WOOSTER, Ohio – It's not uncommon to use grafting – the process of fusing the rootstock of one plant variety to the top, or scion, of another plant variety – to strengthen resistance against soil-borne diseases. But Ohio State University researchers have discovered that the method can also fight off foliar diseases.
Brian McSpadden-Gardener, a microbial ecologist with the Ohio Agricultural Research and Development Center, found that grafted tomato plants appear to be more resistant to foliar diseases than ungrafted tomato plants of the same genotype. McSpadden-Gardener's work is part of a larger three-year multi-state project that is exploring the use of grafting to improve tomato production, specifically in high tunnels and organic production.
"We chose tomato because it is susceptible to a number of field stresses, is very important economically, and responds to grafting," said Matt Kleinhenz, an Ohio State University Extension vegetable crops specialist.
OARDC researchers involved in the project include Kleinhenz, McSpadden-Gardener, geneticist David Francis, and plant pathologist Sally Miller. The group genetically bred 46 rootstocks and grafted them to the tops of two popular tomato cultivars: ‘Cherokee Purple' and ‘Celebrity'. They then compared field and high tunnel grafted tomatoes to non-grafted tomatoes and found that in both production systems, grafted tomatoes out-yielded non-grafted tomatoes anywhere from 5 percent to 30 percent.
The yield increases were greatest in the presence of soil-borne diseases and in drought conditions.
"Grafting plants is done primarily to protect them from soil-borne diseases by using vigorous rootstocks and scions from often less hardy, but more flavorful varieties," said McSpadden-Gardener. "Because root microbiology is known to be affected by cultivars and such differences can affect the plant's immune system, we were interested in studying the relationships between grafting and disease resistance. In last year's field study, we observed that grafted plants tended to be less severely affected by foliar diseases than ungrafted plants, which supports our hypothesis that root-microbe interactions can affect the scion."
Grafting-induced disease reductions were observed for Septoria leaf spot and late blight. However, because of the relatively small scale of the study, it is not clear how significant those reductions might be in economic terms.
In addition, when researchers examined the biochemistry of the grafted tomato leaves they found increased levels of two flavonoids. Flavonoids are antioxidants found naturally in plants that serve a wide variety of functions including giving flowers their color and protecting plants from insects.
McSpaddener-Gardener said that one of the flavonoids has been identified as alpha tomatine and is a component of the plant's immune system.
"The second flavonoid has yet to be identified but appears to be even more responsive to grafting," said McSpadden-Gardener. "What was even more surprising was that the differences were observed over eight weeks after the initial grafting event, an indication that grafting has long-lived effects on host physiology prior to fruiting."
Through fruit and vegetable consumption, flavonoids are known to be beneficial to the human body by triggering the production of natural enzymes that help fight off diseases such as cancer, heart disease and age-related degenerative illnesses.
"At this time, it is not clear if the observed changes in flavonoid content in the foliage also occur in the edible fruit," said McSpadden-Gardener. "Future work will need to be done to characterize the types of biochemical changes that occur in both the fruit and foliage in order to determine if such responses have impacts on fruit quality. But this is a good starting point."
OARDC researchers are collaborating with researchers from the University of Minnesota, West Virginia State University, North Carolina State University and Penn State. Farmers have also contributed significantly to the project from the beginning and continue to test the performance of grafted plants on their farms.
"Most farmers have a favorite tomato variety and heavily rely on just a few varieties that produce fruit the market wants," said Kleinhenz. "Consumers are looking for specific characteristics – weight, size, color, taste, how well it keeps, etc. Grafting means a greater ability for farmers to provide these qualities under challenging field conditions while maintaining ties to sustainability for consumers."
Future work will help determine the range of diseases impacted by grafting and the magnitude of the disease reductions in biological and economic terms.
For more information on grafting, log on to http://oardc.osu.edu/graftingtomato/graft.htm.