VIEW ARTICLE

Occurrence of Fire Blight of Pears in Relation to Weather and Epiphytic Populations of Erwinia amylovora. S. V. Thomson, Assistant Professor, Department of Plant Pathology, University of California, Berkeley 94720; M. N. Schroth(2), W. J. Moller(3), and W. O. Reil(4). (2)Professor, Department of Plant Pathology, University of California, Berkeley 94720; (3)(4)Extension Plant Pathologist and Staff Research Associate, respectively, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 65:353-358. Accepted for publication 21 October 1974. DOI: 10.1094/Phyto-65-353.

Applications of bactericides are often made in California pear orchards long before Erwinia amylovora is present in flowers, and apparently before such measures are warranted. Monitoring pear flowers for the occurrence and population of E. amylovora in 1973, revealed that the bacterium did not colonize flowers during early or full-bloom periods, but subsequently colonized secondary (rattail) flowers in most orchards. In some orchards as many as 18 applications of bactericides were made, even though bacteria were not detected in the flowers. E. amylovora multiplied in healthy pear flowers, and was detected in flowers 14 days prior to disease occurrence in some orchards. Although over 50% of the healthy flowers in some orchards were infested with approx. 10⁶ cells of E. amylovora per flower, subsequent disease incidence was only 1-3 infections per tree. Where populations of fire-blight bacteria in healthy blossoms were low or absent, the incidence of disease was of no economic importance. Precipitation was not requisite for colonization of flowers by E. amylovora, nor for disease development. Fire blight bacteria were on the surface and interior of overwintered cankers, but were not detected in healthy buds from healthy or diseased trees. Erwinia amylovora was detected on leaves, but only after fire blight was common in orchards. Populations of E. amylovora were present on flowers of the alternate hosts, Malus pumila and Pyracantha spp., even though the disease was not observed. The susceptibility of pear flowers to disease varied from year to year, during the season, and among flowers of a cluster. Accordingly, the capacity of E. amylovora to multiply in flowers varied greatly regardless of inoculum concentration. Monltoring epiphytic populations of E. amylovora is now used to improve the timing of bactericide applications for fire blight control.