Putrescine (0.5 mM in water) was applied to roots and leaves of plants 3 h before inoculation with R. solanacearum via (A-C) stem inoculation or (D) leaf infiltration. (A and B) Effect of putrescine on wilt symptom development after stem inoculation of (A) susceptible tomato with 50 CFU R. solanacearum GMI1000 (P=0.0183, repeated measures ANOVA; values are mean ± SEM; n=60 plants/treatment) or (B) quantitatively resistant tomato with 5,000 CFU R. solanacearum resistance-breaking strain UW551 (P=0.0403, repeated measures ANOVA; Values are mean ± SEM; n=60 plants/treatment). Effect of putrescine on R. solanacearum (C) growth and spread in tomato stem after stem inoculation of putrescine-treated (red) or control (black) plants, (*P<0.05 Mann-Whitney test, n>80 plants/treatment) and (D) growth in tobacco leaf apoplast after putrescine (red) or water (black) treatment (*P<0.05 t-test; n>12 plants/treatment).