The literature on resistance in tomato to TMV is reviewed . The chronological sequence of publication and the relationships between the material of different workers is show graphically. The sources of resistance and the genotype/environment interaction is discussed . The author asks for an increased standardisation of techniques, greater attention to tests for allelism and a responsible attitude when resistant varieties are to be released .There are 54 references INTRODUCTION

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SUMMARYThe symptoms and virus content of isogenic tomato genotypes differing by three tobacco mosaic virus (TMV) resistance factors, Tm‐I, Tm‐2 and Tm‐22, were studied in relation to various isolates of TMV and four strains were identified. The common strain induced no symptoms on plants with any of the factors for resistance, one strain caused symptoms on Tm‐I plants, one on Tm‐2 plants and one on both Tm‐I and Tm‐2 plants and also on Tm‐I Tm‐2 plants. No strain induced symptoms on Tm‐22 plants.The gene, Tm‐I, was found to be dominant or incompletely dominant for preventing symptom development but was recessive or intermediate for limiting virus multiplication of the common strain. Both Tm‐2 and Tm‐22 were dominant for a hypersensitive response to the common strain. Virus multiplication was temperature‐dependent. The background or varietal genotype did not affect virus multiplication.A systemic necrosis of Tm‐22 plants occurred only when heterozygous Tm‐22 was not protected by other factors against specific strains of TMV.The complexity of the host genotype, pathogen genotype and environment interactions are outlined and the exploitation of the resistance factors in tomato breeding discussed.