coastal California fields.
On the plant resistance front, he said research continues on the second race of Verticillium on lettuce discovered in mid-2005. Progress has been made by USDA breeders searching for potential plant resistance to Race 1, but none has been found for Race 2.
A new test to rapidly distinguish the two races has been developed by USDA scientists, and a wilt-infested field near the USDA station at Salinas is being used exclusively for uninterrupted resistance evaluation.
In his research for the CLRB on another lettuce disease that is spreading, Fusarium wilt, Thomas R. Gordon, plant pathologist at UC Davis.
Fusarium has been most severe in California's desert growing region and Arizona, likely because higher temperatures favor its development. Growers have also found it in the Salinas and San Joaquin valleys during warmer planting periods.
Using a Fusarium-infested field at the UC, Davis campus, Gordon conducted trials where the disease inoculum is uniformly distributed. Trials were planted June 27, July 25 and Sept. 9. Disease progression, he said, was most rapid in the first planting and slowest in the third.
“This correlated with mean temperatures, which were highest following the June planting and progressively lower thereafter.”
He went on to say the results confirmed prior research in Japan and “these effects on temperature are potentially important as they could allow growers minimize damage by selecting appropriate planting windows in areas where the disease is present.”
Most head lettuce varieties are susceptible, but varieties such as Salinas and Grand Max are relatively resistant, and higher levels of resistance are seen in some leaf and romaine varieties.
He said he has learned that the only practical way soil populations of the disease can be reduced is by crop rotation.
“In order to determine how much time is required for a meaningful reduction in inoculum to occur, we are studying the durability of pathogen propagules in microplots of naturally infested soil.”
For example, he found that a sample of 3,482 propagules per gram of soil declined after seven months to 1,022, a reduction of about 70 percent. After an additional nine months, the count was down to 285 or about 8 percent of the original level.