Lettuce Continued from Page 6
noted that they discovered “a novel race variant” characterized by its resistance to the Banchu Red Fire lettuce variety and susceptibility to the commercial cultivar San Miguel. In a survey of 23 lettuce culti- var locations, Putnam said, “we observed visible symptoms of either internal vas- cular discoloration or external root rot at all locations.” He said the discoloration was indicative of Fusarium or Verticillium wilt, and the root rot was evidence suggesting the pres- ence of Pythium wilt, a pathogen that can survive in the soil without a host plant. It enters lettuce plants through the roots and causes the crop to wilt and die. Studies of the new Fusarium race and its possible connection to Pythium continue. “In many cases, lab analyses couldn’t confirm the suspected disease based on visual symptoms,” Putnam said. “Ongoing work aims to further confirm the novel race variant and evaluate isolates from more locations.” He said an “analysis of Fusarium- Pythium coinfections is ongoing.” Researchers say they are making progress on detecting Pythium in the soil before it infects lettuce and aim to someday predict whether there is enough Pythium in the ground to be problematic for lettuce crops. (Bob Johnson is a reporter in Monterey County. He may be contacted at bjohn11135@gmail.com.)
UCCE plant pathologist Krishna Subbarao, based in Davis, is heading efforts to contain new varieties of Verticillium wilt. After the disease was first discovered in lettuce 30 years ago, researchers found sources of genetic resistance to race 1 of the disease, which causes leaves to turn yellow and die. Researchers found sources of resistance to Verticillium race 1 in the La Brillante let- tuce variety, and that resistance is available in commercial lettuce varieties. But races 2 and 3 of the disease have developed and spread in the Salinas Valley. Now, grow- ers must manage for three races of lettuce Verticillium, and plant breeders struggle to keep up with the disease. Subbarao screened lettuce varieties for resistance to Verticillium found in Watsonville and areas of the Salinas Valley, from Salinas south to King City. He said efforts are continuing to find lettuce variet- ies for sources of resistance to Verticillium races 2 and 3. The pathogen that causes Verticillium has long been an issue in strawberries, artichokes and spinach. It began spread- ing in lettuce fields after strawberry growers stopped fumigating their soil with a combination of chloropicrin and methyl bromide. The pathogenic fungus, Verticillium
A lettuce field is infected with Verticillium wilt, for which no effective treatment exists once the pathogen spreads. Researchers are also targeting Pythium wilt, which can live in the soil without a plant host and later infect lettuce through the roots.
dahliae, “infects and damages numerous crops, including artichoke, spinach and strawberry,” according to the UC pest man- agement guidelines, which said the patho- gen “can persist in the soil for an almost indefinite period of time.” Putnam said there is concern that Fusarium may also be associated with Pythium wilt, a water mold that has
increased in Salinas Valley lettuce during the past five years. “It’s unclear whether Fusarium is im- pacting the impatiens necrotic spot vi- rus-Pythium wilt epidemic,” which has devastated coastal lettuce crops in recent years, Putnam said. Researchers have collected 278 Fusarium isolates from 47 locations. He
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June 5, 2024 Ag Alert 7
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