Management

Detection

The ability of this pathogen to remain in a latent stage as an endophytic fungus or mild pathogen makes it to be unnoticed in asymptomatic trees. The future threats of susceptible hosts under these climate change scenarios urge the application of early detection methods for identifying these latent infections of C. corticale. DNA-based approaches are then essential for early detection (Kelnarová et al., 2017; Brooks et al., 2023). Sampling of asymptomatic living trees for applying molecular detection or cultivation methods consists of extraction of wooden cores with an increment borer (Kelnarová et al., 2017; Schlößer et al., 2023; Brooks et al., 2023). Traditional culture cultivation of the wooden cores serves as a method to isolate the pathogen; however, DNA-methods are more sensitive for confirming the presence of C. corticale in wood tissue.

Although identification of SBD stroma of typical brown-black colour by naked eye is possible, it is sometimes difficult due to very common presence of other mostly saprobic fungi creating black stroma (Hülsewig, 2019). Namely, Eutypa maura or Stegonsporium pyriforme produce stroma on the surface of dying or dead stems of sycamores. It is always recommended to confirm the determination of the stroma by microscopy and identify typical brown, ovoid (or sometimes distorted due to pressure) conidia, 4-6.5 x 3.5-4 µm in size (Gregory and Walles, 1951; Ogris et al., 2021).

Spore trapping is an effective method for early detection and surveillance of airborne fungal pathogens. Burgdorf et al. (2022) studied the influence of season and vertical factors on abundance of conidia by installation of passive spore traps at different heights and distances from the host tree. Muller et al (2023) performed an extensive aerobiological study using 12 active vacuum-pumped suction traps (Hirst-type air samplers) from the existing European pollen-monitoring network in 6 countries. They also developed a species-specific quantitative PCR assay to detect and quantify the spores from these C. corticale aerobiological samples.

Preventive measures and suppression measures

Heat and drought are the main triggers of the SBD development and expression (Cech 2004; Metzler 2006; Bencheva, 2014). Furthermore, other stressful factors such as floodings and those associated with urban environments, like pruning and accidental wounding, together with soil compression promote SBD (Koukol et al., 2015; Kelnarová et al., 2017). Therefore, susceptible hosts such as sycamore trees located in urban areas should be supplied with their demands of nutrients and water for the maintenance of vitality of the tree (Bork, 2018). Especially during drought and high temperature periods, when the disease risk is high, it is important to provide enough irrigation to susceptible hosts (Tanney et al., 2024). Likewise, yearly surveillance for observation of infection symptoms is suggested, notably during these heat and drought periods, when outbreaks occur. Especial attention should be paid to upper branches which can be infected but unnoticed during visual assessments (Kelnarová et al., 2017).  

The choice of other less susceptible tree species to SBD or less demanding in terms of nutrients and water should be also considered for the planning of suitable species for planting and reforestation programs, especially in hands with the future climate change predictions. Within urban settings, non-host tree species should be prioritized for planting (Tanney et al 2024). When planting susceptible maple species, selection for those ones more endured is to prevail. Thus, although the Norway maple is also a susceptible maple species, it requires less supply of nutrients and water than sycamore and therefore, its planting should be more suitable for certain sites (Grüner et al., 2020).

In general, removal of SBD-affected trees from the area seems to slow down the spread of the pathogen (Kelnarova et al., 2017) although at an endophytic state with asymptomatic infected trees (Schlößer et al., 2023), they might remain unnoticed. Furthermore, felled infected trees are reported to be source of new infections (Gregory and Waller, 1951). This implies a source for sporulation and spread of the pathogen to other trees and areas. Therefore, the stumps from infected trees should be removed and either buried on-site or transported in covered containers for subsequent incineration. It is also recommended to schedule pruning and felling in winter and early spring (Burgdorf et al., 2022) and under damp weather conditions (Bork, 2018), when the sporulation is minimal, and the conditions are not conducive for the spore dispersion (Burgdorf et al., 2022). It is important to note that infested wood is unsuitable for firewood and should not be chipped to prevent the spread of spores (Bork, 2018; Niesar et al., 2019; Tanney et al., 2024). Also, if the wood does not show any sign of infection but is still collected from a SBD site, it should be discarded for firewood because it can develop a stromata during its storage (Plate and Schneider, 1965; Burgdorf et al., 2023).