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Lettuce breeders track crop change

Growers aren't the only ones who react to changing consumer preferences in lettuce. Plant breeders must also track the ever-changing disease and pest complexes that accompany cropping shifts.

As growers have expanded their product lines beyond traditional iceberg varieties in meeting demand for romaine and other lettuce types in recent years, they have learned new production skills and sought new plant material.

That's why Becky Grube, research geneticist at USDA's Agricultural Research Station at Salinas, Calif., says she and her three colleagues at the station expect their lettuce breeding research will remain a never-ending task in meeting growers' needs.

Other geneticists working toward improved lettuce varieties in a dozen projects at the station are Ryan Hayes, James McCreight, and Beiquan Mou.

With basic funding from USDA and support for specific projects from the California and Arizona lettuce industries, they develop germplasm to be used by commercial seed companies in new varieties having resistance to diseases and insects. Whenever possible the breeders try to keep a step ahead, looking for backups or alternatives to the original material released.

Often they find resistance genes in exotic lettuces from the Mediterranean region or in non-crop plants related to lettuce. Once resistance is found, they continue to cross primitive plants with other plants to achieve horticultural traits that more closely resemble commercial varieties. Resistance may be linked to an undesirable trait, such as early bolting, or unacceptable shape or texture.

Major changes

“The industry,” Grube said, “has changed significantly in the last 10 to 20 years. Twenty years ago it was predominantly iceberg varieties, and — without any real decrease in iceberg — romaine, red leaf and mixed lettuces have really taken off and will likely continue to expand.”

The public's interest in diversity in its salad ingredients and how marketers respond, she added, will drive the extent of lettuce varieties.

“Along with the new lettuces, we also have a lot of new pests and pathogens and new production issues. It's been a real shift, and as more diversity comes we will see more new problems.”

One problem for romaine growers has been dealing with aphids that reside deep in the crown of the plant where pesticide sprays cannot reach them. The approach to a solution has been to develop new plants that resist the aphids, and progress is being made at the station.

Grube said a production issue for plant breeders is higher plant density practices, such as 80-inch beds with six rows of romaine. While the densities may offer advantages in management of pests, breeders ponder whether the practice is sustainable over the long term.

“We ask ourselves how much can be done culturally to control diseases vs. how much can be done, and should be done, genetically.”

Genetic resistance has its virtues but is not a permanent solution, she reasons. “It's a pretty cheap way to go, but it is also vulnerable, just like chemicals, to being overcome by pathogens. Pathogens will win, so we have to buffer the genetic resistance, just like we have to buffer chemicals, so it will be long lasting.”

The real answer, she adds, is a combination of plant resistance, chemicals, cultural practices, and even location to manage pests. “The best growers are the ones who take this integrated approach, using all the tools and knowledge available to them.”

She predicts that even when a successful combination of practices is applied, it's only a matter of time until pests will mutate and require new defenses. And, she adds, things will get more complicated as new production systems adapt to new pressures.

Among Grube's several projects now in progress is a search for resistance to Verticillium wilt, a continuing threat to iceberg production. She has several breeding lines developed during the past five years at the station in a large trial in a grower's field having a history of the disease. Data will be evaluated later this year.

Sclerotinia minor

In a search for resistance to the deadly, soil borne fungus, Sclerotinia minor, or lettuce drop, she is screening commercial cultivars instead of primitive sources.

“We've looked for so long for sources with high levels of resistance, but no one has looked at the lower levels of resistance in cultivars that have been developed over the past 50 years.”

She has learned that certain redleaf cultivars have a significant amount of Sclerotinia resistance and could be used immediately without delays of additional breeding of experimental plants remove unwanted horticultural traits.

“I hope this will bring us back into ‘the cultivated realm.' At the very least, it may give growers some real information they can use in the short term.”

Grube is also continuing work on lettuce dieback, a disease associated with romaine fields close to rivers, which appeared about five years ago. This project demonstrates the need for breeders to have continued review of released materials.

About two years ago USDA released dieback-resistant romaine germplasm, which has been incorporated into commercial romaine varieties. It was thought that iceberg carried one resistant gene and romaine another.

As a backup, she has continued testing on other sources having higher resistance than the released material but having less desirable horticultural traits. In related work, she is also screening redleaf cultivars for resistance.

Her field trials on lettuce dieback this season suggest thus far that the source of resistance in iceberg and romaine cultivars is not from separate genes but the same one, making it the sole defense against the pathogen.

“This means that we may have all our eggs in one basket,” she said.

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