Studies of the interaction between horse chestnut leaf miner (Cameraria ohridella) and bacterial bleeding canker (Pseudomonas syringae pv. aesculi)

The white flowering horse chestnut tree (Aesculus hippocastanum L.) was first introduced to the UK ca. 500 years ago. Over the past eight years however, this tree has suffered from severe attack by a mining insect pest known as the horse chestnut leaf miner (HCLM; Cameraria ohridella) and, concomitantly a gram negative bacterium (Pseudomonas syringae pv. aesculi; Pae). Although studies have investigated the influence of each problem individually on tree growth and vitality the interaction between HCLM and Pae on horse chestnut remains unknown. For this reason four year old horse chestnut seedlings were artificially inoculated with Pae in the presence and absence of HCLM. Effects on tree vitality were assessed by monitoring alterations to leaf chlorophyll content, leaf chlorophyll fluorescence (Fv/Fm ratios) as well as key defensive enzymatic activity (β-1,3-glucanase, peroxidase) of woody tissue at the site of Pae infection. With respect to mean lesion size, the main proxy of Pae success or aggressiveness, lesion size was significantly increased in the presence of HCLM compared to trees inoculated with Pae but where HCLM was controlled using insecticide sprays. Results of this study also indicate that suppression of two key defensive enzymes, β-1,3-glucanase and peroxidase, within woody tissue at the site of Pae infection, possibly as a result of reduced photosynthetic productivity caused by HCLM defoliation, maybe the underlying reason as to why Pae severity is greater in the presence of HCLM.