“…The second step, inhibition of virus multiplication, has already been reported in several cases: the N resistance to tobacco mosaic virus (TMV) in tobacco (Loebenstein & Gera, 1990), the Tm-1 resistance to TMV in tomato (Fraser, 1985;Meshi et ai, 1988), and the resistance to cowpea mosaic virus in cowpea (Ponz et ai, 1988). At the whole-plant level, we have observed that virus multiplication takes place following mechanical inoculation of leaves but not of the root.…”