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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 * Corresponding author.

Forty grape-vine rootstocks were inoculated with five strains of Agrobacterium tumefaciens biovar 3 in a field trial. Grape-vine material like Paulsen 775 was immune to all strains whereas Kober 125 AA, Freedom, 3309 C, Harmony, Solonis, 101-14 Mgt., Berlandieri 13/5 and Kober 5BB appeared to be resistant. Jacquez, US 2-1, US 24-23, Muscat d' Alexandrie and US 24-10 were the most susceptible. Host range differences occurred among the five strains of A. tumefaciens on the different rootstock cultivars. Five different host range patterns were evident. A. tumefaciens biovar 3 strain 1771 was the most virulent and strain 2221 was the least virulent of the strains used.

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