Control of lettuce necrotic yellows virus disease by the destruction of common sowthistle (Sonchus oleraceus)

Control of the spread of necrotic yellows virus to lettuce was attempted on one vegetable farm in the ~Melbourne vegetable area destroying sowthistles (Sonchus oleraceus L.), the only known source of the virus and its specific vector, Hyperomyzus lactucae L. Thistles were destroyed in an area 470 yd x 170 yd surrounding a February-sown plot of Imperial D lettuce, at first by power-spraying with diquat weedicide but mostly by manual methods. Approximately 60 man-hours were spent on thistle control between February 21 and April 3, 1962. At maturity, 6.0 per cent of lettuce plants was lost through necrotic yellow infection, which was randomly distributed throughout the crop. On a neighbouring farm where sowthistles were not controlled, 75.6 per cent of a crop of the same variety sown at the same time was lost through the disease. In this crop the pattern of disease spread was typically marginal from thistles located along one boundary.