Even though losses to verticillium wilt in lettuce fields of California's Salinas Valley were less serious in 2003 than in the past, the fungal disease may be spreading, alarming growers and researchers alike.

Krishna Subbarao, University of California plant pathologist, announced his latest findings on disease transmission, control strategies, and other research at the recent meeting in Seaside of the California Lettuce Research Board.

“Unlike previous years,” he said, “there were not fields with dramatic losses from verticillium wilt this year. However, appearance of the disease in fields previously fumigated with methyl bromide and chloropicrin was a worrying development.

“The disease appeared in two new fields in the Salinas area and it may have spread to as far south as Soledad.”

Stationed at the USDA Agricultural Research Station in Salinas, he is continuing CLRB-supported trials begun four years ago.

Confirmation of the appearance of wilt in Soledad, he added, is pending. Soil tests in fields where it reappeared after fumigation and a crop of strawberries revealed very high levels of the disease.

Verticillium wilt first appeared in 80 acres of lettuce in Santa Cruz County in 1995 and a later expanded into Monterey County.

The seed borne nature of Verticillium dahliae that causes wilt of lettuce are well known, as is the role of certain weed species in the transmission.

Infestation test

Subbarao wants to learn the critical number of infested seed required to bring on an infestation in clean soil. He has set up trials for infestation ranging from zero to 100 percent in micro plots fumigated with methyl bromide and chloropicrin.

In separate but related trials supported by the CLRB, Subbarao is also investigating fungicides to control two sclerotinia species, S. minor and S. sclerotiorum, which reside in the soil and cause lettuce drop.

During the spring, disease pressure was too low to indicate effect from treatments, but in the fall infestations were more evident.

BAS 510F and Switch, applied two weeks after thinning and again two weeks later, both provided reductions greater than 75 percent. Botran and Rovral were also rated in the trials.

BASF's BAS 510F, marketed as Endura, has federal registration, and clearance of it for lettuce in California is expected by the end of this year. Switch, a Syngenta product registered on strawberries and onions in California, is expected to have the state registration for lettuce late in 2004 or early in 2005, he said.

Trials on the effect on S. minor by fumigant treatments, including chloropicrin, Telone plus chloropicrin, and methyl bromide plus chloropicrin, included assessment of disease in lettuce following strawberries.

Subbarao said S. minor was reduced by 90 percent following the treatments and no increase was seen during the strawberry-growing season.

However, he added, during the lettuce crop, fruiting bodies of that disease and crown rot were significantly higher for all three treatments. The incidence of lettuce drop was not significantly different in the three treatments.

Subbarao noted that the increase is consistent with earlier studies indicating more Sclerotinia and other diseases, such as verticillium, show up in lettuce following soil fumigation and a strawberry planting.

Trials are also devoted to learning why both species of sclerotinia have been reported significantly higher in lettuce on 80-inch planting beds than that on the 40-inch configuration.

Moisture is key in the incidence of the disease, and 35 days of continuous moisture is ideal for release of ascospores.

Such is uncommon from normal rainfall in the irrigated Salinas Valley, but, as Subbarao noted, drainage is less on the 80-inch pattern, creating conditions for the disease to thrive.

He has begun irrigation treatments and will use them with data on the timing of ascospore releases and the distance they travel. S. sclerotiorum releases ascospores throughout the day, and they may travel some 36 feet, according to results of earlier trials.

Collection of soil samples showed four “compatibility” groups of S. minor present in the Salinas Valley. Two groups occur throughout the valley while the other two are found mostly in the northern or southern portions. This may be linked with cropping patterns in the different sectors, and further tests will be aimed at learning the relationship between groups and host crops.

Biocontrols

Biocontrol agents do not have great effect against lettuce drop in the Salinas Valley but have shown promise in the Imperial Valley, where Contans, a German product developed from a soil fungus, has been highly effective.

Subbarao said such materials “have provided encouraging results on lettuce drop caused by both S. minor and S. sclerotiorum in Yuma. Trials in Yuma have also shown the effectiveness of Rovral.”

In Imperial Valley trials, he evaluated four biocontrol treatments for lettuce drop and found none effective on S. minor.

“However, as has been observed in the Midwest on soybeans and in Arizona on lettuce, Contans reduced the incidence of lettuce drop by more than 85 percent, relative to the control.”

He plans to continue the trials this fall.

In another avenue to control verticillium wilt and sclerotinia, Subbarao is collaborating with USDA geneticists who are using, among other sources, backcrossing of plants having partial resistance to the diseases.