[Audy70]

Please tell me it's not. Only one plant, out of 7, has leaves at the bottom that look like this.

[JayG]

Don't think so - they're old leaves which have done their job - take them off and do the same to any others which go the same way. Decaying leaves are potential homes for moulds and fungi which you definitely don't want to encourage.

[AnneB]

I agree with JayG that lower older leaves often go yellow.  However, I think the brown patches look a bit ominous to me.  I can't see the stem from your picture, but if that has brown patches I would say it is blight.   Let's hope I am wrong and JayG is right!

[mumofstig]

I've just been taking off bottom leaves that look exactly the same as that

[Audy70]

Thanks all.

@Anne There are no brown patches on the stem but I'll keep an eye on them.