The occurrence of lines tolerant to the causal agent of bleeding canker, Pseudomonas syringae pv. aesculi, in a natural horse chestnut population in Central Europe

Abstract

Pseudomonas syringae pv. aesculi (Pae) causes bleeding canker, a destructive disease of horse chestnut trees (Aesculus hippocastanum) that limits the breeding of horse chestnut genotypes. The susceptibility of 322 F1 progeny from 20 horse chestnut parent tree sources to the pathogen Pae was evaluated using different inoculation methods. The resulting infection rates, in decreasing order, were caused by inoculation of apical buds, inoculation of annual stems, insertion of bacteria into cut wounds on trunks and foliar spraying of the pathogen. According to the resistance index (RI) results of the field pathogenicity tests, at 2 or 3 years after inoculation 40, 40 and 20 % of horse chestnut parent tree sources were resistant, tolerant and susceptible, respectively, to Pae. Microbiological analyses of plant tissues from buds, phloem in the main trunks and roots of infected F1 progeny regardless of parent tree source showed long-term survival of Pae bacteria in 70 % of tolerant F1 progeny plants that lacked bleeding canker symptoms. Initial RI results and microbial analyses indicated that there was some frequency of resistance in the natural horse chestnut population. Altogether, 41 F1 progeny were the most promising lines based on their viability and the absence of Pae in their tissues.