FACTORS AFFECTING THE SPREAD OF APHID-BORNE VIRUSES IN POTATO IN EASTERN SCOTLAND III. EFFECTS OF PLANTING DATE, ROGUING AND AGE OF CROP ON THE SPREAD OF POTATO LEAF-ROLL AND Y VIRUSES

Experiments at Invergowrie, south-east Perthshire, showed that the extent of spread of potato leaf-roll and Y viruses varied from year to year and that virus Y consistently spread more than leaf roll. Most spread of Virus Y occurred before the end of June and of leaf-roll virus before the end of July. Both viruses spread slightly more in late-than in early-planted crops. When plants with leaf roll and already colonized by Myzus persicae were placed in a healthy crop of Majestic potatoes at intervals during the season, the amount of virus spread decreased rapidly with increasing age of the crop. Spread of leaf roll occurred in all of twenty-five 'seed' crops in different districts of eastern Scotland in 1955 but in only twenty out of thirty-six similar crops in 1956. Annual and regional differences in virus spread appear to reflect differences in the time at which migrant aphids reach potato crops in early summer and the rate at which infestation builds up in the crops.