Endophytic survival of Erwinia amylovora in symptomless pear scions

Autumn infections of pear scions through wounds in nursery might be responsible of latent infections and the infected plants could not be recognized by the nurserymen during the uprooting and selection by late autumn. Consequently, symptomless pear scions harbouring cells of Erwinia amylovora sold at the end of the winter can introduce the pathogen in the farm where the new plantation is set up. In this research work the E. amylovora survival in symptomless scions was investigated to evaluate if it were compatible with the role of primary source of inoculum in new plantations. 274 pear scions were contaminated in autumn through a pruning cut in the main stem with a virulent strain resistant to rifampicin. The young pear plants grown singularly in pots were inspected for two consecutive vegetative seasons and after one and two years, pieces of stem and shoot (10 cm in length) were cut in slices (2-3 mm in thickness) and analyzed to detect the presence of the antibiotic resistant strain. The pear plants were symptomless in both vegetative seasons. The rifampicin resistant strain was re-isolated after one year (8 out of 51), but not after two years (0 out of 146). Several pear plants used as a control excluded the endophytic presence of local strains, and the absence of an environmental inoculum. The results of this and of an analogous experiment indicate that latent infections of scions may cause visible infections during the months following the plant out, but not in the following year