Number of TMV Particles Required to Infect Locally or Systemically Susceptible Tobacco Cultivars

Determinations of the efficiency of the process of mechanical inoculation of tobacco mosaic virus (TMV) have usually been made with local lesion hosts. To our knowledge, the greatest efficiencies were reported by Steere (1955), who found that one lesion was produced for every 50000 characteristic particles in the inoculum, and by Schramm & Engler (1958), who reported that the minimal TMV concentration detectable on the local lesion test plant Nicotiana glutinosa was 10-12 to 10-13 g/ml (about 1500 to 15000 particles/ml). In the latter paper, Schramm & Engler (1958) also reported that 50% infection of N. tabacum cv. Samsun, a plant which reacts with systemic symptoms, could be obtained by rubbing leaves with 5 ml of an inoculum which contained 10-16 g/ml of TMV (about 7.5 particles/plant). This indicated that the minimal infective dose for systemically infectible hosts might be less than that for local lesion hosts.