USDA plant breeders at Salinas, Calif., are improving resistance to big vein, an important cool-weather disease of lettuce, but it will be several years before tougher new varieties find their way to commercial fields.
Geneticist Edward J. Ryder, in a recent report to the California Lettuce Research Board at Seaside, said the ongoing project made crosses of resistant, cultivated lettuce and the wild species, Lactuca virosa, which is highly resistant to big vein.
The big vein project is only one of more than a dozen projects funded by the board and under way by Ryder and other scientists at the USDA station to develop lettuce able to stave off diseases and insects.
Principal symptoms of big vein, thought to be caused by Olpidium brassicae, a virus carried by a soil fungus that invades roots, are enlarged, clear veins in leaves.
The amount of the disease varies from season to season, and severely infected plants produce deformed, unmarketable heads, although plants with less infection can be harvested.
Varieties with some resistance are available, but cultural practices for control are limited. Crop rotations to avoid the disease have not been successful, and susceptible varieties are not recommended for planting in fields having histories of chronic and severe big vein.
1995 cross start
In 1995 the big vein-resistant variety Pacific was crossed with Pavane, a resistant cultivar from France and similar to romaine, with the aim of developing a head-type lettuce. In the next step, crosses with the wild species were begun to improve resistance.
Among trials during 2001-2002, Ryder said, progeny having head-type traits showed resistance equal to or better than Pavane. However, he added, it will be several years before genetic material can be released to seed industry.
In another project several years from completion, breeding continues to incorporate a gene resistant to lettuce mosaic with genetics from an Egyptian stem lettuce, which, while not completely resistant to the disease, shows milder reaction.
“By combining these, we hope to develop significantly higher resistance to lettuce mosaic. We will be doing more tests this year and make selections,” Ryder said.
Lettuce verticillium wilt, a disease first detected in Watsonville in 1995 and since found at various locations in the Salinas Valley, is the target of another project. Plants infected with the wilt, caused by the same V. dahliae that goes to many crops, turn yellow and wilt before dying, and dark streaks show on the taproot and crown.
Working in concert with University of California plant pathologists tracing transmission of the wilt by weeds, Ryder made several crosses of iceberg and romaine, butterhead and other types having resistance. He has several lines with good resistance and will be selecting for the best of them.
During the board meeting, Ryder announced he would retire from USDA as of Jan. 3, 2003, capping a career begun in 1957 at the Salinas station.
He is best known for the breeding of the variety Salinas, which gave the industry breakthroughs in improved handling qualities and resistance to tipburn. It became the most widely grown lettuce variety in the world.
He said he would stay on as a collaborator until a successor was named in November and for about six months afterward.
In addition to developing genetics for a host of disease and insect resistant lettuce varieties used by the California and Arizona industries, Ryder has been curator of USDA's Lactuca Germplasm Collection at the Salinas station. The 14,000 items in the collection, the largest lettuce seedbank anywhere, are exchanged with breeders around the world to improve cultivars.
The board heard good news about screening of fungicides to control crown rot disease of lettuce. The disease was first found in 2000 in romaine at Watsonville and subsequently spread to portions of iceberg fields at numerous sites in Monterey and Santa Cruz counties and later in the Santa Maria Valley.
Identified by distinct blackened and sunken portions on the crown and upper taproot, it causes plants to be broken off easily at ground level.
A team of scientists, including Steve Koike, Monterey County farm advisor and Krishna Subbarao, University of California plant pathologist, linked crown rot to the fungus, Phoma exigua. Although the strain on lettuce may be distinct, its relatives go to more than 60 plants, including cotton, pepper, bean, potato, and vinca.
In trials at Watsonville this year, the investigators applied nine fungicides to plots, one spray a day after thinning and then two additional treatments at one-week intervals.
They found good control with a trio of materials. Compared to an untreated check showing 30 percent infection, treatments of the fungicides BAS 516, Switch, and Quadris showed 0.7 percent, 6.3 percent, and 9.7 percent infection, respectively.
Researchers are still wary of the uncertain nature of crown rot, and the lettuce research board continues to support studies on how soil, weather, nutrition, and seasonality interact with the pathogen.
William M, Wintermantel, USDA plant pathologist at Salinas, is probing lettuce dieback, a disease of romaine in California and Arizona associated with poorly drained soils having high concentrations of salts, especially near rivers. His studies trace salt content by electro-conductivity of soils.
Identified by symptoms of small, cupped and curled leaves and necrotic leaf tips has been found from year to year in fields of riverbottom soils along the Salinas River in Monterey County. Another symptom is a flush of lateral shoots that produce a bushy and stunted shape to infected plants.
Field samples show the disease, also known as lettuce dieback or riverbottom disease, is caused by tomato bushy stunt virus and lettuce necrotic stunt virus. These viruses enter via damaged roots, and since they do so without a biological vector, soil fumigation has no effect. Pathologists suspect they are conveyed by irrigation water.
While Wintermantel monitors Salinas Valley soils for salt levels that encourage virus infections, USDA geneticists screen for resistant cultivars, which include most crisphead types and some butterhead and greenleaf types.