Watermelon Grafting with Disease Resistant Rootstocks

By Kent Cushman, Southwest Florida Research and Education Center

Grafting of watermelon scions on squash, pumpkin, or bottle gourd ( Lagernaria spp. ) rootstocks is practiced many of the major watermelon production regions of the world (Choi et al. 2002; Lee 1994, 2003). The primary reason for grafting of vine crops is to provide protection against soil-borne diseases (Edelstein et al. 1999; Paplomatas et al. 2002), but some rootstocks have the added advantage of being resistant to nematodes, especially root-knot nematode Meloidogyne spp. Additional benefits of grafting include increased yield, increased fruit quality—especially flesh firmness, and more vigorous plants that can be grown using lower plant populations (Core 2005; Yetisir 2003). Disadvantages included increased production cost and altered horticultural characteristics of cultivars used as scions. Mechanical grafting aids are widely used in Korea and Japan and have the potential to greatly reduce costs (Lee 1994, 2003).

There is interest in Florida to explore the use of grafted watermelon plants for commercial production. Growers are concerned about replacing proven practices with new and costly technology, but seed companies and transplant production facilities are currently experimenting with grafted plants under Florida conditions. The following is a brief description, with images, of four grafting techniques used for vine crops and a summary of the advantages and disadvantages of each.

Splice Graft

Rootstock seedlings should have at least one true leaf and scion seedlings should have one or two true leaves (Fig. 1a).

With a single cut, remove one cotyledon with the growing point attached (Fig. 1b). It is important to remove the growing point and the cotyledon together so that the rootstock seedling is not able to grow a new shoot of its own after being grafted. This is one of the advantages of using this type of graft. It is also important, when removing the cotyledon and growing point together, not to remove too much tissue because the remaining cotyledon must be well attached to the stem of the seedling.

Cut the scion and match the two cut surfaces, rootstock and scion (Fig. 1c). Hold in place with a grafting clip (Figs. 1d and 1e). Place the grafted seedling in a chamber with high humidity at about 77 °F and discard the unused parts.

Advantages:

1) Simple technique, almost anyone can do this type of graft.

2) The only task after grafting is to remove the clip – no trimming of unwanted plant parts after healing of the graft union.

Disadvantages:

1) Requires careful control of humidity, light, and temperature after grafting. Can experience high loses due to poor environmental control and possible disease under high humidity conditions.

Side Graft

Rootstock seedlings should have at least one true leaf and scion seedlings should have one or two true leaves (Fig. 2a).

With a sharp knife or razor blade, cut a slit all the way through the stem of the rootstock (Fig. 2b). The cut doesn't need to be too long, just long enough to insert the scion.

Cut the scion and insert into the slit of the rootstock (Fig. 2c). Hold in place with a grafting clip (Fig. 2d). Place the grafted seedling in a chamber with high humidity at about 77 °F and discard the unused parts.

Advantages:

1) Simple technique.

Disadvantages:

1) Requires careful control of humidity, light, and temperature after grafting. Can experience high loses due to poor environmental control and possible disease under high humidity conditions.

2) After healing of graft union, requires removal of top portion of rootstock. This requires additional time and labor but allows scion alone to establish plant canopy.

Approach Graft

Rootstock and scion seedlings should have one or two true leaves (Fig. 3a).

With a sharp knife or razor blade, cut an angled slit half way through the stem of the rootstock and an oppositely angled slit half way through the stem of the scion (Fig. 3b). Match the slits so that they overlap and then seal with aluminum foil or specialty materials available for this purpose (Figs. 3c and 3d).

Place the grafted seedlings in a seedling tray with larger cell size than what they were grown in. Place root balls of both rootstock and scion together in the same cell and add potting media if needed to fill the larger cell. Return to greenhouse or other growing area. High humidity and low light is not necessary to ensure success with this type of graft.

Advantages:

1) Relatively simple technique.

2) High humidity and low light environment not required for successful healing of the graft union. A normal greenhouse environment is sufficient.

Disadvantages:

1) After healing of graft union, requires removal of top portion of rootstock about nine days after making graft. Also requires severing of scion roots after an additional two or three days. (Can then be planted to field in about three more days.)

Hole Insertion Graft

Rootstock seedlings should have one small true leaf and scion seedlings should have one or two true leaves (Fig. 4a).

With a pointed probe, remove from the rootstock the true leaf along with the growing point (Fig. 4b). It is important to remove all of the growing point to prevent future shoot growth of the rootstock. This is one of the advantages of this type of graft.

Use the probe to open a slit along one side on the upper portion of the rootstock's stem, where the stem connects to the cotyledons. Cut the scion and insert into the rootstock (Fig. 4c). Hold in place with a grafting clip (Fig. 4d). Place the grafted seedling in a chamber with high humidity at about 77 °F and discard the unused parts.

Advantages:

1) The only task after grafting is to remove the clip – no trimming of unwanted plant parts after healing of the graft union.

Disadvantages:

1) Requires slightly more skill than most other grafting techniques.

2) Requires careful control of humidity, light, and temperature after grafting. Can experience high loses due to poor environmental control and possible disease under high humidity conditions.

References

Choi DC, Kwon SW, Ko BR, Chou JS. 2002. Using chemical controls to inhibit axillary buds of Lagernaria rootstock for grafted watermelon ( Citrullus lanatus ). Acta Hort. 588:43-48.

Core J. 2005. Grafting watermelon onto squash or gourd rootstock makes firmer, healthier fruit. Agric. Res. July issue.

Edelstein M, Cohen R, Burger Y, Shriber S, Pivonia S, Shtienberg D. 1999. Integrated management of sudden wilt in melons, caused by Monosporascus cannonballus , using grafting and reduced rates of methyl bromide. Plant Disease 83:1142-1145.

Lee JM. 1994. Cultivation of grafted vegetables I. Current status, grafting methods, and benefits. HortScience 29:235-239.

Lee JM. 2003. Advances in vegetable grafting. Chronica Hort. 43:13-19.

Paplomatas EJ, Elena K, Tsagkarakou A, Perdikaris A. 2002. Control of Verticillium wilt of tomato and cucurbits through grafting of commercial varieties on resistant rootstocks. Acta Hort. 579:445-449.

Yetisir H, Sari N, Yucel S. 2003. Rootstock resistance to fusarium wilt and effect on watermelon fruit yield and quality. Phytoparasitica 31:1-7.