloom varieties. is closely linked to a negative quality, such

as small fruit size. For example, grafting is Grafting: An Old Technology often used to reduce bacterial wilt (caused with a New Technique by Ralstonia solanacearum) in tomatoes. Grafting to manage soilborne This disease complex is particularly dif- disease is a simple process. The below- ficult to manage due to its wide range of

ground portion of a plant—the root- hosts and its ability to persevere through stock—is chosen for its genetic ability to long crop rotations. In addition, tomato resist or tolerate soilborne disease. The cultivars that are resistant to bacterial wilt above-ground portion of the plant—the are typically not capable of producing scion—is chosen based on fruit quality. large, marketable fruit. The worldwide

“Japanese top-grafting” or “tube graft- use of grafting with resistant rootstock ing” is a new technique that is especially has significantly decreased bacterial wilt popular for tomato production in com- incidence while keeping fruit quality high, Distributed in furtherance mercial greenhouses worldwide because of the Acts of Congress of even in severely infested soils. May 8 and June 30, 1914. the process is very fast and large numbers North Carolina State Uni- of grafted seedlings can be managed eas- versity and North Carolina A&T State University com- ily throughout the healing process (Figure mit themselves to positive 1). Each seedling is severed just above action to secure equal the cotyledon. The above-ground portion opportunity regardless of race, color, creed, national (scion) of the heirloom variety is secured origin, religion, sex, age, or to the root system (rootstock) of the dis- disability. In addition, the two Universities welcome ease-resistant seedling. Once the grafted all persons without regard transplants heal, they can be planted in to sexual orientation. North Carolina State University, the field and managed according to the North Carolina A&T State grower’s production system. University, U.S. Department Although vegetable grafting is rela- of Agriculture, and local governments cooperating. tively new, it relies on an old principle. Figure 1. Silicon tube-shaped clips are used for “Japanese top grafting.”

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Benefits sistant rootstocks was four times that of the nongrafted Grafting can be used where growers need to increase susceptible plants. Grafting can be a valuable tool for soilborne disease resistance and keep fruit quality high eliminating bacterial wilt in tomato, pepper, and egg- to compete in fresh-produce markets. Grafted tomato plant production. transplants are able to combine the high-quality fruit Stress tolerance of heirloom cultivars with the disease resistance, stress tolerance, and vigor of modern rootstock cultivars. (For Grafting has been highly effective at producing plants further details on how grafting has been used to in- that can overcome abiotic stressors—the environmental crease crop production worldwide, see “Recommended stressors that can lead to decreased yields. More than Reading” at the end of this publication.) one-third of all irrigated land worldwide is affected by high salinity. Grafting with salt-tolerant rootstocks Disease resistance can be instrumental in decreasing yield losses. It has Researchers around the world have demonstrated that also been used to reduce the negative effects of excess grafting can be effective against a variety of soilborne moisture in the soil. Grafted plants also have shown fungal, bacterial, viral, and nematode diseases. It has effective tolerance to soil temperature extremes, and been used to eliminate verticillium and fusarium wilt grafting with certain rootstocks can allow the growing in tomato and cucurbit production systems in Japan, season to be extended in either direction. An extended Korea, and Greece. In New Zealand, it has been used season can help growers to raise the selling price of to reduce levels of corky root rot. In Morocco and their goods as well as add to annual and long-term eco- Greece, grafting is used to control root-knot nematodes nomic stability.

(Meloidogyne species) in both tomatoes and cucurbits. Increased productivity Researchers have proposed using grafted plants instead of methyl bromide to manage soilborne disease in these The use of grafted vegetables is associated with in- regions of the world. creased resistance to diseases, abiotic stressors, or both. Grafting has been essential in Asian horticultural But grafting can also increase yields without the pres- production for eliminating bacterial wilt incidence in ence of these stressors. For example, yields increased solanaceous crops, such as tomatoes and peppers. It has by as much as 106 percent with the use of certain root- also been used in tropical environments, like the South- stocks for production in Australia. Some east Asian kingdom of Brunei, where bacterial wilt inci- rootstock varieties have been bred specifically to be dence is so high that tomatoes cannot be planted unless used as rootstocks, such as the Maxifort rootstock used the soil is sterilized or resistant rootstocks are used. In in greenhouse tomato production systems. Use of vigor- India, wilt-resistant rootstocks were used in one experi- ous rootstock varieties can increase water and nutrient ment to reduce bacterial wilt in tomatoes. By the end uptake in grafted plants. Many growers worldwide are of the season, none of the control plants had survived utilizing these rootstocks to increase fruit yields, even while 100 percent of the grafted plants continued to pro- where little disease pressure is evident. duce. Furthermore, the yield of the tomatoes with re-

Table 1. Disease resistance and vigor of commercial tomato rootstock cultivars. (Ratings are based on seed supplier information. Vigor is measured on a scale of 1 to 5, where 1 represents poor and 5 represents excellent.) Disease Resistance Tomato Verticillium Seed Supplier/ Mosaic Fusarium Wilt Wilt Bacterial Rootstock Virus Corky Root Race 1 Race 2 Race 1 Wilt Nematodes Vigor deRuiter Seeds Maxifort High High Moderate High High Susceptible High 5 Beaufort High High Moderate High High Susceptible High 3 Takii Seeds Anchor-T High Susceptible High High High Moderate High 5 Survivor High Susceptible High High High Moderate High 5 Aegis High Moderate High High High Moderate High 4 Bruinsma Seeds Body High High Susceptible High High Susceptible High 5 Robusta High High Susceptible High High Susceptible Susceptible 3

2 Table 2. Traditional and 2005 international resistance codes for tomato cultivars

2005 International Scientific Name Common Name Traditional Code Code Tomato mosaic virus Tomato mosaic Tm ToMV Tomato spotted wilt virus Spotted wilt TSWV TSWV Ralstonia solanacearum Bacterial wilt R Rs Fusarium oxysporum f. sp. lycopersici Fusarium wilt (Races 0 & 1) FF or F2 Fol: 0, 1 Fusarium oxysporum f. sp. radicis-lycopersici Fusarium crown and root rot Fr For Pyrenochaeta lycosersici Corky root rot K Pt Verticillium albo-atrum Verticillium wilt V Va Verticillium dahliae Verticillium wilt V Vd Meloidogyne spp. Root-knot nematodes N Mj, Mi, Ma

Grafting Step-by-Step specifically for rootstock production, typical hybrids or Although grafting is a simple process, it requires careful other modern varieties may also be valuable as disease attention to rootstock selection, seeding dates, healing, resistant rootstocks for heirloom cultivars. The key to and planting in the field. To produce healthy transplants choosing the right rootstock is to understand how to from grafting, follow these steps. “break the tomato code.” As breeders have developed genetic lines with resistance, they have designated Choose the scion and rootstock cultivars international resistance codes (Table 2) that can be dis- Any cultivar that provides the desired fruit character- played on labels to identify a cultivar’s resistance to istics can be used to produce scions for grafting. For specific diseases. For example, a “Roma VdFol:1” va- heirloom tomato growers, popular scion choices in- riety is one that has been shown to resist race 1 of Ver- clude German Johnson, Cherokee Purple, and Kellogg’s ticillium dahliae (Vd) and race 1 of Fusarium oxysporum Breakfast. Select the rootstock based on its potential for f. sp. lycopersici (Fol:1), which are soilborne pathogenic resisting the soilborne diseases on your farm and its fungi. Many modern hybrids have been bred to resist ability to enhance marketable yield, even in the absence several pathogens. By using the resistance available in of disease pressure. Rootstock selection is probably the modern tomato breeds, growers may be able to grow single most important step in grafting heirloom toma- some of the tastiest heirloom varieties, even in severely toes for disease resistance. infested fields. To choose the right rootstock, identify the potential pathogens on your farm. Fields where solanaceous Construct a healing chamber crops (tomatoes, tobacco, potatoes, and peppers) have After the rootstock and scion are grafted together, the been grown often have recurring problems with bacte- resulting plants must reconnect vascular tissue so that rial wilt. This disease is widespread in Eastern North water and nutrients can be supplied to the scion. This Carolina, and many farmers have abandoned fields due process occurs in a chamber where humidity, light, and to total crop failures from bacterial wilt. In the Appala- temperature can be regulated. Although construction is chian growing regions, verticillium wilt causes severe relatively simple and inexpensive, proper placement on damage to tomato crops because the climate favors the a farm can be somewhat difficult. development of this pathogen in the soil. Root-knot While the grafts are healing in the chamber, they nematodes can be diagnosed through soil sampling pri- must receive 80 to 95 percent humidity, minimal direct or to planting, but occur most frequently in sandy soils. sunlight, and a temperature range between 70 and 80oF. Your county Extension agent can help you learn about Daily temperature variation must remain low, as ad- basic diagnosis so you can identify potential pathogens ditional stress can decrease grafting success. The best in your fields that can hinder productivity. place for a healing chamber would be indoors, in a Many Asian and European companies are releasing heated storage area or garage, where fluorescent lights varieties that have been bred for rootstock use. Sakata can be used during the final days of healing. Healing Seed Company and Takii Seeds (Japan) and Bruinsma chambers can also be maintained inside a greenhouse Seeds and deRuiter Seeds (the Netherlands) have de- during the spring and fall so long as sufficient shading veloped lines specifically for rootstock use, but their devices are set up to keep the grafts from being exposed distribution in the United States may be limited. Table to excessive heat inside the chamber. 1 lists some rootstock varieties that are available in the A simple healing chamber consists of a frame covered U.S. and their levels of disease resistance. by polyethylene sheeting, which keeps the humidity Although the ideal solution is to find varieties bred level high during the healing process. The floor of the

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Figure 2. A healing chamber for tomato grafts. Once grafts are placed in the chamber, black plastic is used to keep out all available sunlight. chamber should hold water. During the first days after and the temperature should be a constant 70 to grafting, an opaque covering is used to keep all light out 80oF. Use black plastic to block all available sun- of the chamber. light from entering the chamber until the leaves At N.C. State University, a number of chambers have of the newly grafted transplants attain normal been built for tomato grafting. One of the most success- turgor levels—until they no longer show signs ful chambers is the simplest and least expensive: of moisture stress.

1. Stretch a tarp or dense shade cloth above the Plant the seeds greenhouse bench to reduce the full sunlight in When planting the scion and rootstock seeds, use good the area where the healing chamber will reside. sanitation practices and a sterile, lightweight potting Be sure that the shaded area is much larger than mix. The grafting process and the subsequent heal- the chamber in order to provide reduced light ing time require that seeds be sown two weeks before levels throughout the day and reduce the risk typical, nongrafted transplant production begins. This of excessive heat from building up inside the allows the newly grafted seedlings to spend up to chamber. one week in the chamber followed by one week in the 2. Place a layer of plastic sheeting on the surface greenhouse to re-acclimate to normal light conditions of the bench so that the raised edges of the bench before they are put into the field. can provide a shallow pool of water on the cham- To achieve a successful graft, make sure that the ber floor. If a raised lip is not available to help rootstock and the scion stems are the same diameter. hold water in the chamber, shallow pans of water Because different varieties require different germina- can be distributed on the bench among the grafts. tion periods, seeding times may need to be altered The use of cool-water vaporizers is an excellent to grow different cultivars to the same size. Stagger way to increase the humidity within the chamber plantings to offset the effects of variable germination as long as it does not increase the internal tem- periods between rootstock and scion varieties. In many perature of the chamber. cases, rootstock varieties take two to five days longer to germinate than heirlooms. However, hybrid root- 3. Construct a frame using 1-inch polyvinyl chlo- stock varieties may germinate faster than heirlooms. ride (PVC) piping as illustrated in Figure 2. The Plant a few test seeds of each variety (both rootstock frame should have a peak to keep condensation and scion) early in the year to determine how long the from dripping onto the newly grafted transplants. germination period is for each in your greenhouse or 4. Cover the PVC frame with a layer of clear plas- propagation facility. If seedlings have already emerged tic so that the sides and ends can be easily pulled and either the rootstock or scion is much larger than its up to check on the grafts during healing. corresponding variety, decreasing the environmental temperature can help to slow the growth of the variety 5. Make sure humidity, light, and temperature that may be ahead of schedule. levels inside the chamber are constant before be- ginning the grafting procedure so that the grafts Choose the best time to graft will be placed into a well-functioning chamber. Tube grafting should be carried out when the seedlings As noted above, the relative humidity level have two to four true leaves and the stems are 1.5 to 2 should be high—between 80 and 95 percent, millimeters in diameter (Figure 3). For proper healing to

4 take place, the vascular degree of each angular cut does not have to be exact, tissue in the rootstock all cuts should be made at a consistent angle to pro- and scion must align vide more surface area for the vascular tissue to meet so that their tissues can and grow together. Other methods have attempted to easily grow together, increase this surface area even further by making V- forming a strong union shaped cuts, but the results are similar to those of the for water and nutrient tube grafting technique. Locate the graft union above uptake. Again, an es- the cotyledon to prevent adventitious roots from form- sential component for ing and leading to infection of the susceptible scion tis- grafting success is to sue (Figure 5). use rootstock and scion Once the transplants plants that have similar have been grafted, put stem diameters. them directly into the Grafting should take healing chamber and place when there is cover the chamber to little water stress upon prevent any light from the plants. Early in the reaching the plants.

morning or just after Figure 3. Seedlings are grafted dark are excellent times Monitor the healing when two to four leaves are process present. to graft as transpiration has typically slowed Immediately after graft- ing takes place, the to reduced levels. Additionally, the grafting process should be carried out indoors or under some sort of plants must form callus shading device. If daytime grafting is essential due to tissue and reconnect timing and labor concerns, move the plants to a shady vascular bundles to area in the morning before transpiration increases to provide the scion with nutrients and water. The prevent unwanted water stress during the process. purpose of the healing Make the grafts Figure 5. Adventitious roots from the scion could lead to chamber is to keep the Sanitation is extremely important during grafting. infection. scion from becoming Wash with anti-microbial soap, and use latex gloves water stressed during and sterile tools to reduce the exposure of the plant to this process. This can be accomplished by slowing the pathogenic bacteria, fungi, and viruses. transpiration stream—the movement of water from in- To make a graft, sever the bottom half of a rootstock side the plant tissue into the atmosphere. The best ways seedling from its top at a 45-degree angle, and sever to do this are to increase humidity, decrease light, and the top half of a scion seedling from its bottom at a 45- decrease temperature. By decreasing the temperature, degree angle. Attach the rootstock to the scion with a however, the development of newly forming callus tis- rubber or silicon clip (Figure 4). Although the specific sue may be hindered. Therefore, the keys to successful chamber operation are high humidity and no direct sunlight while the graft union is develop- ing—between two and four days after making the graft. The first major con- straint to proper graft healing is water stress. Because the scions have been physically sepa- rated from their root sys- tems, they tend to wilt, especially in the first few hours after grafting. By adjusting environmental conditions to keep the Figure 4. Rootstock and scion stems are severed at a 45-degree angle and united with a scions from wilting se- silicon clip.

5 ~ 5 weeks

Humidity 85-959%5%RHRH

Dark Full Sunlight Partial FullFSuullnSliugnhltight

7 days 7 days 7-10 days 2 to 5 days

Figure 6. A typical timeline for grafting. (Rivard and Louws)

verely, the grafts will have a better chance of survival. should return to normal turgor levels and display no Typically, a small amount of wilt during the first day of evidence of moisture stress. healing is acceptable. The high humidity level inside the Once the grafts reach normal turgor levels, light can healing chamber will help the scions regain sufficient be slowly introduced, and eventually humidity lev- turgor pressure and show little sign of wilting. Grafts els can be reduced over the following week. There is should be left in the chamber for the first two to four a danger that this process can be carried out too fast. days with absolutely no light and high humidity. After When light is first introduced back into the chamber, these first days, the successful grafts will become appar- it is best to simulate indirect light. If the chamber is in- ent as they will regain normal turgor levels while those doors, mount fluorescent lights above the chamber. In that were not successful will wilt permanently. the greenhouse, use a shading apparatus that will sig- Any movement that pulls the scion stem away from nificantly reduce the amount of sunlight without elimi- the rootstock will decrease the contact surface between nating it altogether. As soon as the transplants have these tissues, ultimately diminishing graft success. Place the grafts into the chamber carefully. Inspect the grafts after placing them in the chamber to be sure that the sci- on has not been pulled away from the rootstock. Excess water on the leaves can also physically pull the scion away from the rootstock. For this reason, do not mist grafts once they are in the chamber. While the grafts are still weak and healing, apply water from the bottom so that the grafts are not subjected to damage from over- head irrigation.

Acclimate the grafts to normal conditions Throughout the healing process, open the chamber at least twice a day to replenish carbon dioxide for the grafts. After two to four days in the healing chamber

with absolutely no light and high humidity, the scions Figure 7. The clip falls after healing.

6 acclimated to the reduced light level with no signs of This is not a serious stress, gradually decrease humidity by removing pans problem. Move the of water or the cool-water vaporizer and lifting up the transplants into an en- sides of the chamber. vironment with lower Typically, the grafts will require two days at medium relative humidity, and light and humidity levels before they can be moved into the edema should go a low-humidity and high-light environment. Even after away. the plants have been moved out of the chamber and If the graft union into a standard greenhouse environment, it is best to did not maintain good water from the bottom to prevent any physical damage contact, it may become to the grafts while the graft union is still weak. Careful disfigured. This may overhead watering is possible as long as the seedlings cause problems in the are not exposed to high water pressure that could sepa- field if the union is not rate a scion from its rootstock. strong enough to hold As the grafted transplants develop, the grafting clip the weight of the vines. will expand with the growing stem, and eventually fall Avoid this problem by off (Figure 7). Once the grafted transplants have been in taking precautions dur- the greenhouse for five to seven days, they can be man- ing grafting, planting, aged and planted in the field or greenhouse similarly to and training. nongrafted transplants. Plant and monitor Figure 9. Transplant so the graft the transplants Avoid the pitfalls union is well above the soil line. Although grafting with resistant rootstock increases the Plant the transplants so disease resistance of plants in the field, the process may that the graft union is well above the soil line (Figure expose transplants to diseases, edema, and disfigure- 9). Many tomato growers hope to give transplants an ment, all of which could weaken them. advantage by burying transplants deep into the ground The wound created by cutting the stem provides with the stem bent. This practice is not recommended an easy entry point for bacteria, fungi, and viruses to with grafted transplants as susceptible scion tissue may be exposed to soilborne pathogens below ground. Monitor the grafted tomatoes once they are in the field to keep suckers from robbing the scion of water and nutrients. Suckers are branches that form below the graft union. Often they have a much different mor- phology than that of the scion. Fruit from the rootstock- borne sucker branches will have the genetic makeup of the rootstock rather than the scion. By removing these branches, the scion will be provided with more water and nutrients and increase production of the desired high-quality fruit.

Summary Grafting is a valuable disease management tactic for heirloom tomato growers. This practice originated as

a way to ensure fruit quality while keeping disease Figure 8. Edema on leaves. resistance high for melon production systems with soilborne disease pressure. This same principle lends invade the plant tissue. Therefore, keep the working itself well to heirloom tomato production. Furthermore, area, chamber, and all tools extremely clean during research worldwide has demonstrated increased yields this process. Use good sanitation practices to prevent from grafted vegetable plants in comparison to non- soil contamination, and use lightweight potting mixes grafted plants. As Southeastern growers realize its po- that reduce the moisture retention in the soil for proper tential benefits and relative ease, grafting will become grafted transplant production. a more popular disease management technique in the Edema, which appears as swollen areas of plant tis- United States. sue, may form on the leaves if the grafts are allowed to stay in the chamber too long (Figure 8). Edema is a physiological disorder caused by excessive humidity.

7 Recommended Reading Oda, M. 1995. New grafting method for fruit-bearing vegetables in Japan. Japan Agricultural Research Quarterly. 29: 187-194. Abdelhaq, H. 2004. Integrated production and protection in green- house crops. Extension Publication. Institut Agronomique et Oda, M. 1999. Grafting of vegetables to improve greenhouse pro- Vétérinaire Hassan II. Morocco. duction. Extension Bulletin (December). College of Agriculture,

Osaka Prefecture University. Japan: Osaka. Black, L. 2003. Grafting tomatoes for production in the hot-wet season. International Cooperators’ Guide. Asian Vegetable Re- Peregrine, W. 1982. Grafting - A simple technique for overcoming search & Development Center. Pub. no. 03-551, May. Republic bacterial wilt in tomato. Tropical Pest Management. 28:71-76 of China: Taiwan. Rivero, R. 2003. Role of grafting in horticultural plants under stress Bradley, J. 1968. Tomato grafting to control root diseases. New conditions. Food, Agriculture & Environment. 1:70-74. Zealand Journal of Agriculture. 116:26-27. Tikoo, S. 1979. Successful graft culture of tomato in bacterial wilt Giannakou, I., and D. Karpouzas. 2003. Evaluation of chemical and sick soils. Current Science. 48:259-260. integrated strategies as alternatives to methyl bromide for the control of root-knot nematodes in Greece. Pest Management Yetisir, H., and N. Sari. 2003. Effect of different rootstock on plant Science. 59:883-892. growth, yield, and quality of watermelon. Australian Journal of Experimental Agriculture. 43:1269-1274. Grimault, V., and P. Prior. 1994. Grafting tomato cultivars resistant of and susceptible to bacterial wilt–Analysis of resistance mechanisms. Journal of Phytopathology-Phytopathologische Acknowledgements Zeitschrift. 141:330-334. The authors wish to thank Dr. Mary Peet, Dr. Mike Benson,

Ioannou, N. 2001. Integrating soil solarization with grafting on Jim Driver, Bob Stuart, and the N.C. State University De- resistant rootstocks for management of soilborne pathogens partment of Plant Pathology for their assistance with this of . Journal of Horticultural Science & Biotechnology. publication. Cary Rivard provided the cover photograph of 76:396-401. a Cherokee Purple heirloom tomato and all other photo- graphs except Figure 8 (by Mary Peet). Lee, M. 1994. Cultivation of grafted vegetables. I. Current status, grafting methods, and benefits. HortScience. 29:235-239.

Prepared by Cary Rivard, Graduate Research Assistant Frank Louws, Ph.D., Associate Professor Department of Plant Pathology

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