Latent Distribution of Cryptostroma corticale in Asymptomatic Woody Tissues of Sycamore Maple Trees

ABSTRACT

Global change and associated abiotic stresses, such as drought and heat waves, are increasingly predisposing forest trees to fungal pathogens. Many of these pathogens persist in a latent state within their hosts, hindering early detection and limiting accurate assessments of their current distribution. The fungus Cryptostroma corticale , the causal agent of the Sooty Bark Disease (SBD) on maple trees, is one such organism: it can remain latent within the host then become pathogenic and sporulate under abiotic stress. The fungus then colonises weakened trees, leading to dieback and mortality. Despite its emergence across Europe and the North American West Coast, there is a lack of knowledge about its latent distribution within host trees and the reliability of diagnostic approaches. In this study, we investigated the presence of C. corticale within healthy‐looking sycamore maples, sampling at different tree heights and depths. During sampling, we observed numerous stains within the vascular tissues which were associated with the presence of C. corticale , particularly in the lower trunk, even though stains were more frequently observed in branches. Greenish stains were strongly correlated with fungal detection. The pathogen was predominantly detected at the collar and at 1.30‐m height, suggesting that sampling at 1.30 m is a reliable proxy for rapid assessment without felling the tree. Our findings indicate that latent infections are widespread, since almost all sampled healthy‐looking trees tested positive for the pathogen. This work shows that latent presence of C. corticale within infected maple stands is high and could potentially trigger severe outbreaks under abiotic stress conditions. An efficient survey method based on sampling the outer 1 cm of wood and bark from the trunk at 1.30 m above ground would improve monitoring and management of this emerging pathogen.