Bacterial leaf spot of Hydrangea : on a “new old” disease and the importance of diagnostic specificity to reduce cross-reactivity and improve surveillance

Abstract

The causal organism of bacterial leaf spot of Hydrangea L., Xanthomonas hydrangeae, was the subject of a disease report and a new bacterial species description in 2021. These publications, along with the development of a specific isothermal diagnostic assay for X. hydrangeae, and the recent awareness of this pathogenic bacterium, combined with the wet weather conditions during the summer of 2021 in Europe, have revealed a broader historical and current prevalence of this disease. Global trade of plants appeared to play an important role in the dissemination of this pathogen. Furthermore, scouring the literature revealed multiple instances, mainly in the USA, of bacterial leaf spot of Hydrangea, previously attributed to other Xanthomonas species (e.g., Xanthomonas campestris, Xanthomonas hortorum). The first known indication of bacterial leaf spot of Hydrangea dates to 1995 in Georgia (USA). However, these 1995 isolates are no longer available and their identity as X. hydrangeae cannot be confirmed. This mini-review outlines the challenges encountered when studying X. hydrangeae, as related to 1) the development of a diagnostics assay targeting X. hydrangeae, especially given its phylogenetic similarity to X. hortorum, and 2) the information discontinuity regarding historical and current incidence of bacterial leaf spot of Hydrangea