First report of <i>Cryptostroma corticale</i> causing sooty bark disease of sycamore in Ireland

Cryptostroma corticale is a pathogenic fungus that causes sooty bark disease in various Acer tree species, particularly under drought and heat stress conditions (Muller et al., 2023). The fungus is also harmful to human health, causing maple bark disease, a form of hypersensitivity pneumonitis through inhalation of fungal spores (Braun et al., 2021). Prior to the disease the fungus has an endophytic phase within the tree and does not become pathogenic until stress activation. Once pathogenic the fungus causes significant infection of the bark and sapwood (Braun et al., 2021). The fungus causes total economic loss of the infected trees and the added cost of removing and processing infected trees increases the impact on growers and landowners. Acer pseudoplatanus bark samples displaying symptoms of sooty bark disease were collected from Phoenix Park, Dublin in September 2024 (53.349, -6.340). The trees exhibited a black, sooty growth underneath the bark (Fig. 1). The bark had fallen away in places exposing fungal growth (Fig. 2A). Macroscopic and microscopic pictures of the conidia were captured. A qPCR assay was used to confirm the presence of C. corticale. Total DNA was extracted using a DNeasy PowerLyzer PowerSoil Kit (Qiagen, USA). The qPCR was performed on a MyGo Pro (Novacyt, UK) using the protocol developed by Muller et al. (2023) and primers for confirmation of C. corticale (Kelnarová et al., 2017). Positive control consisted of C. corticale DNA from Germany, validated by ITS sequencing (Brenken et al., 2024). Negative controls included template-free and negative extraction controls. Each sample was analysed in triplicate. Macroscopic analysis revealed dense sooty black fungal growth on the bark samples from Phoenix Park (Fig. 2B). Microscopic examination identified oval conidia characteristic of C. corticale (Fig. 3) (Brenken et al., 2024). qPCR analysis confirmed the presence of C. corticale in all the bark samples (Table 1), with consistent amplification across multiple replicates. To our knowledge, this study provides the first confirmed report of C. corticale in Ireland. The confirmation of this pathogen underscores the need for continued surveillance to protect broadleaf tree species. The authors would like to thank Teagasc for funding this work, and the Office of Public Works in Phoenix Park for their assistance with sample collection.