What are these black spots on my plant?

From what I can see in the picture, I think this is a variegated Yucca (could be certain with a picture taken further away to show the whole plant) but I'm pretty sure that's what it is. And if it is, I think the spots are due to something called Yucca leaf spot. My book says to remove severely affected leaves and spray with a chemical called mancozeb. The condition is a fungal infection and the spores are spread by water splash. the spots should have concentric ringing on close inspection, and pin prick sized raised, black fruiting bodies will develop on them. Then the spots enlarge and join up. Oh deep joy, sounds like a right pain doesn't it?

Well i don't know how old Bamboo's book is, coz mancozeb has been banned for years!
I would use a modern chemical such as Systhane which is a systemic fungicide. This sould do the trick nicely.

The book's not that old, only about 5 years and it's an RHS one, but I'm not suprised, half the things mentioned in it are now banned, including malathion.