Breeding Seeks to Boost Disease Resistance in Lettuce

Written by: Bob Johnson for AgAlert

Thanks to expanded breeding trials, lettuce plants may become tougher and better able to hold up if assaulted by diseases such as fusarium, verticillium, pythium, impatiens necrotic spot virus and downy mildew, researchers say.

Lettuce and spinach growers have devoted a majority of their more than $1.1 million research funding this year to breeding and genetics projects, said Jennifer Clark, executive director of the California Leafy Greens Research Board.

“This investment ultimately gives growers the best protection against pests, diseases and abiotic stresses,” she said.

Plant pathologists and crop geneticists are looking for variety resistance to verticillium and fusarium, soil-borne lettuce diseases that became more pronounced after growers stopped rotating crops into ground fumigated with methyl bromide for strawberry production.

The decades-long effort to keep up with downy mildew, the most important foliar disease in the crop, continues.

Researchers are also evaluating lettuce lines for the resistance or susceptibility to impatiens necrotic spot virus, the disease that has recently decimated Central Coast lettuce fields. They’re also studying lettuce’s attractiveness to thrips, the insect that carries this disease.

Once spot virus infects a lettuce plant, there is no cure for the disease. And once the disease and western flower thrips are in an area, growers can only hope to slow its spread, because they cannot stop it.

The plan is to identify the genetic basis for tolerance of the disease and for being nonpreferred hosts for thrips and help seed breeders stack both in future generations of lettuce varieties, according to U.S. Department of Agriculture research geneticist Kelley Richardson, one of several experts who submitted reports for the California Leafy Greens Research Board’s annual conference in Pismo Beach in March.

Lettuce lines are being compared to a red Romaine lettuce variety, which has been attractive to thrips and susceptible to more immediate impatiens necrotic spot virus infection.

Among lettuce varieties screened at a USDA test field in Salinas last October, nine had lower rates of the disease, Richardson reported.

These nine lines are being studied for possible germplasm release to the seed companies.

The Salinas researchers are also screening Romaine and crisp head lettuce lines for their resistance to a combination of pythium and wilt virus.

Clark said the Leafy Greens Research Program and its handler members “invest research dollars in genetic research and pre-breeding efforts for resistance and other horticulturally important traits that are then available to seed companies developing commercially available leafy green varieties.”

Additionally, the Lettuce Genome Sequencing Project at the University of California, Davis, is compiling genetic information on cultivated lettuce to provide resources for other researchers seeking to address crop threats, including fusarium.

“Lettuce now has a very high-quality reference genome,” Richard Michelmore, a UC Davis plant pathologist, said in July.

Posing a question that inspires the effort, he asked, “Can we look at many of the genes available to breeders in this species rather than just one cultivar?”

He added, “We’re getting there.”

Michelmore’s report for the Leafy Greens research conference said 43 lettuce lines with fusarium resistance were released over the past year, with researchers identifying 17 other lines that may have resistance.

Fusarium wilt is a soil borne vascular disease that kills infected seedlings and causes many plants that survive to become stunted and disfigured.

The fungus, which is particularly troublesome in warmer lettuce regions of Fresno or southern Monterey counties, survives in the soil for many years.

USDA research plant pathologist Frank Martin reported that while only one race of lettuce fusarium has been found in California, three other races have been identified in Japan, Taiwan and the Netherlands.

In other studies, researchers are preparing to release lettuce lines that show resistance to both races of verticillium wilt that have developed and spread in the Central Coast region in recent decades.

Verticillium wilt first appeared in the region’s lettuce fields in 1995. The disease has since spread to more than 10,000 acres.

The fungus also survives in the soil, and rotation is difficult because it can affect other common Central Coast crops, including strawberries, spinach and artichokes.

Work also continues in the never-ending struggle to keep up with lettuce downy mildew as the disease evolves to overcome variety resistance.

For the last decade, Michelmore’s lab has tracked the rise and fall of five distinct races of downy mildew.

Over the last five years, however, a major share of the lettuce downy mildew in the state has been of novel strains that cannot be identified as belonging to any of those races.

Michelmore’s lab has evaluated 30 commonly grown lettuce varieties for their resistance or susceptibility to the disease.

This article first appeared in AgAlert – Bob Johnson is a reporter in Monterey County.