Research on lettuce drop caused by Sclerotinia species in California fields indicates that plantings on 80-inch beds irrigated twice a week are at greater risk of the fungal disease, says a University of California plant pathologist.

Krishna Subbarao, in a recent mid-year report to the California Lettuce Research Board at Seaside, said his observations in the Imperial Valley during this year and the previous two seasons evaluated the 40- and 80-inch bed configurations.

Sclerotinia minor is most common in lettuce along the coast, while S. sclerotiorum occurs more often in interior valleys. Under wet conditions, both pathogens attack the lower leaves and stems, leading to decay which collapses plants nearing maturity.

Both can survive for up to 3 years in soils without susceptible hosts. In cool, moist weather, S. sclerotiorum generates aerial spores that also infect upper leaves.

“Results obtained thus far,” he said, “clearly indicate the potential for 80-inch beds to increase lettuce drop incidence caused by both species. This, in turn, can lead to greater numbers of sclerotia in the soil and the establishment of the airborne phase of S. sclerotiorum in the Salinas Valley.”

Subbarao, who is based at Salinas and heads the project's team of scientists, said their studies revealed that lettuce drop caused by S. minor was greater on 80-inch beds irrigated twice a week. However, two other irrigation frequencies, once a week and once in two weeks, had low development of the disease, regardless of the bed width.

Bed width concept

The 80-inch bed concept, which requires custom-built, wider equipment, has been adopted by several farming operations as a means of reducing labor and tractor costs and soil compaction.

Another objective of the project was to gauge performance of a number of biocontrol products. Contans was found this year to give significant control of lettuce drop caused by S. sclerotiorum, although none of the products reduced the disease brought on by S. minor.

Subbarao also collected 243 samples of S. minor which he segregated into four groups. About 50 percent of the samples, or isolates, were in the first group, while about 25 percent, 10 percent, and 4 percent occurred in the other three groups, respectively. The top two groups are more virulent than the others.

He said the greatest diversity of groups was found in the Salinas Valley, with the first group prevalent throughout the valley and the second group most prevalent in the northern portion of the valley. The third group was mainly in the north, and the fourth group was found mainly in the southern portion.

The first group, he added, produced significantly more fruiting bodies and grew faster in the laboratory than isolates of the other three groups. “This perhaps explains why isolates belonging to the first group occur at a far greater frequency than isolates from other groups in the Salinas Valley and other valleys of California.”

50 breeding lines

Since fungicides may control one species but not both and often require multiple applications for satisfactory results, USDA plant breeders at Salinas are evaluating more than 50 breeding lines for use in developing lettuce germplasm resistant to S. minor. Related trials are underway at Yuma to screen lettuce lines for resistance to both S. sclerotiorium and S. minor.

Subbarao also gave his mid-year report on monitoring of Verticillium wilt, another team project sponsored by the research board since 2002.

The disease, caused by the same V. dahliae that goes to several crops, was detected for the first time in a few coastal lettuce fields in 1995. Its symptoms appear at the rosette stage of the crop and include yellowing and wilting of the outer whorl of leaves and dark discoloration of the internal vascular tissues. Its microsclerotia can lurk in the soil practically indefinitely between hosts.

Although the disease when into a lull in 2003, Subbarao said, it showed up in many fields in the Watsonville and Salinas areas in 2004, with an incidence of 30 to 90 percent. It even showed up in fields previously fumigated with methyl bromide with chloropicrin and planted to strawberries.

Only two newly infected fields were detected this year, but losses ran as high as 80 percent in them. Subbarao tested soil samples taken from the fields and evaluated the efficacy of fumigation on the fungus at depths of two inches to 24 inches.

The fields were planted to radicchio in 2004 and incidence of the pathogen was higher at most soil depths. As a result, Subbarao reported, “It is possible that radicchio planted last year supported the formation of V. dahliae microsclerotia without showing visible signs of the wilt on the crop.”

He is continuing monitoring to learn the connection of a related fungus, V. tricorpus, which was found on lettuce roots early in the season. High populations of V. tricorpus compete with V. dahliae in potatoes and the same may be true for lettuce. Subbarao said, however, that further studies would be needed to confirm this.

The soil sampling on one field also indicated significantly greater amounts of V. dahliae at the edge where tillage normally begins. This field was also fumigated and planted to strawberries. The analysis of data is not complete, but he said the pathogen might have been introduced by tillage equipment.

Developing lines

USDA plant breeders at Salinas have collected dozens of plants having resistance to the wilt and are developing lines for new sources of resistance in various types of lettuce, from romaine to butterhead.

The research board also wants to know more about the post-harvest effects of Verticillium wilt. Subbarao set out trials for several lettuce types from infected fields this year and then evaluated the mature heads for post-harvest quality after packing and cold storage.

“Quality parameters included those that consumers normally look for and also for signs of wilt and other anomalies,” he said.

“For the most part, the quality of all lettuce types was excellent for two weeks in storage. However, there were varietal differences by the third week.”

Subbarao said he could not determine whether the wilt caused the decrease in quality by the third week since there was no data for corresponding varieties from non-infected fields. As a follow-up, he will be circulating a grower questionnaire to collect data for comparisons.