The establishment of a new strain of tobacco mosaic virus resulting from the use of resistant varieties of tomato

SUMMARY Tomato 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.