Five isolates (2, 3, 4, 5 and 6) were isolated from fire blighted shoots of pear and identified as E. amylovora. The performed tests for confirm identification revealed that isolates 1, 2, 4 and 6 were equal and most virulent whereas isolate 3 was the least virulent on both slices and sound immature pear fruits. However, E. amylovora isolates 4 was the most pathogenic to the in vitro communis plantlets followed by isolates 6, 5 and 2, respectively. The disease incidence was greatly varied according to inoculation method and tested isolate. In leaf spray method the disease incidence was obviously higher than injection one. In the injection method, isolate-6 was the most virulent while isolate-4 was the most virulent in case of the leaf spray method. Typical disease symptoms were incited on the in vitro communis pear plantlets either inoculated with filtrates of E. amylovora-cultures or the attenuated cells of E. amylovora.
The communis and betulaefolia rootstocks responded differently against infection with fire blight (E. amylovora -6). The first rootstock was more susceptible to infection than the latter one. Thus, plantlets developed from betulaefolia rootstock were more resistant to infection than those developed from communis one.
The free and total phenols content were obviously higher in the diseased-responded plantlets of communis rootstock than betulaefolia one while, the opposite trend was noticed in the sugar content. Comparing with healthy-responded plantlets, the reduction in the free phenols in the diseased plantlets was higher in betulaefolia rootstock than communis rootstock while the opposite trend was noticed in the conjugate phenols comparing with the healthy responded plantlets. However, reduction in the sugars content in the diseased plantlets of communis rootstock was obviously higher than plantlets of betulaefolia rootstock. The acclimatized plantlets of betulaefolia rootstock showed lower disease incidence and higher survivals than communis rootstock.
Key words: Fire blight, E. amylovora isolation, Pathogenicity, Erwinia attenuation, pear rootstocks.