ISHS Acta Horticulturae 273: V International Workshop on Fireblight
POSSIBLE ROLE OF NATURALLY OCCURING NON AGGRESSIVE ISOLATES OF ERWINIA AMYLOVORA IN THE EPIDEMIOLOGY OF FIRE BLIGHT |
| Authors: | A. MENDOZA-HERRERA, J. PAULIN |
| Abstract:
It has been previously shown (Mendoza-Herrera and Paulin, 1989), that certain lesions of fire blight may contain a mixture of aggressive and (in low proportion) non aggressive isolates of E. amylovora. The possible role of these non-aggressive populations in the epidemiology of the disease has been studied with two types of experiments, supposed to simulate cases of mixed infections in natural conditions :
The inoculations of a mixture of two non aggressive isolates may give infections (on pear seedlings) at least in some cases. A level of aggressiveness comparable to that of a normally aggressive strains has been obtained with the inoculation of a mixture (9/1) of a capsulated isolate with a non capsulated one (both avirulent). This could support the observation of Hignett and Roberts (1989) showing that a balance between capsulated and non capsulated cells in the bacterial population of the pathogen is needed for pathogenicity. Restoration of pathogenicity by a mixture of two non virulent strains had been formerly described (Bennett, 1980). The inoculation of a mixture of an aggressive isolate with a non aggressive one usually gave a significant reduction of the aggressivity of the virulent isolate inoculated alone, provided the ratio between aggressive and non aggressive isolates was higher than 1/10. This protective effect was not due to direct antagonism between isolates on the plant : it has been checked that the population of the aggressive isolates remained present, even 14 days after inoculation, at a high level, at the infection site. So did the population of the non aggressive isolate. At least two roles of the non aggressive part of natural populations of E. amylovora could then be suggested : either, if mixed in suitable proportion, they may regain a certain level of pathogenicity, or, mixed with normally aggressive isolates, they may to a certain extent prevent the expression of this aggressivity. | |
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