Grafting is a useful technique when soil sterilization is not available or when certain diseases, e.g., fusarium crown and root rot, cannot be controlled by soil steaming. Wild species closely related to the tomato, or even tomato cultivars with resistance to a number of diseases, are used as rootstocks. Rootstocks are currently available with resistance to corky root rot, fusarium and verticillium wilt, root knot nematode, and fusarium crown and root rot. Figure 12 shows various grafting methods suitable for tomatoes.

All types of grafting require a sharp knife and a clean working surface so that cuts are not contaminated with soil; a razor blade or scalpel are ideal tools for grafting. Type A grafting is the fastest but is associated with the most check in the growth of the transplant. Type B grafting is also fast and results in a stable grafting union, but some check in the growth of the transplants can be found. Type C grafting is the slowest but is usually associated with the greatest success in grafting.

Because the germination of rootstock seed is slow and nonuniform, sow rootstock seed before the scion cultivar so that by grafting time both scion and rootstock are of similar size and stem thickness.

If the diseases to be controlled include fusarium wilt or verticillium wilt, remove the scion root system by cutting through the stem of the cultivar below the graft union. Gradually extend the cut through the stem of the scion over a period of 1 week, to minimize the wilting of the grafted plants; some misting during this time may also be needed to aid recovery.

Although grafting can be very helpful in saving soil sterilization costs and in allowing the cropping of cultivars that are productive but devoid of adequate disease resistance, it poses its own problems. The graft union is an inherited obstacle in the movement of water and nutrients from the roots to the top of plant and of photoassimilates from the top of the plant to the root. Grafting is therefore a potential limiting factor in maximizing yield.

Grafting also requires skilled labor, which is either expensive or not readily available. The difference in vigor between scion and rootstock can r result in significant differences in stem diameter (a minimum diameter of 3 mm is desirable), which slows down the speed of grafting and reduces the success rate.

Finally, the repeated handling of plants at grafting may help spread tobacco mosaic. It is therefore extremely important to make sure that the knife is cleaned regularly and that the hands are washed frequently with milk during grafting. Of course, every effort should always be made to eliminate tobacco mosaic virus by starting with heat-treated seed.

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