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Big Vein of Lettuce: Infection and Methods of Control. R. N. Campbell, Professor, Department of Plant Pathology, University of California, Davis 95616; A. S. Greathead(2), and F. V. Westerlund(3). (2)Farm advisor, Agricultural Extension Service, Monterey County, Salinas, CA 93901; (3)Plant pathologist, Moran Seed Company, Salinas, CA 93901. Phytopathology 70:741-746. Accepted for publication 31 January 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-741.

Olpidium brassicae, the vector of the lettuce big-vein agent (BVA), is uniformly distributed to a depth of 6090 cm in big vein-prone soils in the Salinas Valley of California. Olpidium infects ≥ 50% of the lettuce seedlings as early as 8 days after summer plantings or 15 days in winter plantings that emerge more slowly. Soil temperatures at the 10-cm depth were not different between big-vein-prone soils and nearby non-infested soils at any season. Fungicides were tested at 100 μg a.i./ml for effects on zoospore motility and infectivity, and on growth and maturation of thalli in vivo. Fenaminosulf generally was ineffective; metalaxyl stopped zoospore motility but not infection or reproduction in vivo; pyroxychlor, captan, or ethazole (5-ethoxy-3-[trichloromethyl]-1, 2, 4-thiadiazole) stopped motility and infection by zoospores, but were ineffective in vivo; triadimefon or benomyl did not stop motility, but did prevent infection or reproduction. Benomyl was systemic in the roots and prevented reproduction in seedlings for >7 days after 1 day of uptake. In four field trials, transplanted lettuce had less big vein than did direct-seeded lettuce and in two of the four trials a benomyl drench prior to transplanting reduced the incidence still further. Annual soil fumigation with methyl bromide (224 kg/ha) was tested in field plots for 3 yr; it controlled Olpidium, reduced the incidence of big vein in two succeeding lettuce crops, and increased the rapidity and uniformity of maturity. Chloropicrin (336 kg/ha) or Vorlex (80% chlorinated C3 hydrocarbons + 20% methylisothiocyanate) (234 or 468 L/ha) did not control Olpidium or big vein, but increased plant vigor, size, and the rapidity and uniformity of maturity in some trials.