Sixty-five strains and isolates of Agrobacterium tumefaciens representing each of the known biotypes, were tested for in vitro and in vivo susceptibility to the
agrocin-producing strains Agrobacterium radiobacter 84 and A. tumefaciens D286. No biotype 3 strain was susceptible to the effects of either of the agrocinogenic strains in vitro. On datura and tobacco, the best inhibition of
tumor formation was obtained when the agrocinogenic strains were applied to
wounds 24 h before the pathogens and by the concomitant application of
agrocin producer and pathogen at cell ratios of 10:1 or 3:1; inhibition of
infection tended to decrease progressively as the cell ratio decreased from 10:1 to 3:1 to 1:1. Generally, strain 84 was superior to D286 in inhibiting
tumor formation. A combined cell
suspension of 84 and D286 was as effective as 84 alone. The overall pattern of inhibition of
tumor formation by biotype 1 and 2 pathogens resistant to the agrocinogenic strains in vitro was similar to that obtained with strains that were susceptible in vitro.