The use of insecticides to find when leaf roll and Y viruses spread within potato crops

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SUMMARYAttempts to find when leaf roll and Y viruses spread within Majestic potato crops, made during 4 years at Harpenden by spraying with aphicides to restrict virus spread during four different periods per season, succeeded with leaf roll but not with virus Y.I n 1957, 1958 and 1961 leaf‐roll virus spread mostly before mid‐July, very little between mid‐ July and early August, but a little more from early August to the season's end. In 1959, when the summer was exceptionally fine, more virus spread during July. Four sprays with persistent aphicides: (I) when 50 yo of the crop has emerged from the ground; (2) 10 days later; and (3) and (4) at fortnightly intervals, will prevent leaf‐roll virus spreading within a potato crop until the end of July and in most seasons will therefore prevent almost all spread; the earliest sprays are usually the most important. Systemic soil insecticides applied to prevent leaf roll virus spreading should make the plants lethal to aphids as early as possible.Little was learned about the time of spread of virus Y because in the experimental conditions the insecticides did not prevent its spread.Further evidence was obtained that leaf roll virus is spread within crops mostly by alatae colonizing the potatoes in spring and early summer.