ONE YEAR SURVIVAL OF ERWINIA AMYLOVORA IN SYMPTOMLESS PEAR SCIONS
F. Traversa, U. Mazzucchi
Pear scions cv. ‘Abate Fetel’ on B29 rootstock were raised individually in pots from dormant buds. In autumn 1999, the main stems were divided into 5 groups and inoculated at 10 day intervals starting from 21 September with a virulent RifR mutant of Erwinia amylovora. For the inoculation, 30,000 cells of E. amylovora 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. Scions treated with water were used as a control. All symptomatic scions were eliminated by mid-April 2000. The frequency of scions developing symptoms (21.3%) decreased from the first to the last inoculation date (from 5/15 to 1/15). In autumn, a year after inoculation, the plants remaining symptomless were monitored for the presence of endophytic E. amylovora. Re-isolation on selective medium was successful in 13 plants out of 41. Reisolation was also successful in 7 segments taken from the side shoots developed from the first and second nodes below the inoculation point. The results show that E. amylovora can survive a year in symptomless planted scions. Isolation of the mutant from symptomless shoots developed during the growing season following inoculation indicates that E. amylovora may have a prolonged symptomless endophytic phase.