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Recovery of Erwinia amylovora from Symptomless Stems and Shoots of Jonathan Apple and Bartlett Pear Trees. H. L. Keil, Research Plant Pathologist, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705; T. van der Zwet, Research Plant Pathologist, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705. Phytopathology 62:39-42. Accepted for publication 12 July 1971. DOI: 10.1094/Phyto-62-39.

Highly virulent Erwinia amylovora was readily isolated from the internal tissues of symptomless side shoots on artificially inoculated apple and pear trees in the greenhouse. These shoots did not exist at the time of inoculation, but developed from axillary buds immediately below the base of the cankers when blight progress ceased. Bacteria were easily recovered from the symptomless lower stem below the blighted portion also, and the pathogen survived in both types of tissues for periods up to 6 months. In addition, fire blight bacteria were isolated from 60% of apparently healthy suckers from blighted Bartlett trees in the orchard. Of these, 93% showed bacteria in the upper two-thirds of the suckers. Virulent bacteria were also recovered from symptomless shoots of resistant Magness pear buds and from branches of two other pear cultivars, all collected from orchard trees without any record of visible fire blight. Thus, E. amylovora seems to be a resident in symptomless apple and pear tissue, which may explain the frequent, unexpected appearance of the disease in nurseries, young plantings, and well-managed bearing orchards.