Exobasidiaceae

Exobasidium

Rhododendron ferrugineum

Rhododendron hirsutum

Rhododendron x intermedium

Exobasidium rhododendri

Conspicuous, subglobose, fleshy, chlorophyll-less, often red gall apples, up to 3 cm. across, with narrow base . . . 

Exobasidium, a taxonomic reassessment applied to the  European species

This is NOT a North American species. Its only hosts are species native to Europe or cultivars derived from those European species. Although there are numerous published sources that treat Exobasidium galls on native American rhododendrons as caused by E. rhododendri or E. vaccinii, those works are based on studies that predate the modern understanding that Exobasidium species are host-specific--often narrowly so. 

Please ID galls on the many Rhododendron species that lack entries on Gallformers' Exobasidium pages as "Exobasidium." There is far too little recent research on N. American species of Exobasidium. If we have missed new research, please let us know.