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Monitoring the Epiphytic Population of Erwinia amylovora on Pear with a Selective Medium. T. D. Miller, Assistant Research Plant Pathologist, Department of Plant Pathology, University of California, Berkeley 94720, Present address of senior author: Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster, Ohio 44691; M. N. Schroth, Professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 62:1175-1182. Accepted for publication 17 April 1972. DOI: 10.1094/Phyto-62-1175.

A selective differential medium devised for monitoring epiphytic populations of Erwinia amylovora inhibited growth of most other microorganisms. Although all Erwinia sp. that were tested grew on the medium, each species was identified by its distinctive colony morphology and color. The few pseudomonads and other bacteria that grew on the medium were blue or green in contrast to the red-to-orange hues characteristic of Erwinia sp. Generation times and the percentage of E. amylovora cells producing colonies were similar for the selective medium and standard media. E. amylovora existed as an epiphyte in pear flowers and other plant parts during spring. The bacterium was not detected in leaf or flower buds during winter and early spring months. However, it was detected in flowers and on the surface of cankers (no visible ooze) prior to evidence of flower infection. Epiphytic populations of E. amylovora varied among orchards and trees, and were related to disease severity. Every flower assayed during an epiphytotic was infested with bacteria at populations commonly ranging from 10⁴ to 10⁶ cells/flower. However, relatively few flowers became infected (100 flower infections/tree). In one orchard, 7% of the aborted flowers had a surface population of E. amylovora. Neither the population of E. amylovora nor disease incidence was affected by the occurrence of saprophytic bacteria in flowers. Four to 33% of the flowers were colonized only by E. amylovora. The insects, Pegamya sp. and Minettia sp., carried surface populations of fire blight bacteria ranging from 10¹ to 10⁵ cells/insect. The first naturally occurring streptomycin-resistant strain of E. amylovora was found in a severely diseased orchard. Concentrations of streptomycin up to 200 µg/ml did not affect growth of this strain.