THE CORRELATION OF CERTAIN QUANTITATIVE CHARACTERS WITH THE VIRULENCE OF ERWINIA CAROTOVORA.

Abstract

Erwinia strains from several geographic regions and several hosts were evaluated for virulence, sensitivity to siderophores produced by Pseudomonas fluorescens (Pf) and bacteriocins produced by Erwinia, and for the presence of plasmids. Selection of virulent strains of Erwinia for use in plant breeding programs for resistance to disease might be based upon quantitative characters which are correlated with disease severity rather than the biochemical reactions used to distinguish Erwinia carotovora subspecies. Quantitative assays for motility, polygalacturonate degradation, potato tuber infection, and tuber decay revealed that motility was correlated with infection (r = 0.83, p = 0.01) and polygalacturonate degradation with decay (r = 0.84, p = 0.01) of potato tubers. Siderophores produced by Pf and bacteriocins produced by Erwinia yielded variable results in bioassays against the Erwinia strains studied. Six of the 12 strains of Erwinia tested were resistant to Pf siderophores. The growth inhibition of sensitive strains was bacteriostatic and reversible by addition of iron (Fe II or Fe III) to the culture medium. Additionally, only one strain of Erwinia was sensitive to the bacteriocins produced by the other 12 strains. The resistance of Erwinia strains to Pf siderophores and Erwinia bacteriocins severely limits the potential for widespread application of these agents in biological control of Erwinia. Bacteriocin-like structures were detected in culture extracts of all 12 Erwinia strains studied. The presence of bacteriocins is indirect evidence that these strains harbor plasmids. Bacteriocin-coding plasmids may be the source of genetic and phenotypic variability demonstrated by the erwinias. The status and value of subspecific classification of Erwinia carotovora may therefore require re-evaluation.