by Sally Colby
Tomatoes are a big draw at farmers markets and every year, growers aim for a crop that delivers what customers want.
Lori Hoagland, of Purdue University, says that while the demand for organic produce is rising, growers face challenges in terms of meeting consumer expectations for produce that differs from produce grown for supermarkets.
“Tomatoes are a popular crop in local and organic food systems, and when we surveyed tomato growers to understand their needs, they overwhelmingly rated flavor as their top priority,” said Hoagland. “A lot of growers are turning to heirloom varieties, which are often perceived to have superior flavor. The challenge is that many heirloom varieties may not have the traits to be the most productive in organic cropping systems.”
Growers who were asked identify their biggest production challenges listed disease management, especially foliar diseases such as late blight, early blight and Septoria leaf spot. Many heirloom varieties are not resistant to these pathogens and although some new resistant hybrids are available, these varieties don’t meet the flavor profile fresh-market consumers are seeking.
To address these challenges, a group of interdisciplinary researchers from across the country came together in 2014 to establish the Tomato Organic Management and Improvement Project (TOMI). The components of the project include identifying biofungicides and biostimulants to control foliar diseases, investigating induced systemic resistance (soil management practices) and a varietal development program that focuses on new varieties that have the flavor both growers and consumers want along with resistance to the most problematic tomato diseases.
Dr. Jim Myers, Oregon State University, said one challenge was in screening resistant material on the farm — farmers don’t want diseased experimental material in their fields, so a combination of on-farm and research station work was needed. The other aspect is seeking broad adaptation of varieties, so field trials were conducted in Indiana, North Carolina, Wisconsin and Oregon.
“One thing about tomatoes is it’s a self-pollinated crop, so it’s a matter of crossing materials then selecting during subsequent generations of inbreeding,” said Myers. “We were looking for an open pollinated type and not an F1 hybrid.”
The project began with tomato lines from the North Carolina State breeding program. “They had resistance to early blight and late blight,” said Myers. “A good combination of genes, with resistance to Verticillium and Fusarium wilts (1,1-2) and had a determinate background.” The parents chosen to provide flavor and heirloom traits were Wisconsin 55 and Crimson Sprinter, both indeterminate varieties with excellent flavor but with fruit quality issues such as cracking. Wisconsin 55 has moderate early blight resistance and some degree of polygenic late blight resistance. Crimson Sprinter has moderate Septoria leaf spot resistance, and it has the crimson gene, which results in a higher level of lycopene and more fruit pigment.
The first season in the field was 2015 and families were grown across the United States. “We were looking at total fruit weight, fruit number, marketable fruit number, Brix and flavor,” said Myers. “We were doing sequential harvest across seasons, then evaluating for disease over the season.” Myers said that early blight and powdery mildew were present in Indiana and North Carolina; Septoria leaf spot was present in Wisconsin and Indiana; and Verticillium was present in Oregon. Bacterial speck was present in Wisconsin and late blight was in North Carolina.
Myers said varieties were screened for molecular markers for disease, then 10 selections were chosen for increases in the greenhouse. In 2017, varieties were evaluated in-station, and in 2018, work will continue on regional organic farms.
The tools required for tomato breeding are simple: forceps to remove the anthers, 70 percent ethanol to sterilize the forceps between pollinations and to ensure pollen is not accidentally introduced to the wrong parent; tags for marking crosses and a pencil for notes (which can also be used for emasculation).
“Pick flowers at the optimum stage,” said Myers. “You want to catch it before the anthers have allowed pollen to escape. If you try to emasculate too small a flower, it can be difficult to work with and it becomes mangled. The optimum stage is when it’s showing just a little bit of color. Open the flower up and remove the anthers. If you do it right, you can break the anther cone loose from the rest of the flower and slide it over the style and stigma.”
Once that’s done, pick an open flower from the pollen donor. “You want the anther to have dehisced,” said Myers. “In a tomato, there are slits on the inside of the anther cone from which the pollen is dispersed. Choose that and apply that portion of the anther onto the stigma of the emasculated flower.”
Jared Zystro, of the Organic Seed Alliance, explained the process of making selections from the crosses and identifying plants that are close to ideal. He says the trick is figuring out how to ‘get the signal out of the noise’ — determining whether the plants or lines being selected are genuinely better because of superior genetics versus whether they’re influenced by environmental conditions.
“How you do that depends on how many generations out you are after the initial cross,” said Zystro. “The first generation after a cross is the F1, and in that generation, all the plants will be fairly uniform and will be an intermediate between the two parents. You won’t be able to make a lot of selections from the generation. But if you save seed from that F1 generation, tomatoes are naturally self-pollinating, and you will have F2 seeds to plant the next year.”
When making selections, the challenge is dealing with individual plants and deciding ‘is this plant genetically superior and do I want to save seed from it? Is the fruit truly better looking and tasting, or is that simply because of where it was in the field?’
Consistent field conditions are important in any breeding project trial, especially for the F2 generation. “You want to feel confident that the differences between the plants you are selecting are based on genetics and not that the plant happens to be growing in the more fertile or better watered part of the field,” said Zystro. “When you’re planning the breeding trial location, identify locations that are as uniform as possible in regard to soil type, shade, fertility, prior crops and irrigation. Make sure that throughout the season, you’re treating the trial as uniformly as possible.”
When it comes to selecting single plants in a field with segregating F2 plants, it’s important to select evenly from sections of the field. “Rather than pick your 20 best plants, it’s better to divide the field into four imaginary squares,” said Zystro. “Pick five from each square. This gives you a better chance to capture the best plants from the entire field.”
Zystro suggests saving F2 generation seeds in separate bags. “The next season, take seeds from each of those bags and plant in separate rows,” he said. “These are the family, or progeny rows. This will give you much more ability to see if there are superior families because you’re looking at many plants at the same time and judging them on the overall quality of the related individuals.”