BLOSSOMS OF INFECTED PEAR TREES MAY BE SOURCES OF EARLY INOCULUM FOR FIRE BLIGHT (ERWINIA AMYLOVORA)

Observations in 2006 indicated that blossoms from infected pear trees (Pyrus communis) may be the early sources of inoculum for Erwinia amylovora. Two cider pear trees and two orchards with dessert pear trees, which were infected naturally the year before, were selected for blossom sampling in 2006. The first samples were taken shortly after opening of the first blossoms and tested for epiphytic E. amylovora by nested PCR. Three of four samples were positive. At the second and third sampling date all four samples were positive. Cherry (Prunus avium) and dandelion (Taraxacum officinale) blossoms adjacent to one of the cider pear trees were positive for fire blight at the first and/or second sampling date. Eleven days after the last sampling date single dessert pear trees in the orchards showed many blossom infections whereas adjacent trees were healthy. On the big cider pear trees single or several blossom infections were found. The weather during bloom was cold and rainy. According to the Maryblyt^TM model, propagation and spread of epiphytic bacteria would have been possible at the third sampling date at the earliest. In 2007, E. amylovora was detected in or on flower buds in the green bud stage of a pear tree which was infected in the year before. Furthermore, bacteria were found in pistils of unopened blossoms of a diseased pear tree as well as in internal tissue of the twig where these blossoms were attached to. Blossoms of infected pear trees act as early source of inoculum and pose a high risk to apple trees nearby due to overlapping blooming periods. This corresponds to experiences with fire blight control in Southwest Germany.