INTERACTIONS BETWEEN ERWINIA-HERBICOLA AND E-AMYLOVORA ON THE STIGMA OF HAWTHORN BLOSSOMS

Erwinia herbicola HL9N13 is an effective biological control agent of fire blight disease of hawthorn. The interactions between E. herbicola and E. amylovora on the stigma of hawthorn blossoms were examined to assess the possible roles of competition and antibiosis in the mechanism of biological control of blossom blight. Preemptive and competitive colonization of the stigma by the biological control agent reduced the pathogen growth rate and final population. Scanning electron microscopy indicated that E. herbicola colonized the same sites on the stigmatic surface as E. amylovora in its epiphytic phase of development. The competitive advantage exhibited by E herbicola may have resulted from antibiosis. Although E. herbicola HL9N13 produced a broad-spectrum antibiotic on potato-dextrose agar, it was not determined whether this antibiotic was produced in planta. The results suggest that stigma colonization by E. amylovora is prevented by preemptive or competitive occupation of colonization sites by E. herbicola and by the reduction in availability of a resource required by the pathogen for growth at these sites.