Howard,

The orange spots on the underside of Plumeria leaves are probably one stage of plumeria rust, Coleosporium plumeriae.  Three spore stages are involved in the life cycle of this rust.  The one you see now is the urediniospore stage which produces masses of yellow to orange spores capable of infecting other leaves on the same Plumeria or different plants in the area.  Later in the season, teliospores, which have the same colour as urediniospores, are produced.  These generate the third spore stage, basidiospores, which initiate the cycle again.  Bees may collect uredinio- and teliospores..  


Steve Mitchell
Bee Haven Farm
Duncan, BC

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