University of California plant pathologists are trying to identify a new crown-rot disease which causes dieback of lettuce plants and appeared this past spring in a few fields in Monterey and Santa Cruz counties.
In a mid-year report to the California Lettuce Research Board, Steve Koike, Monterey County farm advisor, said Romaine lettuce seems most susceptible to the fungal disease. Iceberg and greenleaf types also showed symptoms, although to a lesser extent.
At the board meeting in Seaside, Koike said the importance of the disease has yet to be assessed. The disease was recovered from about a dozen fields in the two counties. Most of the fields had only minor losses, but in a couple about 50 to 60 percent of the stands were infected.
Observation trials during the summer with several lettuce varieties planted in a stricken field did not produce the disease. He said he couldn't predict whether the disease would occur next year.
Koike refuted parts of Associated Press news coverage in September on the disease. One story erroneously stated the disease had infected 1,000 acres and had destroyed up to 60 percent of a crop.
"A lot of the statements in reports in the media were misquotes. The extent of the problem was extremely over-emphasized. I was amazed at some of the things I supposedly said in the articles," he said.
Early symptoms of the disease are stunting and yellowing of lower leaves, which mimic other lettuce diseases, such as tomato bushy stunt and verticillium. Later the symptoms of wilting and collapse resemble another disease, sclerotinia.
Plant base rot "But the most important feature is seen at the base of the infected plant. There's a very distinctive dark, sunken cavity or lesion at the base, which causes the plant to break off easily at ground level," Koike said.
The rot that occurs is firm, not soft. Soft rot is a sign of a botrytis infection, which is further typified by a gray color without the dark lesion on the crown of the plant. Sclerotinia also produces a rot having a very soft texture.
He added that symptoms may not be visible during inspection of a field, but infected plants break off readily when struck. Another sign of the disease is a lop-sided growth pattern when one side of the plant is infected.
Koike, working with Krishna Subbarao, UC plant pathology specialist at Salinas, found that most of the fields infected were planted to Romaine lettuce but a few greenleaf and iceberg fields also showed symptoms.
"Since this is so new, we can only speculate that iceberg lettuce seems less susceptible, but we are not sure about that," Koike said.
Samples were taken from the fields, which spanned from Watsonville to the Spreckels and Blanco districts of Salinas, and laboratory tests yielded the same gray-green fungus.
Koike stressed that recovery of a fungus from a diseased plant does not necessarily mean that fungus is important to the problem. However, if the fungus is applied to test plants which then develop the same symptoms, the fungus is significant.
"We have done that in greenhouse tests in Salinas," he added. "But unfortunately, we do not know the name of the fungus. It is not producing fruiting bodies that we need to see before we can give it a name."
Until it is identified the source of the fungus, how it is vectored, what plants are susceptible, and other traits cannot be determined.
Koike did say that preliminary tests on how the disease moves in plants showed it is not likely transmitted vascularly, the same manner as verticillium. Meanwhile, he is continuing laboratory plating tests with the fungus and may consider more sophisticated testing methods if no results are found.
Once more is known about the disease, USDA plant breeders at Salinas may be able to search for plant resistance to it.
Botrytis gray mold Turning to other projects he has in progress with Lettuce Research Board support, Koike said his work with Subbarao on a management strategy for botrytis gray mold of lettuce came up with inconsistent results this past season. He said the ambiguities coincide with reports from several coastal growers.
The studies focused on the infection of leaf lettuce transplants made during the spring. Acreage of transplanted leaf and Romaine varieties has increased in recent seasons, so more infections by the disease have been seen.
The disease invades transplants damaged during the normal practice of handling and planting. Damaged tissue that is wet or comes in contact with soil are particularly vulnerable. The botrytis pathogen is widespread, being able to survive on residues of several crops.
Koike said the trials involved treatments with the fungicides Rovral, Ronilan, Botran, Serenade, and Elevate. Applications to Romaine were made both preplant, when transplants were still in their trays, and postplant, after they were in the ground. In-tray treatments with Rovral and with Ronilan were significantly better than the untreated check in only one case.
No significant differences were found between treated and untreated transplants when the fungicides were applied immediately after transplanting.
Koike and Subbarao also scheduled trials for management of lettuce anthracnose, known commonly as ringspot and shothole. The project aimed at rate and timing of fungicide applications, methods of detecting the disease, and evaluation of various cultivars for resistance.
Koike said the dry spring of 2000 prevented threat of disease, as well as collection of data.
In 1998 the disease was very severe and the only product available, Maneb, was not providing satisfactory control, he said.
As a result, the board funded research on alternative fungicides, and Quadris and Folicur rendered excellent control in trials. A Section 18 registration for Quadris was granted and subsequently renewed.
Continuing studies of verticillium wilt by Subbarao, Koike, and others made progress in learning more about how the disease is transmitted by lettuce seed. The disease has symptoms of chlorosis, wilting of mature plants, and vascular discoloration.
Results, said Subbarao, imply that the pathogen is internally seed-borne and that seed production should be only in uninfested fields.
The researchers inoculated greenhouse plants with verticillium, allowed the plants to bolt, and collected the seed, which revealed the disease. Plants grown from the seed showed wilting symptoms within 10 weeks.
Seeds of lettuce varieties Salinas, Little Gem, and Lactuca serriola were soaked in a verticillium inoculum, potted in the greenhouse, and allowed to bolt. Subbarao said nearly 90 percent of the seeds tested had verticillium colonies.
The plant pathologist also said verticillium, detected in a small area in Watsonville in 1995, is moving into the Salinas Valley and in the past year has been found in three fields in the valley.