Interactions in fireblight infections

In the host, Erwinia arnylovora (Burrill) Winslow et al. travels primarily in the inter-cellular spaces of immature cortical tissue; progress in mature tissue is restricted. Only in the later stages of infection are saprophytic bacteria such as Erwinia herbicola (Löhnis) Dye commonly found in association with the pathogen. Their effect on the persistence of the pathogen is still uncertain though commonly assumed (with little supporting evidence) to be inhibitory. Most interaction studies have concentrated on the early stages of infection. The interactants have been inoculated prior to or together with E. amylovora and they have included avirulent E. amylovora, phyto-pathogenic pseudomonads, E. herbicola and other saprophytes or cell components including DNA. Our experience reflects that of other workers in that interactants only inhibit at high doses or at high numbers relative to the pathogen. Prior inoculation is not always necessary for protection. In our preliminary studies of the fate of interactants, the outcome varied. Where infection progressed the interactant disappeared, persisted only at the point of inoculation or progressed alongside the pathogen. Where there was protection, both pathogen and interactant disappeared from the tissues or only the interactant persisted at the site of inoculation. Crown gall apart (possibly a special case), observations with other bacterial plant diseases have been similar to those with fireblight with an added one of stimulation of infection by the interactant. The underlying mechanisms of protection probably vary with different interacting systems and cannot always be attributed to a hypersensitivity reaction or to bacteriophage or bacteriocin activity. If a host response is involved, it seems pertinent to ask whether there are simpler ways of achieving protection of growing tissue other than by using bacteria and their products. With epiphytic bacteria two major problems are the achievement and maintenance of high enough populations at critical sites and the lack of major transmitted or translocated effects beyond the site of interaction.