Powdery Mildew (Brasiliomyces setosus) on Soapberry

Charles Hodges (1985) described as a new species an unusual powdery mildew occurring on, and apparently restricted to, the endemic soapberry tree Sapindus oahuensis.  

The fungus was referable to the genus Brasiliomyces, but had characteristics distinguishing it from the other three species in that genus.  Brasiliomyces is a little-known genus first described in Brazil, but subsequent species were described from South Africa and elsewhere.  The fungus produces conspicuous, spreading white colonies on the under surfaces of leaves.  The colonies can reach 6 cm in diameter.  The area on the upper leaf surface corresponding to the colony eventually turns yellow.  As its name implies, B. setosus is characterized by numerous setae (stiff, thick-walled hairs) that break off readily at a basal septum.   The ascocarps (sexual fruiting bodies) of Brasiliomyces are hyaline and are distinct from those of other genera of powdery mildews by their lack of ornamental appendages.  They contain 5-6 ascospores.  Hodges' paper (1985) further describes the microscopic characteristics of B. setosus.  As with other newly recognized fungi on endemic hosts, B. setosus is considered to have evolved with its host and thus is itself thought to be endemic to Hawai‘i.

   White colonies of Brasiliomyces setosus on leaves of Sapindus oahuensis (Photo by C. Hodges).  

   Ascocarps of B. setosus containing asci and ascospores (Photo by C. Hodges).