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Tomato Blight: Breeding for Resistance (page 2 of 2)

by Pat Stone

Talking With the Gene Farmer

Stone: How did you get started working on blight?

Gardner: I grew up on a farm in Virginia, and blight was always a big problem. So when I was new to research, I decided that it would be a good long-range problem for me to tackle. I began in 1976 and released my first early-blight-resistant variety, Mountain Supreme, in 1992.

Stone: How do you breed for blight resistance?

Gardner: Basically, it's done by crossing varieties that show some degree of the desirable trait. It's tough because the factor that controls blight resistance is controlled by several genes. When a characteristic is linked to several genes, chances are it's also linked to other qualities you may not want, like small fruits or late maturity. It takes a lot of crossing to break through those connections.

Stone: Just how resistant to early blight are the Mountain varieties?

Gardner: You'll see some blight on those varieties, but not enough to cause a problem. Whenever a factor is conditioned by several genes, you generally end up with varieties that have moderate but stable resistance. When a characteristic is controlled by a single gene, you often get a higher degree of resistance in less breeding time, but it probably won't last long. A fungus or virus can mutate around a single genetic block in just one step.

Stone: Isn't this what's happened with Fusarium wilt?

Gardner: Exactly. At one time we had tomato varieties that were resistant. Then Fusarium race 2 developed, and then race 3. A tomato's ability to resist Fusarium is controlled by only one gene, so each time the tomato becomes resistant, the fungus mutates.

Stone: Late blight didn't used to be much of a problem. What's the story?

Gardner: As recently as five years ago, late blight appeared infrequently. And there was a commercial fungicide, Ridomil, that controlled it with one application. The fungicide must have attacked something very specific, like one enzyme, because the fungus mutated and Ridomil is no longer effective.

In the last four years, late blight has become a severe problem in many areas of the U.S. and Mexico. I'm not sure when I'll have resistant varieties, but fortunately, some of my early-blight-resistant varieties show some resistance to late blight as well. It's likely that late blight resistance is also controlled by several genes, meaning new varieties won't stop the disease completely, but will slow it down and reduce fungicide use.

Stone: What about flavor?

Gardner: My varieties are used by home gardeners, but my primary responsibility is to commercial growers. For them, I have to breed for many characteristics in addition to blight resistance. Some, like firmness, good size and smooth skin, are purely cosmetic but important. Shoppers won't buy tomatoes that are too soft, or that are scarred by catfacing.

For determinate types, my varieties have good flavor, but they'll never match the best backyard tomatoes for taste. There's a reason for that. Determinate tomatoes give big yields on smaller plants, and that lower foliage-to-fruit ratio limits the amount of sugars and acids in the fruits. Indeterminate tomatoes, the kind that keep on growing and bearing fruit instead of producing their whole crop at once, have a higher ratio of foliage to fruit. Those old heirloom tomatoes like Brandywine and Ponderosa may not give you a lot of fruit, but they will have a lot of flavor. It's the same principle as thinning apples or peaches to get fewer, better-flavored fruits.

Stone: Why don't supermarket tomatoes have better flavor?

Gardner: One of the main reasons is the way tomatoes are marketed. Different varieties aren't labeled, so if you bought one last week that had really good flavor and went back this week to get some more, you wouldn't have any way of knowing you were getting the same variety or not. Tomatoes need to be stickered the way bananas are. That's beginning to happen; Del Monte is labeling some of its tomatoes, and there's Devine Ripe out of Mexico that's picked mature, not green.

Stone: Is bioengineering going to replace conventional breeding?

Gardner: I doubt it. Flavr Savr cost more than $20 million to develop. Those tomatoes will have to be labeled and they'll have to sell for $2 to $3 a pound. How many people will pay that when they can buy other varieties for $1 a pound-- Besides, some genetic engineering goals can be accomplished conventionally for a lot less money. The Flavr Savr's claim to fame is slow ripening, which allows later harvest of more ripe fruits. But conventional breeders are already working with two genes -- one called rin" for "ripening inhibitor" and the other called "nor" for "nonripening" -- that do much the same thing.

By now, we were riding back from Gardner's research plots, bouncing along a dirt road in the cab of a well-used pickup truck, baskets and stakes jiggling in the back. I couldn't help but remember my one visit, a few years back, to Monsanto's biotech complex in Missouri, with its gleaming multistory building, pristine, antiseptic facilities and omnipresent security staff. I was struck by the difference between agriculture in test tubes and agriculture in test fields.

But Gardner didn't seem to mind a bit being a hands-on breeder instead of a laboratory one. As he put it, "I know that varieties like Mountain Supreme and Mountain Fresh may not get me a lot of respect in professional circles. I can tell you one thing, though. The growers, they are really appreciative."

Photo courtesy of University of Wisconsin-Madison, Dept. of Plant Pathology

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