SUMMARYTomato varieties with monogenic resistance (tolerance) to tobacco mosaic virus (TMV) were used by British growers for the first time in 1966. Samples of TMV collected before 1966, and in 1967 and 1968, were tested for their ability to produce virus symptoms on a series of isogenic tomato differential hosts differing in three factors for TMV resistance. Samples collected before 1966 yielded only strain o of TMV, which was unable to overcome any of the three factors. Strain 1, able to infect the newly‐introduced resistant varieties, was found with increased frequency after 1966. It was found less frequently on nurseries which had resumed growing susceptible varieties, suggesting that in these it may be unable to compete successfully with strain o. The dangers of indiscriminate release of varieties protected from disease by only one gene are discussed.

SUMMARY 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.

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