The hunt for tomatoes with disease resistance

Checking tomato variety plotsat the University of Florida’s Gulf Coast Research and Education Center, Sam Hutton stops and fingers leaves, squeezes an occasional fruit and riffles plant canopies. On this day he is particularly interested in plants showing resistance to yellow leaf curl virus, which can destroy 100 percent of the tomato plants in an infested field.

“This one I call Ty-3,” he says. “It holds up against lots of strains of the virus. There are many strains of tomato yellow leaf curl virus, and you never know ahead of time which one you’re going to have to deal with. We have the Israeli strain, but there are others from South America, Guatemala and other places. The virus strains can mutate — they may move and they may evolve on their own. It makes breeding for resistance tricky.”

Ty-3’s predecessors, Ty-1 and Ty-2, have little resistance against these other strains of the virus, he says. He still uses them in the breeding program, but is moving forward with Ty-3 because of its multiple resistance.

Hutton has just finished his first year as a full-fledged tomato plant breeder on the staff of the station, which has a mailing address of Wimauma, Fla., although it is actually located in the small community of Balm.

He did six years of Ph.D student and post-doctoral work here before landing what he calls his dream job. Altogether, he has been involved with this breeding project focusing on tomato yellow leaf curl virus for several years, along with another geared toward finding genetic resistance to bacterial spot.

“All our resistance is coming out of wild tomato species, which we have growing here, too. We cross them and grow out the progeny, then select for the resistance gene,” Hutton says.

The wild tomatoes offer disease resistance but have few characteristics that would attract growers, so he has to bring those traits in from other, more standard varieties.

“The wild ones originated in South America and are very vigorous growers,” he says. “They have small fruit you wouldn’t want to eat. It never turns red, the yield is low; there are lots of negatives with them. But they have genes we can use for something good.”

For the yellow leaf curl virus work, he likes resistance found in a section of chromosome 6 in a wild plant. “Then we crossed that part of chromosome 6 with a Florida breeding line known as 7776, which is a good parent. That let us develop a 7776 tomato with Ty-3 in it, which has multiple yellow leaf curl virus resistance.

“There is a pretty big differencein appearance between Ty-1, Ty-2 and Ty-3. Some of the biggest complaints about Ty varieties are that the vines look terrible and that they have greater susceptibility to foliar diseases. But in this case, we took nothing except that one gene from the wild plant and put it in 7776. We really used only a little sliver of the chromosome. I can’t tell the difference in appearance between Ty-3 and 7776, which is a good thing.”

Hutton quickly points out that all this work is being done with traditional breeding techniques. “This is the climax of 20 years of work,” he says. “Back in the 1990’s, Jay Scott, our well-known tomato breeder, did the first crosses with wild species.

“Out of thousands of seed, maybe 20 would germinate. Then he’d take those 20, grow them out, and see what he got. These plants here represent literally years and years of breeding work, of Dr. Scott finding the best resistance and running disease screens.”

Work like this gets intense and requires a lot of man-hours to accomplish, to say nothing of patience.

“In genetics, we’re looking to use a small percentage of a chromosome,” Hutton explains. “We grow out a whole bunch of progeny and look for those with a portion of this chromosome, or even less. We wind up with different lines, with all sorts of different sizes of plants and fruit. We grow them out, then inoculate them with the disease and figure out which one — if we’re lucky — is resistant.”

In the spring of 2009, Hutton grew out 11,000 seedlings in this fashion. He picked a leaf from each and screened every leaf for disease resistance. Of those, he found 300 that had the short piece of chromosomal resistance he wanted.

“We inoculated them all and found exactly where the gene was. We crossed the plants that had resistance and then screened 2,000 of the plants from the crosses to find three plants that had resistance.”

“We’re trying to get this part from the mother and another part from the father, both with resistance, to get a recombinant event. We’re hopeful this will usher in a new generation of Ty-resistant varieties that are every bit as good horticulturally as non-resistant varieties. This is something we haven’t had the choice of doing before now.”

Disease-resistant varieties, in general, are reputed to yield less than non-resistant ones, but Hutton hopes varieties coming out of this latest program will avoid the so-called linkage drag.

“Anywhere you have a diseaseresistance gene with a linkage drag, it’s in everybody’s best interests to get rid of them,” he says. “With respect to the major resistance gene Ty-3, we have addressed this problem by determining the precise location of the gene, then developing material that has Ty-3 within a minimal segment of introgressed chromosome — one which has very little ‘foreign’ DNA flanking the gene.”

He thinks the resistant lines are not too far from being commercialized.

“They’re really close. We hope to soon release hybrids and breeding lines with high levels of resistance to tomato yellow leaf curl virus and no associated linkage drag. We’ll be looking at test crosses this coming spring. As far as getting it out, we could potentially have varieties produced in two years. The seed companies are going to be right behind us on it.

“We’re three crosses into it right now. Four crosses should be enough. Next fall, we’ll have hybrids. Then we’ll get the data, and growers should have them in two years. We are really excited about it.”

Hutton and his colleagues won’t stopthere, however. They’ll continue refining the new varieties in order to improve them.

“Within three years, we should be able to spin out a hybrid with good yield, good quality and multiple disease resistance. When we do that, all of a sudden the variety becomes much more attractive to growers.”

In addition to the tomato yellow leaf curl virus and bacterial leaf spot programs, Hutton is working on building disease resistance into the newly released Tasti-Lee variety, which came from Jay Scott’s work at the station.

Tasti-Lee has become somewhat of a taste-sensation with consumers, but some growers are reluctant to grow it because it requires extra management. Hutton thinks putting disease resistance into Tasti-Lee will reduce much of that.

“We already have a Tasti-Lee with multiple disease resistance in tests, and it still has premium quality. We still have to test it to make sure everything is right. If everything pans out as we hope, maybe this could help Tasti-Lee capture more of the market for fresh tomatoes.”

Hutton grew up on a farm at Tchula, Miss., where his father, Sam D. G. Hutton, Jr., grew cotton, corn and soybeans. After earning an undergraduate degree at Mississippi State University, he received his master’s degree at the University of Minnesota, mostly working with soybeans, a crop he says bored him. Tomatoes proved to be challenging — and he likes a challenge.

“Tomatoes are kind of a unique crop,Hutton says. “There’s a tremendous list of things to breed for. A lot of things can be flexible to a point, but certain things are givens. Yield is the top factor in breeding. If there are no tomatoes on the vine, there’s nothing a breeder can do with it. Unless you’re breeding for the heirloom market, the tomato has to have good size and shape.

“It has to have a smooth blossom scar, good outside color, good crack resistance, and it has to be firm. The fruit has to set right on the plant. Fruit should not only be of good size, it should have smooth shoulders and uniform color on the shoulders. It should ripen evenly. It should be round — what I call a flat round shape, not a deep round shape. There should be no puffy fruit. As for flavor, for most breeders it has been somewhat of an afterthought.”

Compare all that to Mississippi’s famous old crop, cotton. Breeders push it for quality, along with disease resistance, fiber length and strength, and a few other characteristics. “Cotton has a relatively short shopping list of things breeders need to look for, at least compared to tomatoes,” Hutton says.

“It’s pretty much the same situation with soybeans, which I worked with at Minnesota. While I was in school at St. Paul, I got into some pretty serious gardening with tomatoes. I was more excited about my garden plot than my research plots. (Don’t tell my old professors that).

“I decided if I was going to earn a Ph.D, I’d better do it on something I was excited about. So when the opportunity came to work on tomatoes in Florida, that seemed perfect.

“After getting my Ph.D, I had offers to work in crops such as peppers and cucurbits, but nothing excited me like this job in tomatoes. It’s exactly what I like to do. Tomatoes are a fun crop to work with — and you’d better try to have some fun in life, if you can,”

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