Endophytic survival of Erwinia amylovora in symptomless pear scions

Pear scions cv. Abate Fetel on B29 rootstock were individually grown in pots from dormant buds. In autumn 2001, the main stems were divided into 5 groups and each group was inoculated at 10 day intervals starting from 21th September with a virulent RifR mutant of Erwinia amylovora. For the inoculation, 30,000 cells of Ea were deposited on a transverse cut 10 cm from the tip. After ensuring establishment of the pathogen, the five groups of plants were placed in the open and inspected weekly for the presence of fire blight symptoms. While 20% of the scions became symptomatic before the winter of 2001-2002, every other scion did not show symptoms of fire blight in 2002 and 2003. In autumn, 1 and 2 years after the inoculation, the asymptomatic plants were analyzed for the endophytic presence of the mutant. Reisolation of Ea on a selective substrate was successful after 1 year in 8 out of 40 plants, but after 2 years 0 out of 146 plants. After 1 year, the reisolation was successful in only one case from a shoot developed from the first node under the inoculation point. The results show that E. amylovora can survive for 1 year in asymptomatic pear scions. The data are discussed in relation to temperature changes during the critical periods of the experiment.