Management of Verticillium wilt remains a lingering concern for coastal California lettuce growers, but research during 2005 found no link between the level of the disease and postharvest quality of lettuce.
Krishna Subbarao, University of California plant pathologist, said he did not detect any loss of postharvest quality attributable to Verticillium dahliae in two trials in the spring followed by a third in the fall.
At the recent California Lettuce Research Board (CLRB) gathering near Coalinga, Subbarao said the first trial was a limited effort dealing with two romaine and two crisphead cultivars only.
The second and third, however, were expanded to include three varieties each of red, greenleaf, and butterhead types and six varieties each of romaine and crisphead.
Each cultivar was rated for disease at appropriate harvest times, harvested to commercial standards, boxed, and transported to the cooler. Boxes were retrieved every week and rated for postharvest quality with parameters that consumers normally look for and signs of wilt or other problems.
“Results from all three trials,” he said, “suggest there is no relationship between the level of Verticillium wilt on the various cultivars and postharvest quality.”
The trials were a new element in Subbarao's long-term efforts to learn more about the disease, which first appeared in a few coastal lettuce fields in 1995. The CLRB has supported his concentrated research since 2002.
The soilborne pathogen is the same that strikes several crops, and on lettuce its symptoms are yellowing and wilting of outer leaves at the rosette stage. Its fruiting bodies persist between hosts.
After subsiding in 2003, the vert reappeared in several fields near Watsonville and Salinas in 2004, some having incidence of 30 to 90 percent.
Only two newly infected fields were found in 2005, but losses in them were as high as 80 percent. These had been planted to radicchio during 2004, indicating that the disease may have been supported without symptoms in that crop. Meanwhile, monitoring has shown no additional infected fields.
After collecting and analyzing soil samples from infected fields following fumigation in 2005, Subbarao said the preliminary data “suggest dramatic reductions in the microsclerotia at all sites by fumigation but not total elimination.”
He has also been collaborating with USDA lettuce breeders and growers in Salinas in field evaluations of new cultivars and experimental lettuce germplasm for resistance to the vert.
Find disease race
In the 2005 research they identified a second, more virulent race of the disease, and although several plant materials showed resistance to race 1, none was resistant to race 2. Breeders say this discovery signals the need for additional screening to find resistance to the new race.
In observations tracing another Verticillium species, V. tricorpus, found on lettuce roots early in the season, Subbarao set up greenhouse trials with lettuce plants exposed to both species.
“None of the V. tricorpus isolates,” he reported, “caused significant disease individually or increased the severity when inoculated with V. dahliae. Results from these coinoculations suggest at least the potential of some isolates of V. tricorpus to suppress V. dahliae infections as has been observed with certain cultivars of potatoes. Further studies are needed to confirm this.”
In reviewing other developments in his 2005 research on lettuce drop caused by Sclerotinia species, Subbarao said work with biocontrol agents in the Imperial Valley showed that “Contans provided significant control of lettuce drop caused by S. sclerotiorum and the number of healthy heads was similar to that in the uninfected control plots.”
The product, he added, “provided excellent control of S. sclerotiorum in two of three years by not only reducing disease incidence but also the survival of sclerotia between seasons.”
However, he noted, none of the materials evaluated was effective on lettuce drop caused by the related pathogen, S. minor.
While S. minor is found mostly in coastal lettuce, S. sclerotiorium shows up more often in inland valleys. In high moisture, both move to the lower leaves and stems of lettuce plants, causing them to decay and collapse as the plants near maturity.
For the past three years, he has been investigating the incidence of disease caused by both species in lettuce planted to 40-inch or 80-inch beds, with different irrigation frequencies, in the Salinas Valley.
“The incidence of lettuce drop cause by S. sclerotiorum was significantly higher on 80-inch beds than on 40-inch beds on plots irrigated twice weekly,” he said. At once per week and once in two weeks disease development was low, regardless of bed configuration.
“In contrast,” he continued, lettuce drop caused by S. minor was greater on 80-inch beds receiving both twice-weekly water and weekly water.
“Results clearly indicate the potential of 80-inch beds to increase lettuce drop incidence caused by both species. This, in turn, can lead to greater numbers of sclerotia in soil and the establishment of the airborne phase of S. sclerotiorium in the Salinas Valley.”
William M. Wintermantel, a USDA research plant pathologist at Salinas, described new developments in his CLRB-supported studies aimed at finding resistance to lettuce dieback disease.
Lettuce dieback, which has symptoms of severe plant stunting leading to necrosis and eventual death, is caused by a pair of viruses in the genus Tombusvirus, lettuce necrotic stunt virus (LNSV) and tomato bushy stunt virus (TBSV).
The disease is closely connected with riparian areas in California and Arizona and is commonly found along river beds where drainage is poor. Flood or irrigation water is associated with outbreaks in lettuce fields, which occur sporadically with varying severity.
The dieback is also linked to Salinas Valley sites where the viruses coincide with lettuce plants in soils having high salt content.
“The viruses can be present in soil, but unless stress factors such as elevated salinity are present, plants may not develop disease symptoms,” Wintermantel said.
He said his recent studies indicate that although the viruses may be transmitted by lettuce seed coats, the amount of disease carried is not likely to be enough to infect fields to significant levels.
His collaboration with J. E. Schoelz, a professor of plant pathology at the University of Missouri, over the past year and a half discovered a gene in tobacco plants that halts LNSV, TBSV, and other viruses.
Further research with that gene, Wintermantel added, might help in developing resistance to the virus in lettuce plants.