Sources of Potato Blight (Phytophthora infestans) in the East of Scotland

Since 1965, 146 early occurrences of blight on field crops were recorded in the East of Scotland before the end of July. Of these, 119 were investigated to determine the source of infection, and 78 were found to be associated with heaps of discarded potatoes on which Phytophthora infestans was actively sporulating. From samples of bright‐affected foliage taken from such dumps a wide range of races was detected. In 1971 and 1972, attempts were made to control growth on discard heaps throughout East Lothian, a county in South‐east Scotland relatively isolated with regard to spread of blight, to determine whether this would delay the onset of blight in this area. In 1971, blight was recorded first on 17 August whereas further north in the counties of Perth, Angus and Fife it was noted on 5 July. The same pattern obtained in the latter counties in 1972 but no field cases of blight were recorded in East Lothian during the whole season. However, neither season was a ‘blight year’. The lack of sources on potato dumps may have contributed to this delay, or even absence, of blight but other factors may well have had more influence. In the counties where blight occurred early i.e., in July, there was a highly significantly greater total acreage of potatoes, and also the proportion of this acreage devoted to the tuber‐susceptible cultivar King Edward was very highly correlated with the incidence of early reports. The influence of such a cultivar was illustrated by the fact that 45 per cent of the outbreaks of blight recorded up to the end of July related to King Edward.