Tomato Blight: Breeding for Resistance

Late blight on tomato leaves

It's got to be an axiom of home gardening that nothing ruins a good garden like a vacation. Go away, come back and something has gone wrong. Still, even I didn't expect last summer's disaster. When my family returned from our annual week at the beach, I discovered that an arsonist had torched my tomato plants! All eight vines looked like pine trees after a forest fire.

The firebug was no human, though, but a disease: blight. In much of the East and Midwest, early and late tomato blight infestations are very hard to hold at bay. And when it rains every day for two months, as it did last summer in western North Carolina, it's impossible.

So it was with zeal that I drove out to North Carolina State University's Mountain Horticultural Crops Research Station in nearby Fletcher last September to visit the one person I knew would still have tomatoes on the vine: breeder Randy Gardner. Since 1976, Gardner has made battling blight his full-time job. He's been successful. Over the last several years, he has developed the early-blight-resistant Mountain series, including Mountain Pride, Supreme, Gold, Fresh and Belle.

Gardner did, indeed, have tomatoes. Row upon row of them -- red, green, yellow, orange, big, small, pear, plum -- his field beds a living gene library of Lycopersicon esculentum: the tomato. All had the stop-at-three-feet height of determinate varieties. Many were so laden with large fruits they looked like pint-size Christmas trees. He even had one I nicknamed the "anti-tomato." It was everything a tomato shouldn't be: a solid green, softball-hard globe that has the flavor of watermelon rind and, best of all, never ripens. Why keep such a gardener's nightmare alive? "It has a ripening inhibitor gene that's useful for crossbreeding," I was told.

Gardner was a pleasure to talk with. A straightforward, down-to-earth man, a farmer of genes, not crops, he talked in layman's language and exhibited a patient determination that reminded me of the land itself. We started off, of course, on the subject of b-l-i-g-h-t, but by the end of the day, we had also talked about tomato marketing, flavor, genetic engineering and more.

Early Blight (Alternaria solani) occurs wherever tomatoes are grown. It is most common in humid or semiarid climates where frequent dews provide sufficient moisture to permit disease development

Symptoms: Small, brownish black lesions form on older foliage first. The tissue surrounding the spots may become yellow, and by the time spots are about 1/2 inch in diameter, concentric rings may be discernable in the dark brown portion of the spot. When a spot is located on one of the primary leaf veins, the area beyond the spot soon dies and becomes brown. In the latter part of the season, the lesions become numerous and under favorable conditions, affected plants become defoliated, exposing fruit to sunscald. Stem lesions on seedlings are small, dark and slightly sunken. They enlarge to form circular or elongated lesions with pronounced concentric rings and light centers. If lesions continue to enlarge at the ground level, plants will become girdled and die. This phase of the disease is called collar rot.

Fruits are infected at either the flower or stem end, in either the green or ripe stage. Lesions attain considerable size, occasionally involving the entire fruit, and usually have concentric ringing. Diseased areas appear leathery and may be covered by a velvety mass of black spores.

Disease cycle: The soilborne fungus survives between crops on debris in the soil and on seed. In mild locations it survives from season to season on volunteer tomato plants and other related host plants, such as potato, eggplant, horsenettle and black nightshade. Primary infection occurs during periods of mild, 75? F to 85? F, rainy weather. Older leaves are affected first. High soil fertility reduces severity of early blight.

Control: Plant resistant varieties an use pathogen-free seed and healthy transplants. Use long crop rotations and eradicate weeds and volunteer tomato plants. Fertilize properly to keep plants growing

vigorously.

Late Blight (Phytophthora infestans) is the fungal disease that caused the Irish potato famine in the 1840's. It's also very serious on tomatoes when the weather is consistently cool and rainy.

Symptoms: The fungus attacks all aboveground parts of the tomato. At first, leaf lesions appear as water-soaked spots. They enlarge rapidly into pale green or brown spots that cover large areas of the leaf. In moist weather, the undersides of small lesions may be covered with a gray to white moldy growth. Infected foliage becomes brown, shrivels and dies. Petioles and stems are affected similarly. Fruit lesions appear as dark, greasy spots, which enlarge until the entire fruit is invaded. Soft rot often follows late blight infection and leads to fruit disintegration. Decaying vines may have a foul odor.

Disease cycle: The fungus survives between seasons in volunteer and abandoned potato and tomato plants in fields and gardens. Cool nights and warm days (temperatures below 85? F) are ideal for late blight development. The spores spread readily by wind to other plants and when weather is favorable, the infection moves so rapidly that affected plants appear as though damaged by frost.

Control: Eliminate all potato cull piles in the vicinity of tomato plantings, and destroy volunteer potato plants that grow from overwintered tubers.

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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