Black spots on leaves on grape tree...

Hi all

I have noticed the leaves on my grape tree have developed black spots:



I've also noticed some of the leaves appear to have just turned black:



Does anyone know what causes this? Is it sick? Not all the leaves are like this but if there is something going on I would like to nip it in the bud (no pun intended) before it spreads to the healthy leaves. 

Thanks for any help you can share