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Erwinia alni, a New Species Causing Bark Cankers of Alder (Alnus Miller) Species

¹Istituto di Patologia e Zoologia forestale e agraria
²Dipartimento di Scienze e Tecnologie Alimentari e Microbiologiche, Sezione Microbiologia Applicata
³Università di Firenze, 50144 Florence, Italy, and Central Science Laboratory, Hatching Green, Harpenden AL5 2BD, England

^* Corresponding author. Mailing address: Istituto di Patologia e Zoologia forestale e agraria, Università di Firenze, P. le delle Cascine 28, 50144 Florence, Italy.

ABSTRACT

The causal agent of an undetermined disease of black alder (Alnus glutinosa) and Italian alder (Alnus cordata) was identified as an Erwinia species. This alder bacterium induces dark brown necrotic cankers on alder plants; these cankers are often longitudinally elongated and occur in the bark of the trunks and also in the bark of branches, twigs, and suckers. A dark watery liquid often exudes from small cracks in the cankers and stains the bark surface. Disease symptoms were produced on the trunks of 2-year-old black and Italian alder trees after artificial inoculation of selected strains of bacteria obtained from typical bark cankers (G. Surico and L. Mugnai, Inf. tore Fitopatol. 12:41-43, 1992). Morphological examination of the pathogen by electron microscopy showed that it is a motile rod-shaped organism. All of the strains examined were gram negative, oxidase negative, and facultatively anaerobic with fermentative metabolism and had the general characteristics and fatty acids of members of the family Enterobacteriaceae. Strains PVFi 20^T (T = type strain), PVFi 23, PVFi 25, and PVFi 27 were chosen for further characterization. These four strains exhibited 96 to 100% DNA homology in hybridization experiments performed at 40 and 50°C. They were most closely related to Erwinia nigrifluens (levels of homology 40°C, 49 to 65%). Phenotypic differentiation from E. nigrifluens, which induces a similar disease on Persian walnut but is nonpathogenic on alder, is based on positive reactions by the alder strains for acetoin (Voges-Proskauer reaction), endoglucanase activity, and acid production from maltose and negative reactions for esculin hydrolysis and acid production from raffinose, melibiose, sorbitol, and inositol. The fatty acid profiles of the alder strains were quantitatively different from those of all previously validly named plant-pathogenic species belonging to the Enterobacteriaceae. On the basis of data described above, the name Erwinia alni is proposed for the new organism.