Ventura County lettuce growers and PCAs have a new tool for managing Sclerotinia lettuce drop with survey maps of fields by disease species and severity.
Maren Mochizuki, staff research associate with Ventura County Cooperative Extension, explained development of the maps and how they are used during the recent meeting of the California Lettuce Research Board (CLRB) near Coalinga.
Cooperating growers in the county allowed their fields to be surveyed for Sclerotinia minor and S. sclerotiorum for the board-supported project in 2005 and early 2006.
About a week prior to harvest, researchers walked a 500-foot, diagonal transect in each field and recorded GPS coordinates as they identified and counted infected plants and collected 20 infected plants from each transect. Ten of the plants were plated for identification of disease locally, and 10 were sent for confirmation in the laboratory.
Mochizuki joined Oleg Daugovish and Jim Downer, Ventura County farm advisors, Steve Koike, Monterey County farm advisor, and Krishna Subbarao, University of California, Davis plant pathologist, in the project.
S. minor was the species found most often, although some areas were entirely free of both species. Of 20 fields sampled since January of 2005, 13 were found to have S. minor, four had S. sclerotiorium, and three had neither.
Amount of disease
Fields having S. minor infections were identified according to low, medium, or high severity of disease and mapped. In a production area roughly between Port Hueneme and Camarillo, one field had a high rating count of 467 infected plants per 500 feet in sampling in March of 2005.
Mochizuki said the count in the same field was reduced to 182 in sampling in November of 2005. While that was still significant disease pressure, she added, only four infected samples were found there by February 2006.
“That is good news in that even areas with severe infection can possibly be brought under control with management of the disease, even though weather has much to do with it,” she said. The January-February 2005 period was very wet and conducive to disease development, while the same period of 2006 was relatively dry.
She said growers and PCAs now have a tool to use in determining how severe a potential outbreak of Sclerotinia might be.
“It also shows how important accurate field identification of species can be,” she said. Botrytis cinerea, for example, also occurs in lettuce and has the same overall plant symptoms as both Sclerotinia species: wilting and yellowing of lower leaves.
However, when the undersides of those leaves are examined, Botrytis infection shows as dusty gray spores and occasional black sclerotia, S. minor has white mycelia and black sclerotia, and S. sclerotiorum has thick, fluffy white mycelia and black sclerotia. Size, shape, and precise color of sclerotia also differ between the two species.
Although available fungicides are generally useful for controlling both species, location and timing of treatment makes a difference. In the case of S. minor, that is soil application after thinning, while for S. sclerotiorum it is a foliar spray at the rosette stage.
Other management steps are removal of infected plants and crop residue for S. minor and removal of weeds for S. sclerotiorum, along with level land and well-drained beds. Another cultural practice to discourage S. minor is plowing to put sclerotia 10 inches below the soil surface.
For both species, irrigation should be scheduled to keep the soil surface as dry as possible.
“Rotation with non-host to manage the disease is difficult but not impossible,” Mochizuki said. Broccoli is one alternative.
The traditional rotation crop with lettuce in the area has been strawberries because Sclerotinia was controlled with methyl bromide fumigation, but now that alternative fumigants must be used, greater vigilance and cultural practices for the disease are necessary.
Ventura County lettuce crops in 2004 were valued at $12 million, or about 3 percent of the county's total vegetable output.
In a CLRB-supported project on biological control of lettuce aphid, Bill Chaney, Monterey County farm advisor, said Hugh Smith, an entomologist with Monterey County UC Cooperative Extension, has compiled information on syrphid flies, natural enemies of lettuce aphid, in the Salinas Valley.
The aphid is difficult to control with insecticides because it resides deep in the head of romaine.
Smith's survey is primarily aimed at suppressing lettuce aphid on commercially grown, organic romaine, but it may also be useful in conventional production of the crop, Chaney noted.
Although adult syrphids feed on nectar and pollen of various “insectary” plants seeded in strips or windbreaks in or around lettuce fields, their nymphal stage preys on the aphids.
Obstacles with biological control are maintaining a certain level of aphid prey to support the predators and waiting for syrphids populations to build up during a romaine crop.
Chaney said Smith's research showed that syrphids of a half-dozen or so species make up a complex and Smith's future work will investigate how the different egg-laying behaviors of the several species interact to suppress the aphid.
In continuing studies of Fusarium wilt in lettuce, Thomas R. Gordon, plant pathologist at UC, Davis, told the board he has learned that higher temperatures favor development of the disease in many crops.
“These effects of temperature are potentially important as they could allow growers to minimize damage by selecting appropriate planting windows in areas where the disease is present,” he said.
Gordon is also observing how temperature relates to a number of Fusarium-resistant leaf and romaine cultivars. So far, he has found that their resistance remains in higher temperatures.
Another element of his project, investigation of how long Fusarium persists in soil, seeks to learn which crop rotation patterns might help growers schedule lettuce crops to minimize impact of the disease.
“We have now monitored viability of the pathogen in artificially infested soil over a period of about 21 months. By the end of that period, the pathogen was detectable at approximately 2 percent of the initial inoculum level,” Gordon said.
Fusarium wilt first appeared in California lettuce in 1990 in the Huron district of the San Joaquin Valley. It has since spread elsewhere in the SJV and in Arizona and coastal California.