ROLE OF THE AGGRESSSIVENESS OF STRAINS OF ERWINIA AMYLOVORA IN THE EXPERIMENTAL EVALUATION OF SUSCEPTIBILITY OF APPLE CULTIVARS TO FIRE BLIGHT

ISHS Acta Horticulturae 338: VI International Workshop on Fireblight

**ROLE OF THE AGGRESSSIVENESS OF STRAINS OF ERWINIA AMYLOVORA IN THE EXPERIMENTAL EVALUATION OF SUSCEPTIBILITY OF APPLE CULTIVARS TO FIRE BLIGHT**

Authors:

J.P. Paulin, G. Lachaud, Y. Lespinasse

Abstract:
Determination of susceptibility to fire blight of genotypes is a difficult problem. Among the causes of variations in the assessment of the plant response to inoculation the role of the characteriscs of the inoculated strain of Erwinia amylovora is of key-importance. In the case of apple genotypes Norelli et al (1984) demonstrated differential virulence between strains of E. amylovora, caused by interactions between strains and cultivars.

We shoot inoculated 5 selected strains of the pathogen [US (Cornell) standard : 273, strains with differential virulence (Norelli et al, 1984) : 266, French past and present standards : 2045 and 1430, and a highly virulent strain from Canada : 265] to 48 genotypes of apple in the field and/or in the glasshouse. Results of the inoculations were assessed according to 2 criteria : Frequency (F), and Mean Length of Necrosis (MLN).

Our results can be summarized as follows :

A scale of aggressiveness between strains was apparent, when results of shoot inoculations of each strains on the whole set of cultivars were compared with each others. The rating of strains from the most to the less virulent was : 265, 266, 2045–273, 1430. This rating was similar, either based of F or an MLN values.
This general pattern gave a schematic view of the behaviour of each strain on each cultivar. Strains may behave differently on different cultivars. Three typical cases could be identified :

cultivars showing the same level of resistance to the 5 strains (e.g. Florina x Prima),
cultivars showing the same level of susceptibility to the 5 strains (e.g. Reine des Reinettes),
cultivars showing diverse degree of susceptibility, according to the inoculated strain (e.g. Robusta V).

Therefore, a judgment on the resistance (or susceptibility) to fire blight of an apple genotype can not often be based on the inoculation of a single strain. We propose that the difference in response obtained with different strains of E. amylovora is taken into account in the assessement of the resistance of the genotype. This could be expressed through the use of an Index, IMS (Index of Modulation of Susceptibility) calculated

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