Articles
EVALUATION WITH DIFFERENT ISOLATES OF ERWINIA AMYLOVORA OF THE SUSCEPTIBILITY TO FIRE BLIGHT OF APPLE CULTIVARS
Conversely growing very susceptible varieties is not economically feasible, whatever the control measures, as soon as conditions for fire blight infections are too favourable.
The best possible knowledge is therefore needed on fire blight susceptibility, before plantation in an area where infection is likely.
Besides, an accurate scale of susceptibility must be available for screening in breeding programs aimed at selecting resistant plant material.
Assessment of varietal susceptibility is not a simple problem : it can be achieved with (i) the observation of the behaviour of trees, in the orchard, under natural conditions of infection, or (ii) by experimental inoculations of especially planted trees in an experimental field.
Both have their own advantages and disadvantages.
It is likely that the most complete information about fire blight susceptibility is given by a combination of these two assessments : fortunately they are not always conflicting !
Among the factors which prevent each of these observations to be always of general value, is the choice of the indexes used to quantify the disease.
Several criteria (infectivity, severity, varietal index of susceptibility) (ALDWINCKLE and PRECZEWKI, 1976 ; LE LEZEC and PAULIN, 1984) have been proposed, which can tell different things.
Besides, the isolate used to inoculate the plants under test, or naturally infecting an observed orchard, may be at the origin of limitation of interest of the assessment of the resulting infection.
It had long been considered that, in the case of Erwinia amylovora only differences in level of agressivity could be found between isolates.
This has been experimentaly shown by QUAMME and BONN (1981) on pear cultivars.
In the case of apple NORELLI and ALDWINCKLE (1982), NORELLI et al (1984) showed that interactions between isolates and cultivars can be found, thus revealing possible specific virulences among isolates of E. amylovora.
In our experiment, in DAX experimental field, we have attempted (i) to select a convenient disease index and then (ii) to compare with the same varieties isolates of E. amylovora of U.S. and French origin.