Affected leaves

Has anybody an idea as to what this is on the underside of my zonal pelargonium leaves please. I keep ripping off affected leaves but more of these appear.

 

Looks like rust, Rich.

 

That was my first thought Pete, but I've never seen it in this form, i.e. in curved rows. My books and Google pics show nothing like it either.

Edited to say I've just looked up Geranium Rust, that's it.

Now all I have to look up is how to get rid of it.

 

I think if you can keep them going through the winter you might find it goes away on its own accord once growing condition improve.
but I've no experience of it so that is just a guess.

 

You could also use a systemic fungicide approved for Pelargoniums.

 

I didn't know there were different varieties of rust. Anyway, when weather permits, I'll take them all outside, rip off affected leaves again and spray them with an antifungal spray.

 

I know it well. One reason why we (in the Parks Depts) never keep geraniums/pelargoniums from year to year as it usually shows itself in the autumn and winter and we don't take cuttings anymore.