Nothing ruins a salad like lettuce that’s wilted in the fridge, and nothing ruins a farmer’s crop like lettuce that’s wilted before harvest. California leads the nation in lettuce production, and between 2008 and 2010, lettuce was one of the state’s top 10 commodities. Yet in 2010, the state’s lettuce growers planted 1,000 acres more than they harvested. This may be due to one of the top problems for California’s lettuce growers, a fungus that causes the plant to wilt before the growing season ends. To protect the stability of Monterey County’s lettuce industry, USDA researchers in the Salinas Valley are breeding varieties with resistance to the disease.

Infected plants in a commercial field.

Most of the production in Salinas Valley is iceberg and romaine. As it happens, iceberg is the type of lettuce most damaged by verticillium wilt. Verticillium dahliae, the fungus that causes verticillium wilt, infects plants one at a time. Because it is a soilborne pathogen, typically spread via tillage equipment, the fungus can infect an entire crop. “Losses can be complete,” says Dr. Ryan Hayes, a research geneticist at USDA’s Salinas station. “A plant with this disease wilts just as it’s beginning to reach maturity, so there’s nothing to harvest.”

Control of Verticillium dahliae through cultural practices is limited, in part because the economic and environmental costs associated with the routine use of soil fumigants during vegetable production are considerably high. Therefore, in order to maintain agronomic sustainability within the region, growers need lettuce cultivars resistant to the fungus.

An important discovery

Through collaboration with Dr. Krishna Subbarao, a colleague at University of California, Davis, Hayes discovered two isolates of Verticillium dahliae. Dubbed “race 1” and “race 2,” these isolates affect lettuce and other crops. Subbarao identified the gene that determines if an isolate is race 1 or race 2. Although a lettuce cultivar susceptible to both races will exhibit no difference in symptoms between the two isolates, the discovery helped the breeders turn a corner in developing lettuce with resistance to the fungus. In lettuce, one dominant gene confers resistance to the race 1 isolates. Researchers found this gene in ‘La Brillante’, an heirloom Batavia variety with suspected origins in France.

Hayes and Subbarao conducted experiments in fields infested with Verticillium dahliae. Initial results indicated that ‘La Brillante’ and some other heirloom varieties of lettuce were resistant. Follow-up experiments in the greenhouse indicated that all lettuce cultivars were susceptible. Why the contradiction? To sort this data, researchers tested a large number of Verticillium dahliae isolates for their ability to cause disease on ‘La Brillante’ and a few other cultivars. It turned out the strain of Verticillium dahliae used in greenhouse experiments was a race 2 isolate, and not representative of the population of isolates found in fields used in these experiments, which were predominantly race 1.

Because ‘La Brillante’ has the gene that confers resistance to the race 1 isolate, it can grow in soil infested with race 1 isolates of Verticillium dahliae without becoming infected with the disease verticillium wilt. There are no known cultivars with resistance to race 2. Even ‘La Brillante’ will develop verticillium wilt if it is exposed to the race 2 isolate. Therefore, until recently, the only easy way to tell which isolate was in a field was to plant ‘La Brillante’ with other cultivars and see whether ‘La Brillante’ developed the disease. The ability to cause disease on ‘La Brillante’ was the determining factor. That’s why Hayes and Subbarao used it and other heirloom varieties in their breeding program to develop iceberg cultivars with resistance to verticillium wilt.

The cultivar ‘La Brillante’.

The origins of the heirloom material are widespread, but the center of diversity for Lactuca sativa is the Mediterranean region. USDA’s collection at the Salinas station contains a majority of seed lots from the Mediterranean. Hayes and his colleagues are also working with botanists in the Mediterranean to collect wild relatives of lettuce; they hope to discover resistance to race 2 isolates.

Methods and releases

Using traditional breeding practices, Hayes takes pollen from one plant and puts it on the stigma of another to create F1 hybrids that can be grown out and eventually developed into new varieties. Lettuce is an inbred crop, so the F1 generation hybrids are the only uniformly heterozygous generation. “We allow those to naturally self-pollinate, then we are able to select for the traits as we inbreed the plants for four to five generations,” explains Hayes. “When we get to the end, the idea is that we have the new novel combination of traits that we want in something that’s inbred again. When it’s inbred it’s stable.”

A few years ago, Hayes, Subbarao, Dr. Gary Vallad and Dr. Edward Ryder released RH05-0336, RH05-0339 and RH05-0340, three iceberg cultivars selected from the cross ‘La Brillante’ x Pacific. The breeding lines were the first iceberg-type lettuce with verticillium wilt resistance. However, because they lack the necessary yield and quality needed for commercial production, they were released to be used as parents to develop verticillium wilt-resistant cultivars.

In July 2011, Hayes and Simko, along with colleagues from UC Davis, released two varieties of iceberg with resistance to verticillium wilt race 1. RH08-0472 and RH08-0475 are F9 generation inbred lines derived from the cross ‘Tiber’ x (‘La Brillante’ x Pacific). In contrast to the researchers’ previous release, RH08-0472 and RH08-0475 are suitable for commercial production in both healthy fields and in fields infested with Verticillium dahliae.

These two new varieties may be perfect for your operation, but don’t look for either one in your seed catalog. Though released to the public, seed companies usually breed the cultivars with their own favored varieties, and then release them with other names.

The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.