Spots on Courgette leaves?

I'm new to growing courgettes. On my young plants the older leaves are getting these marks, they are not on the younger leaves yet. Does anyone know what this is? Do I need to remove all the leaves effected?  I am growing these in the greenhouse in a growbag and don't want it to spread to other plants if it's a bacteria or mildew. Many thanks for any advice to a novice gardener