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Ecology and Epidemiology

Fire Blight Epidemiology: Factors Affecting Release of Erwinia amylovora by Cankers. Steven V. Beer, Assistant Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; John L. Norelli, former Experimentalist, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Present address: Department of Plant Pathology, University of California, Berkeley, CA 94720. Phytopathology 67:1119-1125. Accepted for publication 17 March 1977. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1119.

Cankers with different characteristics in one pear and two apple orchards were studied to determine which were effective producers of primary inoculum and when inoculum was produced. Erwinia amylovora was recovered at least once from intact canker margin surfaces of 19 of 122 cankers that were swabbed weekly from April through July. Fifteen of the 19 active cankers initially yielded E. amylovora during a 2-wk period after bloom. Cankers from which E. amylovora was recovered generally had indeterminate-type (smooth) margins and were located on wood at least 4 yr old. The pathogen was recovered from a significantly greater proportion of cankers, the margins of which had been covered with moist pads, than from similar cankers that were exposed to ambient moisture. Erwinia amylovora was recovered from blossom buds or blossoms collected from clusters in the immediate vicinity of two cankers that had extended before bloom, but the pathogen was isolated from these two cankers surfaces only after bloom. Only clusters that were included in the pathogen- positive blossom samples became infected. Inoculation of nursery-grown apple trees early during the growing season produced cankers with determinate-type margins; later inoculation yielded cankers with indeterminate-type margins. When these cankered trees were potted, chilled, and then placed under controlled-environment conditions, E. amylovora was recovered from a significantly greater proportion of trees with indeterminate-type canker margins than from those with determinate-type canker margins. More of the cankered trees grown at 21 C yielded E. amylovora than those grown at 17 C or 28 C.