Growing Magazine - May, 2014

FEATURES

Progress in Pest and Disease-Resistant Tomatoes

By Nancy Riggs


Iron Lady tomatoes, which were first marketed in 2013, offer early blight, late blight and Septoria leaf spot resistance.
Photo courtesy of High Mowing Organic Seeds.

Tomatoes are the most purchased vegetable in the U.S. and rank fifth among the most profitable commercially grown vegetables, according to the Northeastern IPM Center at Cornell University. Martha Mutschler-Chu, Cornell vegetable breeder and geneticist, has made major strides in the ongoing battle to prevent tomato crop losses due to viruses spread by thrips and fungal diseases transferred through the air and growing processes.

While estimates of the impacts vary, the University of Georgia (UGA) recently put the tomato and pepper crop losses caused by thrips in the Southeast alone at more than $440 million. Nationally, crop losses due to viruses such as tomato spotted wilt virus (TSWV) are astronomical. Working as part of a USDA-funded team addressing thrips-caused diseases, Mutschler-Chu has developed a tomato with dual resistance: It is resistant not only to thrips, but also to the viruses spread by the tiny insects.



Martha Mutschler-Chu examines blight-resistant tomato plants.
Photo courtesy of Martha Mutschler-Chu.

Mutschler-Chu said, "This brings us closer to the development of a commercial [variety] that essentially eliminates the need for pesticides in many growing regions." She is looking toward future development of tomatoes providing an even broader resistance to pests and diseases, giving producers stronger advantages in growing tomatoes profitably as production costs continue to increase.

Employing materials and technology developed over two decades of research, along with current communication technology to share results, Mutschler-Chu is able to collaborate with other researchers to share, evaluate and further develop findings. In response to the high crop losses throughout the U.S., the USDA is funding a $3.75 million research project to develop and implement a national scientific and educational network to limit thrips-caused crop losses. The team is led by Diane Ullman, University of California, Davis, and John Sherwood, UGA.

Other interdisciplinary researchers working on the project include Scott Adkins, U.S. Horticultural Research Laboratory; Robert Kemerait, UGA; George Kennedy, North Carolina State University; Naidu Rayapati, Washington State University; and Dorith Rotenberg and Anna Whitfield, Kansas State University. Researchers will share results, and the thrips-resistant tomato lines developed at Cornell will be used by the team in ongoing research. The team members integrate their work through monthly cyber conferences and annual meetings.

Developing thrips-resistant tomatoes

The dually resistant tomato line is important in its ability to not only resist thrips and associated viruses, but also to help reduce, or possibly eliminate, pesticides in some tomato-producing regions. Although disease incidence is greater in regions with high moisture levels, viruses such as TSWV occur in all climates.

The tiny insects pierce and suck fluids from plants, including tomatoes, grapes, strawberries and many more. At least 14 species of thrips cause damage by spreading TSWV and 23 additional types of viruses.



A wild tomato plant used to breed thrips-resistant tomato lines, Solanum pennellii, is in the foreground among thrips-resistant plants.
Photos courtesy of Stacey Shackford, Cornell University, unless otherwise noted.

Mutschler-Chu isolated the insect resistance found in a wild tomato plant native to Peru. The plants are covered in hairs, or trichomes, that secrete acyl sugars, which confer the resistance. The acyl sugars prevent insects from feeding on the plants or laying eggs. The resistance was transferred into new lines, and Mutschler-Chu worked through the traditional breeding processes to breed out undesirable traits. Two natural genes that resist viruses including TSWV were added to the new tomato lines, offering additional protection. Genetic modification is not required in this process.

Mutschler-Chu said, "The research also incorporates tools of molecular biology, including PCR-based markers and SNP markers, as well as the full tomato genomic sequence now available." She worked for approximately 10 years on developing the first tomato line that contained high enough levels of acyl sugars, and spent another four years on creating an improved series of 30 derived lines. "Current development is even faster," Mutschler-Chu noted.

The USDA-funded teams are spread out in various locations, so Mutschler-Chu's varieties can be tested in regions with different climatic conditions. Findings at those test sites can be used to modify the lines and help develop new lines that will perform best in various growing environments while resisting thrips-caused viruses. The findings will be shared with seed companies so they can incorporate the effective traits into their varieties.



Solanum pennellii, a wild tomato plant from Peru, was used in breeding thrips-resistant tomato lines.

Developing blight-resistant tomatoes

Mutschler-Chu developed tomato lines that have genetic resistance to three major blights: early blight, late blight and Septoria leaf spot. These lines were released nonexclusively last year. The airborne fungus late blight can spread quickly, decimating tomato crops in just a few days. In 2009, some Northeastern tomato growers experienced total crop loss. In addition to airborne transmission, the fungus can be spread by splashing water from rain or overhead irrigation.

"Septoria leaf spot is almost as pathogenic as late blight and has recently been a particular problem in organic tomato production. Early blight is a common problem in the northeastern United States," Mutschler-Chu said.

She initially developed tomatoes that resisted early and late blight. Septoria could still defoliate the plant, so she looked for ways to create Septoria resistance in early and late blight-resistant tomatoes. She worked with Cornell's Tom Zitter, plant pathologist, and Stella Zitter, research associate, to develop Septoria resistance in the tomatoes.

"We screened putative sources of natural Septoria resistance to find the best one, then transferred it, by breeding, to tomato lines that already possessed control of early and late blight," Mutschler-Chu explained.

A series of hybrids was developed from a cross of the triply resistant Cornell line and a late blight/early blight line from North Carolina State University. A combination of two genes, Ph2 and Ph3, provides a strong resistance to late blight. The hybrids were tested in both organic and conventional settings to meet the needs of diverse growing operations.

"This testing was to determine the benefit of the resistance, which hybrids were of sufficient quality to use, and what additional improvements would be useful," Mutschler-Chu said.



Thrips-resistant tomato plants.

One combined blight-resistant Cornell line licensed by High Mowing Organic Seeds (http://www.highmowingseeds.com) is a parent of the blight-resistant hybrid Iron Lady, first sold in 2013. Additional new hybrids with the combined blight resistance trait are expected to be released by other seed companies starting in 2015. "The process is moving fast," Mutschler-Chu said. "We have even better blight controls coming along. We are testing second-generation hybrids now."

Mutschler-Chu and Tom Zitter also developed molecular markers for the late blight-resistant genes, and these markers are available to seed companies that license the tomato lines. The extended time to breed plants is a significant issue in bringing new tomato lines to completion.



Photo by Laboko/thinkstock.com.

"Use of molecular markers cuts in half the number of generations it takes to breed for the desired trait. Development of similar markers for [the] Septoria resistance gene is nearing completion," said Mutschler-Chu.

"Due to the mechanism of the Septoria resistance genes, the best control of Septoria leaf spot is maximized by minimizing sources of inoculum," she explained. "Septoria-resistant hybrids should be grown upwind and separated from susceptible hybrids. At least 20 feet of separation should be maintained to maximize [the] benefit of the resistance."

Mutschler-Chu and Zitter developed a list of variety characteristics and tolerance capabilities. The list is available on Cornell's Vegetable MD website (http://bit.ly/1iT9p zA). They identified a fungicide strategy for the tomatoes. Tolerance to early blight is not as strong as resistance, and fungicides may still be needed in some environments. It is possible that sprays could be reduced from weekly applications to once or twice a season. Application information is available to growers on the Vegetable MD site (http://bit.ly/1m7pYMr).

Mutschler-Chu is confident her team will continue to develop tomatoes that will withstand a wide range of pests and the diseases that they spread.

Nancy Riggs is a freelance writer and frequent contributor. She resides in Mount Zion, Ill.