Farmer Su Chang‐seng harvests a tomato crop. Disease-resistant varieties developed from AVRDC lines help him meet his production contracts.
In the mid-1990s, however, the Shanhua farmers began noticing that not all was well in their fields. The typical bright green foliage of their tomato plants turned yellowish, and the leaves crumpled in on themselves. Plants were stunted and shriveled. Yields of once-prolific tomato varieties fell from about 100 metric tonnes per hectare to 40 metric tonnes or less.
With such low production levels, farmers could not provide sufficient fruit to keep the Kagome processing factory operating at full capacity. Management either would have to import tomatoes to supply its operations—to the detriment of hundreds of local farmers—or find a better way to deal with the problem.
Taiwan’s long struggle with tomato yellow leaf curl disease had begun.
Tomato yellow leaf curl disease is caused by viruses in the Begomovirus genus, which has more than 200 species. At the time, the begomovirus troubling Taiwan’s farmers was Tomato leaf curl Taiwan virus (ToLCTWV). Although there are many species of begomovirus, there is only one vector, or carrier: the whitefly (Bemisia tabaci). This tiny pest feeds on the undersides of young leaves and growing tips of tomato, and spreads viruses to the plant through its saliva. Because whiteflies congregate in large numbers, susceptible plants can be quickly overwhelmed. Further harm is done by the sticky honeydew whiteflies secrete, which fosters mold growth on the plant’s surface.
If a tomato field was infected with the disease when the tomato plants were young, a farmer might lose the entire crop. Kagome’s tomato farmers fought back by applying pesticides, but the whitefly proved to be a formidable foe. Increasing the number of applications or the strength of the pesticide solution didn’t help, as whitefly rapidly develop resistance to chemical pesticides, and the company was concerned about pesticide residues compromising the safely and purity of its food products.
“It was in 1998 that Kagome first approached AVRDC,” said Hirotaka Ito, Manager of the Plant Breeding Group in Kagome’s Agricultural Research department. “We were aware that the Center did research on vegetables and thought that perhaps the scientists there could help.”
The benefits of good breeding
Producing a good crop of tomatoes in the tropics and subtropics challenges the best of farmers. Fighting pests and diseases is a never-ending battle as pathogens evolve and develop resistance, rendering once-effective pesticides useless. Resistance bred into the plant is the cheapest, simplest and most effective way to control tomato leaf curl viruses.
Since its founding, AVRDC has sought to develop virus-resistant tomato varieties by selecting plants that demonstrated resistance for its breeding programs. By the time Kagome sought assistance, AVRDC’s plant breeders had been conducting research into the genetic diversity of plant viruses for more than two decades to develop tomato breeding lines with improved resistance to pests and diseases. “We had a tomato line from India called H24 that had demonstrated high levels of resistance to ToLCTWV,” said AVRDC Tomato Breeder Peter Hanson. “And we knew the resistance was conferred by a single dominant gene, called Ty-2.”
(left to right) The Kagome‐AVRDC team: Shunichi Shimakawa, Vice‐Director Production, Kagome Taiwan; Shien‐hui Hung, Kagome Taiwan; Jin‐shu Lin, Kagome Taiwan; Hirotaka Ito, Manager, Kagome Plant Breeding Group, Japan; Ayumu Onozato, Kagome Plant Breeding, Japan; Peter Hanson, AVRDC Tomato Breeder; Chee‐Wee Tan and Shu‐fen Lu, AVRDC Research Assistants.
AVRDC breeders and Kagome researchers began collaborating to produce disease-resistant tomatoes with suitable fruit quality for processing. The company maintains a large collection of tomato seed in Japan, with about 7500 tomato varieties from around the world. Breeders selected varieties from the collection to cross with H24 and other AVRDC lines in pursuit of a hybrid that would survive virus outbreaks and produce fruit to meet the company’s quality control standards.
The first Kagome-AVRDC hybrids with resistance to ToLCTWV were planted in Taiwan in the 2001/2002 growing season. The new hybrids proved successful, and by 2005 average yields had bounced back to about 80 metric tonnes per hectare—a little lower than the higher yields of the past, but sufficient to supply the factory and protect a major source of income for farmers.
Once is not enough
As farmers in Taiwan (and all around the world) well know, the history of agriculture is one long battle against organisms that destroy or compete with crops. No sooner had the Ty-2 tomato hybrids proved their worth than a second begomovirus was introduced into Taiwan: Tomato yellow leaf curl Thailand virus (TYLCTHV). The virus began spreading around the island, likely brought in by whitefly on infected plants and produce. Despite import and border quarantine controls in place to stave off such threats to agriculture, it is difficult to fully control the movement of plant pests and diseases.
During the 2010/2011 growing season, southern Taiwan experienced a severe whitefly infestation in tomato fields. Farmers noticed about 10-20% of their tomato plants with the Ty-2 gene were also showing symptoms of TYLCTHV. Distribution of the two viruses varies across the island, but they can occur in the same field, and even in the same plant.
To mount a defense, AVRDC and Kagome began another collaboration in March 2010, this time to evaluate tomato lines with other resistance genes in addition to Ty-2. “Gene pyramiding” is a breeding technique used to introduce multiple genes into a plant, each of which imparts resistance to a specific pest or disease. Because a pest must overcome all of the resistance genes simultaneously to survive, it is more likely the vegetable line or variety will retain its resistance over a longer period—perhaps for several decades.
Breeding lines selected from AVRDC contained different combinations of resistance genes Ty-1, Ty-2, and Ty-3; those from Kagome contained only Ty-2 or no Ty genes. Breeders produced 55 hybrids using the gene pyramiding technique; the lines were grown in AVRDC’s research fields to see how they would hold up under pressure from the viruses. In May 2012 Kagome selected eight to ten lines for further development and distribution to its contractors.
The Ty resistance tomatoes are the latest in a very long line of improved tomato germplasm developed at AVRDC. Since 1978, 172 open-pollinated tomato varieties based on the Center’s lines have been released in 41 countries, offering farmers the opportunity to reduce their crop production costs by saving their own seed.
The reality of growing at ground level and beyond
Su Chang-seng, a farmer in his 60s who has been growing tomatoes for Kagome for 12 years in nearby Xinhua, appreciates having access to the improved varieties. “Growing tomatoes in screenhouses is too costly,” he said. “We really need the varieties that can resist the viruses.” Producing for Kagome brings in a steady income for Mr. Su.
Mr. Su has four children: three girls and one boy. Although all his children will come and assist him during harvest time, none of them—including a daughter who graduated from agricultural school and now works at a factory in the nearby Nanke Science Park—are interested in taking over the farm.
A knowing nod comes from Tseng Ching-hsin, a Kagome extension agent who leads a group of 20 contract farmers, hires workers, and advises farmers on details of their contracts. “It takes three workers to harvest a field—but their ages add up to 200!” he said, commenting on the average advanced age of Taiwan farmers. “And we can’t harvest by machine, because the fields are too small.”
Shien-hiu Hung and Jin-shu Lin, Kagome project managers, meet with their contract farmers about two to three times per week, visiting fields and estimating yield to inform the production lines at the Shanhua factory, which makes tomato concentrate to use as a base for producing juice throughout the year. “Due to the bad virus outbreak in 2011, yields were low and our production of concentrate was down,” said Mr. Hung. He said the company plans to increase the number of farmers in Tainan City, expand the land area under tomato, and hopes to have a better harvest to build up its stock of tomato concentrate. “But so much will depend on the improved tomatoes with better resistance to the viruses,” he observed. “Without those, it is difficult to do business.”
A local community, a Japanese company, and an international research institution: each with different strengths, all willing to work toward a common goal. The ability to find and share solutions will depend in part on similar collaborations to help farmers continue to meet market demand—and on the willingness of governments and donors to support agricultural research. The challenges facing agriculture in the future are many—an unpredictable climate, an aging workforce, the appearance of new diseases, to note just a few. These challenges will always be with us; they are an inescapable part of the dynamic natural environment in which we live. With a global population projected to reach 9 billion by the year 2050, the well-being of future generations will depend on a strong, steady commitment to the essential business of growing crops and processing food.
