CAUTION: The ID Exobasidium arctostaphyli should only be used for manzanita with red blisters on the leaves. If the entire shoot is red, the ID is simply Exobasidium. This was agreed to because otherwise ALL the manzanita Exobasidium would be ID'd as generic Exobasidium by N. Schwab, who is by far the most knowledgeable about Exobasidium of all the major identifiers. If we can get enough samples to C. Benton Willis, currently https://www.inaturalist.org/people/erumpent_epidermis on iNaturalist, but whose personal and iNat names tend to vary, we may eventually determine whether the blister form is caused by the same species of Exobasidium as the whole-shoot form.

The Exobasidium causing galls on Arctostaphylos was long identified as E. vacciniii. However, newer research shows that E. vaccinii is host-specific, infecting only Vaccinium vitis-idaea. The name E. arctostaphyli was originally used for fungal galls observed on A. pungens. But until further research is undertaken, and in light of work by Burt, and later Savile, lumping many Exobasidium galls on different hosts together under E. vaccinii, it seems appropriate to ID the complex of Exobasidium species galling Arctostaphylos leaves as E. arctostaphyli.

SIMILAR GALLS
It appears that galls of Unknown (Cecidomyiidae) a-rainbowensis-leaf-blister can be mistaken for Exobasidium arctostaphyli galls if only the top side of the leaf is viewed. The blister for the midge gall has two convex sides; while the Exobasidium blister gall is concave on one side and convex on the other. See yerbasanta's comments at https://www.inaturalist.org/observations/316616357.

ONE EXCEPTION: the whole shoot discoloration, which gives the appearance of a pseudo-flower, is being identified on iNaturalist as "Exobasidium sp." with the Gallformers Code "a-spp-whole-shoot-discoloration." This is the result of a compromise between (a) identifying all but the A. pungens galls as Exobasidium sp. or (b) identifying all the manzanita galls as Exobasidium arctostaphyli. This compromise may not really satisfy anyone. But it is a huge step forward from the practice by most American sources of mis-identifying these galls as E. vaccinii.

Taxonomic research is desperately needed!