Unseen, but still present in Czechia: Hymenoscyphus albidus detected by real-time PCR, but not by intensive sampling

The native European saprotrophic species Hymenoscyphus albidus seems to be replaced in the recent decade by a closely related, but invasive parasite of ash trees, H. fraxineus. Both species colonize the same niche, fallen petioles of ash leaves. In our study, we aimed at current distribution of H. fraxineus and attempted to find also H. albidus in Czechia. A revision of herbarium material based on molecular data confirmed the past presence of H. albidus in Czechia. Repeated attempts to rediscover H. albidus at four historical localities together with further 87 localities were unsuccessful. However, results of a targeted detection of H. albidus at four different localities using species-specific real-time PCR showed low, but constant presence of H. albidus ascospores. Therefore, H. albidus is not replaced by H. fraxineus, but these two species obviously coexist. Additionally, Cyathicula fraxinophila, another helotialean species commonly colonizing and specific to decaying ash petioles, seems to be unaffected by the spread of H. fraxineus and was present at 21 localities (at nine localities together with H. fraxineus, usually even on the same petioles). Another closely related, but plurivorous species on herbaceous stems, C. coronata, was recorded at five localities on ash petioles. Our results show that helotialean fungi colonizing ash petioles do not appear to be critically affected by the invasion of H. fraxineus.