| Summary: | Bleeding cankers on horse chestnut trees (<i>Aesculus</i><i>hippocastanum</i> and <i>Aesculus</i> × <i>carnea</i>), caused by <i>Pseudomonas</i><i>syringae</i> pv. <i>aesculi</i>, have been reported across Europe. In the present study, we show the successful detection of <i>P</i>. <i>syringae</i> pv. <i>aesculi</i> on symptomatic horse chestnut trees in Switzerland using quantitative PCR (qPCR). However, <i>P</i>. <i>syringae</i> pv. <i>aesculi</i> was also detected by qPCR on trees from which no isolate was obtained through cultivation. Reduced isolation success and low copy numbers of the target gene were correlated with the increasing age of symptomatic horse chestnut trees. The potential of detecting non-viable <i>P</i>. <i>syringae</i> pv. <i>aesculi</i> by qPCR was evaluated using an inoculation experiment with dead bacteria and detection by qPCR and cultivation. The detectability of DNA from <i>P</i>. <i>syringae</i> pv. <i>aesculi</i> cells dropped by 34.5% one day after inoculation and then decreased only slightly until the end of the experiment (22 days after inoculation). In contrast, no bacterial growth was observed at any time point after the inactivation of the bacteria. To protect horse chestnut trees, evaluating the viability and actual infection stage of the bacterium may play an important role.
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