Contamination byPolyscytalum pustulans andPhoma exigua var.foveata of seed stocks derived from stem cuttings in Scotland

Abstract

The incidence ofPolyscytalum pustulans andPhoma exigua var.foveata on virus-tested stocks of seed potatoes derived from stem cuttings (VTSC) was assessed on samples from more than 15 commercial farms in Scotland from 1974 to 1978. Lower class seed stocks (FS3 and AA1) were also examined in 1978 in relation to the region of Scotland in which they were grown. P. pustulans was detected on tubers grown at the DAFS nuclear stock farm only in 1977 butP. exigua var.foveata was present at very low levels in most years. Contamination by both pathogens was much greater on clones grown on commercial VTSC farms than on the DAFS nuclear stock farm. Both fungi were more prevalent on tubers after the third year of multiplication than on those of second year clones. While there was variation between farms in the amount of tuber contamination of VTSC stocks, contamination was reduced considerably by an annual post harvest fungicide treatment. The incidence ofP. pustulans was largely unaffected by the geographical location of the farms.P. exigua var.foveata was more prevalent in the northern counties of Scotland than in Angus, Perth, Fife and Kinross. This difference was not related to either date of haulm destruction or interval between haulm destruction and lifting.