Big Vein of Lettuce: Infection and Methods of Control

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Abstract
Olpidium brassicae, the vector of the lettuce big-vein agent (BVA) [which may be a viral agent] is uniformly distributed to a depth of 60-90 cm in big vein-prone soils in the Salinas Valley of California [USA]. O. infects .gtoreq. 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 .mu.g 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 4 field trials, transplanted lettuce had less big vein than did direct-seeded lettuce and in 2 of the 4 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 2 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.