The invasive ash dieback pathogen Hymenoscyphus pseudoalbidus exerts maximal infection pressure prior to the onset of host leaf senescence

•H. pseudoalbidus was predominant over H. albidus in the stand.•H. pseudoalbidus spreads by simultaneous ascospore discharge in the mornings.•Maximum infection pressure is initiated soon after development of ash leaves.•Build-up of pathogen biomass in attached leaves is terminated before autumn coloration appears.

Shoot dieback disease of European ash caused by the ascomycete Hymenoscyphus pseudoalbidus threatens ash on a continental scale. A spore sampler placed in a diseased ash forest in Southern Norway, coupled with microscopy and DNA-based fungal species-specific real-time PCR assays, was employed to profile diurnal and within-season variation in infection pressure by ascospores of H. pseudoalbidus and the potentially co-existing non-pathogenic Hymenoscyphus albidus. Hymenoscyphus pseudoalbidus was found to be predominant in the stand. Massive simultaneous liberation, by active discharge of pathogen ascospores in the morning, peaked in mid-Jul. to mid-Aug. Accumulation of pathogen DNA on leaflets of current-year leaves reached a high level plateau phase before appearance of autumn coloration, suggesting that pathogen establishment in leaves is terminated before the onset of leaf senescence.