In 1971, irrigation at Gleadthorpe E.H.F. had little or no effect on yields but, when applied early in the season, greatly decreased common scab on the susceptible cultivars King Edward, Majestic and Désirée. Record, and especially Pentland Crown, had little scab with or without irrigation.
Most-scab infection occurred during dry weather in late June and early July. Irrigation for 6 wk from 4 June, at 0.6 in. (15 mm) soil moisture deficit (S.M.D.) controlled scab most effectively, but 6 wk was only marginally better than 4 wk. With other regimes, the severity of scab increased as the S.M.D. allowed before irrigation was increased. King Edward tubers from unirrigated plots were blemished more in 1971 than in the two previous dry seasons, partly because rain caused tubers to swell more than previously and enlarged scab lesions so much that healthy tissue between scabs was ruptured.
More tubers per plant were formed from King Edward when seed tubers were treated with benomyl and oxycarboxin before planting than when untreated, but irrigation even from 75 per cent emergence had no effect on tuber numbers.