Can you post pictures? Does it easily come off? Most of the time, fungal infections start on the leaves and can propagate to stems. Really bad infections of anthracnose can be a problem. But other times, the issue is just leaf spots. Controlling the excessive humidity should be priority one. By viewing what you are seeing, someone might be able to identify THE problem. Leaf spots sometimes automatically self correct once summer weather patterns start (low humidity; high temperatures).

In the meantime, make sure that you do not do overhead watering. Instead, water the soil early in the mornings. Also, water when the soil feels dry if you insert a finger to a depth of 4"; you can also water if it feels almost dry. Dispose in the trash all plant debris found under the shrub.

Little black spots running up and down the stems are very common and obvious on many H. macrophyllas - they are lenticels or tiny pores that allow gas exchange and are not a concern.

Leaf spot from various causes can also be common and is usually more disfiguring than harmful. Follow Luis' advice and the plant should grow out of it.