In recent years collar necroses and butt rot associated with the ash dieback disease occurred with alarming frequency in Fraxinus excelsior. We analysed tree ring structures to identify the year of necrosis initiation on a set of 507 necroses on 155 stem discs from nine severely diseased south-western German stands. The number of first-time infections of trees was highest from 2010 to 2012 and slightly decreased in 2013 and 2014, whereas the total number of newly emerging individual necroses remained high. Logistic modelling of disease progression suggests that collar rot infection has almost reached its maximum incidence and that a fraction of trees will remain healthy at the root collar. On average, Hymenoscyphus fraxineus was isolated more frequently from younger collar necroses, whereas older necroses were more often colonized by Armillaria spp. Advanced stages of rot that may pose a risk to forest workers, visitors and traffic were observed already in two years-old necroses infected by Armillaria spp.