Erwinia herbicola: A Bacterial Ice Nucleus Active in Increasing Frost Injury to Corn

Cell suspensions of an isolate of Erwinia herbicola from a corn leaf were active in ice nucleation at   -2.3 C and below. Ice nucleation activity was affected by the medium on which the cells were grown. Nucleation at -2.3 and -2.5 C was detected in suspensions of cells that had been grown on nutrient agar supplemented with 2.5% glycerol or glucose, respectively, but was detected only at temperatures below -4 C in cells grown on nutrient agar alone. Corn seedlings sprayed with suspensions of this isolate of E. herbicola were severely damaged at -4 C, whereas control plants lacking leaf populations of E. herbicola or other bacteria active in ice nucleation were not injured. If plants were frozen 6 hr or less after they were sprayed with suspensions (10⁷ cells/ml) of E. herbicola, frost damage was not significantly different from that to controls. The amount of damage increased greatly with increase in time of incubation between spraying and freezing, from 12 to about 36 hr. The amount of frost damage measured 48 hr after application of E. herbicola suspensions increased as applied bacterial cell densities were increased. Erwinia herbicola began rapid multiplication about 6 hr after the cells were sprayed onto leaves; populations of about 3 × 10³ cells/g fresh weight of leaf at 6 hr after inoculation had increased to about 5 × 10⁶ cells/g after 36 hr. Probit frost injury was directly proportional to the logarithm of E. herbicola population (P <0.001).