• Bill Wintermantel, with USDA/ARS in Salinas, looked at lettuce dieback caused by tombusviruses.

He said contributing factors for the disease include flooding or poorly drained soil, elevated salinity, plant stress and high temperatures. Wintermantel said the movement and stability of the virus in irrigation water, along with its soil-borne nature, causes this disease to persist in soils for many years.

• Steve Fennimore, with UC Davis, studied use of metam sodium for control of lettuce drop and weeds in lettuce.

Vapam suppressed weed in one crop and lettuce drop incidence in a second crop, Fennimore said.

Fennimore also talked of thinning trials in which a machine much like a highway striper was used. The machine applied various materials – including Scythe, sulfuric acid and AN 20 to thin rows.

Sulfuric acid proved most effective, he said. He said it is not clear that the automated thinning is economically viable, remarking that improvements are needed to increase machine speed and precision.

• Krishna Subbarao, plant pathologist with UC Davis, talked of his research on verticillium wilt.

He said lettuce crops that followed three consecutive crops of spinach were more vulnerable to the disease and spinach seeds tend to be more infested with the agent that triggers the disease.

• Lindsey du Toit, vegetable seed pathologist at Washington State University, also addressed the link between verticillium wilt and spinach.

She said research showed the highest presence of verticillum wilt occurred when non-treated seed was used, whether the soil was fumigated or not, adding that fumigation can exacerbate the problem. She said seed treatments did reduce transmission.

Du Toit added that contamination of spinach seed by phomopsis has put a strain on availability of seed because “$20 million of spinach seed from more than five countries is held up at U.S. ports of entry.”

• Steven Koike, plant pathology advisor for Monterey County, talked of an evaluation of winter cover crops and their susceptibility to verticillium wilt.

He said various legume cover crops are capable of becoming infected, but cover crops in general do not seem to be particularly susceptible. He noted that infected cover crops do not show symptoms and the amount of inoculum in the soil is a likely factor in the degree of infestation.