Population of Erwinia amylovora on External and Internal Apple Fruit Tissues. T. Vander Zwet, Research Plant Pathologist, U.S. Department of Agriculture, ARS, Appalachian Fruit Research Station, Kearneysville, WV 25430. S. V. Thomson, R. P. Covey (deceased), and W. G. Bonn. Professor, Department of Biology, Utah State University, Logan 84322-5305; Plant Pathologist, Washington State University, Tree Fruit Research Center, Wenatchee 98801; and Plant Pathologist, Agriculture Canada Research Station, Harrow, Ontario NOR 1GO. Plant Dis. 74:711-716. Accepted for publication 9 February 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990. DOI: 10.1094/PD-74-0711.

Injury of apple fruit at time of dip inoculation with Erwinia amylovora led to increased disease when compared to injury after or before inoculation. Few fruit expressed symptoms when injured 96 hr after inoculation. Fruit of Delicious, a relatively resistant cultivar, and Rome Beauty, a susceptible one, became infected more frequently when injured with bruising or large punctures than with small punctures. Blight developed on 4% of Rome Beauty fruit that were punctured but not artificially inoculated. Moreover, only 1% of 125 surface-disinfested Rome Beauty fruit, collected from apparently healthy trees, developed blight symptoms during a 4-mo cold storage period, compared to as much as 15% of fruit taken from blighted trees. No E. amylovora was detected on or inside 210 Delicious fruit that had been obtained from two orchards without blight in Washington state. Endophytic populations of E. amylovora were recovered from apples located within 30 cm from blighted shoots but not from those 60 or 200 cm away. The bacterium was not detected in core tissues of 280 apples sampled from four cultivars collected from apparently healthy trees grown in four regions of North America. Thus, chances for the dissemination of E. amylovora to areas or countries without fire blight is extremely unlikely when undamaged Delicious fruit are harvested a minimum of 100 cm from visible blight symptoms or, preferably, from apparently healthy trees located in orchards free of fire blight.

Keyword(s): detection, export, quarantine.