Hymenoscyphus fraxineus persistence in the ash litter

Hymenoscyphus fraxineus causes ash dieback in Europe. It overwinters on ash leaf residue (rachis) within the forest litter. In late spring, the fungus produces apothecia on the rachises and releases ascospores to infect leaves. Previous studies reported that H. fraxineus was able to produce apothecia on the rachis for 5 years after the leaf infection under artificial conditions. However, ash litter is known to decompose rapidly in situ. We therefore monitored the decomposition kinetics of ash leaf debris and the persistence of the pathogen, as well as its ability to produce apothecia in the forest litter. For this, leaves shed in autumn in stands affected by ash dieback were placed in mesh bags and left in the forest litter for 6, 18 and 30 months. At each sampling period, litter mass loss and level of colonization of the rachises by H. fraxineus were measured, as was the pathogen's ability to produce apothecia on them. Despite high fragmentation, about 14% of the rachis dry weight remained in the mesh bags after 30 months, and the pathogen retained the ability to produce apothecia on these rachises. A simulation estimating the age composition of the colonized rachises present in the litter during the fruiting period was developed from these results. It shows that the persistence of H. fraxineus in old rachises of the litter represents a reservoir of inoculum that could compensate for poor colonization of autumn leaves and revive local outbreaks after years of unfavourable weather for the development of the pathogen.