Fungus or mould for ID

[animartco]

I have posted this picture up once or twice on long threads but have had no reply, so am posting it again hopefully.
The strange growths are close together on a section of the stems of hibiscus syriacus 'bluebird'. It has waxy stems anyway, but there seem to be regular sheaths around the stems of a thicker cellulose perfectly cylindrical. On this there are regular slightly oval black splodges. Since the picture was taken they have grown and seem to protrude from the sheath with what look like knobbly sporangia. The plant appears to be unharmed, the leaves on the uninfected stems are developing at the same rate as those on the sheathed stems.