Fire blight. Why do views on host invasion by Erwinia amylovora differ?

Abstract

The fire blight pathogen, Erwinia amylovora, commonly infects flowers and shoots of certain rosaceous hosts and systemic (whole‐tree) invasion sometimes follows. The bacterium may be found in the parenchyma of bark tissue and/or in mature xylem vessels of stem tissue. Views differ on initial sites of multiplication and the optimal route for systemic migration. This article presents the evidence on which the different views are based. There are limited observations on orchard pear and apple trees; in most experimental studies, young apple shoots on potted plants were used. Tissue maturity at the site of shoot inoculation is of prime importance. If xylem vessels are damaged, inoculum may be sucked into the vessels and the bacteria will multiply there. In younger tissue, there is less suction pressure. The critical stem entry site for the invasion of cortical parenchyma seems to be near or above the most recently unfolded leaf. No one has suggested that migration in bark tissue cannot be a major route. If the xylem route is followed, the pathogen needs a means of escape into bark tissue, where typical symptoms develop; means of escape from mature xylem vessels have not been demonstrated and remain a matter for speculation. Published evidence does not seem to support the idea that fire blight is a vascular wilt disease, nor that the extracellular polysaccharide produced by E. amylovora is a toxin and responsible for the wilting symptom seen in the early stages of the disease.

Genomics and current genetic understanding of Erwinia amylovora and the fire blight antagonist Pantoea vagans

“…E. amylovora lacks enzymatic means for penetrating healthy host tissues and infects plants through natural openings (e.g., floral nectaries, leaf hydathodes) and via wounds (e.g., from hail or insect damage). Once inside a host, the pathogen can spread in the plant through the vascular system (Billing 2011), and aggressive sanitation is the key to remove inoculum reservoirs (e.g., tree removal) and prevent further advance within infected hosts (e.g., pruning well beyond visible disease symptoms). Dependent on the infected plant part, the disease develops as flower, shoot or rootstock blight.…”

Complete sequence of Erwinia piriflorinigrans plasmids pEPIR37 and pEPIR5 and role of pEPIR37 in pathogen virulence

“…Care should also be taken when pooling observations of different studies where experimental conditions that can influence disease severity typically vary (e.g. pathogen inoculum concentration, inoculation conditions; Cabrefiga & Montesinos, 2005;Billing, 2011).…”

Erwinia spp. from pome fruit trees: similarities and differences among pathogenic and non-pathogenic species

“…The pathogen needs some means of movement from vessels into the bark tissue, but this mechanism also remains unknown. See Billing (2011) for a re-examination of the available evidence on which these divergent views are based.…”