Graft in the Tomato Patch

Heirloom tomato varieties, such as Brandywine, Cherokee Purple, and Mortgage Lifter, are known for the quality of the fruit they produce, particularly the flavor. This is what makes them popular with gardeners. The problem with heirlooms is that they are prone to soil-borne diseases that inhibit production. The common problems are fusarium wilt, verticilium wilt, bacterial wilt, tomato mosaic virus and root-knot nematodes. These microbes are either taken up by the roots of the tomato plant and cause the plant’s demise through clogging of its vascular system, or they directly damage the roots. If water and nutrients cannot be transported from the roots to the leaves, the plant cannot produce sugars through photosynthesis. Eventually, it will wilt and die.

Modern hybrid tomatoes have been bred for resistance to soil pathogens but often lack the flavor of the heirloom varieties. What if we could combine the best characteristics of the heirlooms – great tasting tomatoes – with the disease resistance of the hybrids? We would really have something, no?

For centuries, growers have grafted the tops of woody plants such as apple trees to vigorous rootstocks. Seldom is an apple produced anywhere in the world that is not from a grafted tree. Early in the 20th century, horticulturalists in Asia began to graft watermelon plants with some success. Over the years, they learned how to graft other cucurbits and eventually solanaceous plants such eggplants, peppers and tomatoes. In Japan and Korea, this has become a mainstream farming technique, particularly for crops that are grown in greenhouses.

In recent years, grafting vegetable plants has become popular in European commercial horticulture, but it has been slow to take hold in the United States. Do commercial tomato growers have a less severe problem with soil-borne pathogens than farmers in other parts of the world? The answer is no. In the U.S., however, farmers still use methyl bromide gas to combat these diseases. Methyl bromide is a soil fumigant that was banned by international agreement in 2005 because it has a similar effect on the ozone layer as the old Freon that was commonly used in air-conditioning systems up to a few years ago. Our U.S. Environmental Protection Agency continues to grant agricultural exceptions to the methyl bromide ban because no effective method to control soil pathogens has been developed. Other countries have not been so generous in granting exceptions to their farmers. Therefore, the farmers have embraced the more expensive grafting approach.

Now grafted tomato plants are available to everyone — commercial farmers and backyard gardeners alike — from such vendors as Burpee, Territorial Seeds, and Johnny’s. Expect to pay a whopping $8 to $10 per grafted seedling. (That is not a misprint; they are very dear.) With a small investment in tools and a steady hand, you can make the grafted plants yourself. Johnny’s and other sources sell the tools inexpensively. Grafting instructions are widely available; but beginners should expect a high percentage of failures at the outset.

Why should you care?

If you have a small garden and cannot do the recommended five-year or three-year crop rotations (See fairfaxgardening.org vegetable gardening), you might be interested in using grafted tomatoes. Although some garden writers such as Barbara Damrosch have written skeptically about grafted tomatoes, some extension services have verified that grafted heirloom tomato plants produce significantly higher yields of tomatoes than the straight heirlooms. Noted heirloom-tomato expert and author Craig LeHoullier calls grafted tomatoes, “promising.” The grafted plants are also better at using water so they do not need to be watered as often. In other cases, depending on the rootstock, they extend the growing season because the roots are not as sensitive to extremes of heat and cold.

Are grafted tomatoes for you?

Grafted tomatoes are in no way a panacea for all tomato problems. Grafted tomato plants remedy nasty problems such as fusarium wilt, verticilium wilt, tomato mosaic virus, root-knot nematodes, and bacterial wilt. However, they are no more resistant to leaf diseases such as septoria leaf spot, early blight, or late blight than regular tomato plants. And, as noted above, they are expensive.

References
Grafting for Disease Resistance in Heirloom Tomatoes, NC State University Cooperative Extension
Epic Tomatoes, by Craig LeHoullier
In Defense of Grafting, Anne Raver, NY Times, May 29, 2013