Principal investigator in a project funded by the California Lettuce Research Board is Carolee Bull, a USDA bacteriologist based at Salinas.
The varieties having the least severity from bacterial leaf spot in tests since 2001 are Little Gem, Batavia Reine de Glace, and Prizehead, all used in USDA’s lettuce breeding program at Salinas. The varieties Vista Verde and Pybas 251 are the most susceptible to the disease.
Reporting on her work during the past two years at the board’s recent meeting in Seaside, Bull said she is also dealing with measurement of severity of the disease. While disease incidence may be high, its severity determines economic effect.
"We measure severity in the field by looking at where the disease occurs on the plant. For example, if it is on the lower leaves, it doesn’t affect quality like it would on the cap leaves."
In observing chemical and biological treatments, she said other researchers are trying to determine the best timing of applications, particularly whether they must be applied before the pathogen reaches the plant. She plans to announce results to the board next March.
Combinations of Serenade, copper, Maneb, Messenger, and compost tea have given control but not consistently. "We see variability with those materials. If growers don’t have the disease every year, they will have to question whether to apply them every year." That’s why, she continued, attention is given to cultivars having resistance to the disease.
In rating severity, she said, a visual symptom on a plant does not necessarily mean it was caused by bacterial leaf spot. Different cultivars may show different symptoms from the same disease, or symptoms might be caused by another disease or even lack of irrigation.
"Particularly with leaf spots on vegetables, there are a lot of things that cause spots. So we follow through with a test for presence of the pathogen with a molecular test."
She is also leading development of greenhouse methods to screen experimental cultivars more economically than can be done with trials in the field.
Among the greenhouse trials were inoculations of the disease in different concentrations in one to three applications. "The only place we see difference is when the concentration of bacteria differs, so higher levels of the disease will mean greater severity, but multiple applications don’t seem to have any effect."
In a commercial field, she added, "if you have a high level of inoculum and everything else is conducive to disease development, that’s what is important. It doesn’t matter that someone brings new inoculum into the field later."
Bull said planning continues for a joint U.S., Canadian, and French research organization to study Xanthomonas campestris, the pathogen that causes bacterial leaf spot on lettuce. An international approach would pool resources and avoid duplication, she said, adding that an application for funding is expected to be made in February 2004.
William M. Wintermantel, a plant pathologist at USDA in Salinas, in reporting on his research for the CLRB, said lettuce dieback has been traced to high salt concentrations in soils of the Salinas Valley.
The disease, also known as tomato bushy stunt virus and lettuce necrotic stunt virus, impacts both Romaine and leaf lettuce, severely in one season and mildly the next, in fields near rivers in California and Arizona.
"The disease is tightly linked to the presence of river water, being found commonly in areas with poor drainage, where flooding has recently occurred, where river water is used for irrigation of fields, and where dredge is deposited on fields," Wintermantel said.
Studies measuring the electrical conductivity of soils showed high salt concentration to be the reason for the viral infections. Fumigation does not deter the virus, which does not need a biological vector to reach lettuce plants.
Soil samples from adjacent lettuce dieback-infested, as well as disease-free, fields were collected and nutrient profile of these related soils were compared, Wintermantel said.
Drainage as culprit
"The studies," he added, "indicated that poor drainage, such as occurs in areas near rivers, can lead to high salt concentrations and that these conditions lead to increased incidence of infection."
USDA geneticists are addressing the lettuce dieback problem by introducing resistance to the disorder into romaine and red leaf germplasm. They have found that resistance in the crisp head cultivar Salinas is controlled by a single dominant gene and they have identified molecular markers associated with that gene.