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Protection of Pear Against Fire Blight by Bacteria and Bacterial Sonicates. John L. McIntyre, Department of Botany and Plant Pathology and Biochemistry, Purdue University, West Lafayette, Indiana 47907; J. Kuc(2), and E. B. Williams(3). (2)(3)Departments of Botany and Plant Pathology and Biochemistry, Purdue University, West Lafayette, Indiana 47907. Phytopathology 63:872-877. Accepted for publication 17 January 1973. DOI: 10.1094/Phyto-63-872.

A delay in the expression of fire blight symptoms occurred in actively growing ‘Bartlett’ pear shoots when inoculation with 1.2 × 10⁶ cells of avirulent Erwinia amylovora, E. herbicola, or Pseudomonas tabaci (inducers) preceded inoculation with the same concentration of virulent E. amylovora cells (challenge) by 24 hr. In some experiments this delay appeared permanent. A delay in symptom expression did not occur when the challenge followed the inducers by 0.5 hr or when Xanthomonas campestris was used as the inducer. Similar results were obtained in 10-day-old etiolated Bartlett pear seedlings. A delay in symptom expression occurred when inoculation with 10⁶ cells of the inducers preceded inoculation with 10⁴, 10³, or 10² cells of the challenge by 24 hr. In several experiments, delay in symptom expression occurred when challenge followed inducer by 0.5 hr. No delay was noted when X. campestris was used as the inducer. The similarities in response between clonal Bartlett pear shoots and etiolated seedlings indicate that etiolated seedlings may be used to study the nature of the protection. A delay in symptom expression also occurred when cell-free sonicates of both avirulent and virulent E. amylovora were used as inducers. In vitro experiments showed that sonicates did not inhibit the growth or virulence of E. amylovora.