A contribution to the identity of Rhododendron powdery mildew in Europe

Rhododendron powdery mildew has caused serious disease on outdoor Rhododend ron in Europe since 1981 yet its identity and origins are still uncertain. Light and scanning electron microscope studies confirmed previous evidence that conidial specimens on outdoor Rhododendron in Europe are morphological ly similar. They could all be placed into the newly erected anamorph taxon, Oidium subgenus Pseudoidium. Cleistothecia of Microsphaera azaleae on deci duous azalea (Rhododendron luteum) and evergreen rhododendron (Rhododendron hybrid and Rhododendron fortunei) are described in detail for the first ti me in Europe from Bonn, Germany, together with the previously undescribed a namorph. An earlier record of 'M. azaleae' from Belgium was shown to be a n ew species, here described as Microsphaera digitata sp. nov. The pseudoidiu m anamorphs associated with both M. azaleae and M. digitata were similar to those examined on outdoor Rhododendron in Europe. They all conformed to th e description of Oidium ericinum from Rhododendron and other Ericaceae. The se two Microsphaera species are the first rhododendron powdery mildews in E urope for which the teleomorph-anamorph connections have been described. Th e possible identities of rhododendron powdery mildews in Europe are discuss ed.