These 5 isolates with a high level of agressivity have been studied with 3 different conidia concentrations: 107, 106 and 105 conidia per ml. Twenty plants of 3 near isogenic Moneymaker genotypes were inoculated - Monalbo, line without Frl - Momor, line with Frl - F1 Momor x Monalbo, F1, hybrid FrI/Frl+ The agressivity of 4 among these 5 isolates was confirmed with the concentration 107 on the 3 genotypes. The fifth having lost its pathogenicity. With the concentration 106 the agressivity of the 4 isolates was always high on Monalbo line and comparatively was reduced on Momor and on the F1, hybrid. With the concentration 105 the agressivity of the different isolates was lower on Monalbo line and near void on Momor. The comportment of the F1 hybrid was intermediate, near the resistant line, confirming thus the incomplete dominance of Frl. This study shows a large variation for the level of agressivity between the FORL isolates. Some of these shown a strong agressivity but not a virulence change. A major gene controlling the expression of resistance to AMV in Lycopersicon hirsutum P1 134417 Parrella, G.1, Laterrot, H.2, Marchoux, G. 3, Gebre-Selassie, K 3, Monti, L.M.1.   1 Department of Agronomy and Plant Genetics - Via Universita, 100 - 80055 Portici - Italy 21NRA - Station d'Amelioration des Plantes Maraicheres - BP 94 _ 84143 Montfavet Cedex France   31NRA - Station de Pathologie Vegetale - BP 94 - 84143 Montfavet Cedex France Resistance to AMV (alfalfa mosaic virus) has been already reported in Lycopersicon hirsutum f. typicum LA 1777 and in L. hirsutum f. glabratum PI 134417 and "Bruinsma" accessions (Parrella et al., 1996; Parrella et al., 1996). In the present study, PI 134417 was used as donor of resistance in interspecific crosses with L. esculentum "Momor" (gene Tm-22), and in intraspecific crosses with PI 247087 Australia accession of L. hirsutum f. glabratum, highly susceptible to AMV. The AMV isolate LYH-1 was used in the study of the inheritance of resistance to AMV by evaluation of F1, F2 and BC1 generations after mechanical inoculation in greenhouse test. All the plants material were checked visually for symptoms development and by DASELISA tests, on the upper non inoculated leaves, 15 and 30 days after inoculation. The age of the plants at the inoculation time was 14 days old.

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