Pathogenicity and Relative Abundance of Dickeya and Pectobacterium Species in Switzerland: An Epidemiological Dichotomy

Abstract

<i>Pectobacterium</i> and <i>Dickeya</i> species are the causal agents of blackleg and soft rot diseases in potatoes. The main pathogenic species identified so far on potatoes are <i>Dickeya dianthicola</i>, <i>Dickeya solani</i>, <i>Pectobacterium atrosepticum</i>, <i>Pectobacterium brasiliense</i>, <i>Pectobacterium carotovorum</i>, and <i>Pectobacterium parmentieri</i>. Ten years ago, the most prevalent Soft Rot Pectobacteriaceae in Europe were the <i>Dickeya</i> species, <i>P. atrosepticum</i> and <i>P. carotovorum</i>, with some variations among countries. Since then, a drastic increase in the abundance of <i>P. brasiliense</i> has been observed in most European countries. This shift is difficult to explain without comparing the pathogenicity of all <i>Dickeya</i> and <i>Pectobacterium</i> species. The pathogenicity of all the above-mentioned bacterial species was assessed in field trials and in vitro tuber slice trials in Switzerland. Two isolates of each species were inoculated by soaking tubers of cv. Desiree in a suspension of 10⁵ CFU/mL, before planting in the field. For all trials, the <i>Dickeya</i> species were the most virulent ones, but long-term strain surveys performed in Switzerland indicate that <i>P. brasiliense</i> is currently the most frequent species detected. Our results show that the pathogenicity of the species is not the main factor explaining the high prevalence of <i>P. brasiliense</i> and <i>P. parmentieri</i> in the Swiss potato fields.