Climatic elimination of mosaic spread in lettuce seed crops in the Swan Hill region of the Murray Valley

Abstract

The elimination of lettuce mosaic disease, formerly an economical important problem of market lettuce in Victoria, has been achieved by the .we of seed produced in the vicinity of Swan Hill. During the last quarter of 1960, trapping records of the incidence of four species of aphid vectors for lettuce mosaic virus were obtained from two localities in that area, and in Melbourne. The aphids Myzus persicae (July.), Macrosiphum euphorbiae (Thos.), Aphis gossypii Glover and Hyperomyzus lactucae (L.), were caught throughout the trapping period in Melbourne, September to December but not during November and December at the Swan Hill sites. The absence of aphids during most of the growing season for lettuce seed crops, explains the progressive reduction of seedborne mosaic which has occurred in varieties grown for several consecutive seasons in the Swan Hill region. During the trapping period more days with temperatures above 90¦F occurred at Swan Hill than Melbourne, and long term records over the same period showed a similar trend. Differences in average relative humidity between the two localities were more significant. Swan Hill records (13 years) showed a progressive and pronounced reduction in humidity during the months September-December, whereas Melbourne records (30 years) were higher and practical constant.