Rose fungicide specifically isn't the problem, its any fungicide on fuchsias. You could buy a fungal spray (Bayer fungus fighter) and try on a small area to see what happens after 4 weeks (bit late in the year now, nearly ready for leaf drop anyway) - otherwise, its simply a case of removing affected leaves as soon as you notice them earlier in the growing season. There is no other treatment.

Thank you Bamboo I will probably give the Bayer fungus fighter a go as I am hopefully growing some plants on over winter.

I use 3 in 1 action Rose Clear Ultra gun. It's marvellous - clears the rust and it doesn't come back either :o)
(but get rid of badly affected leaves)
It also clears white mould ...

Thank you Hywel picked some up from Wilko today worth giving it a try.

I have been using fungicidal spray since I found a few affected leaves. A few days later the whole bushy plant is covered in rust.

It might be that the fungicide you used is one that fuchsias don't like, Alex.

I didn't know that. Mine is Bayer Fungal Spray.

Yea, read my answer at the top of this thread...

Thanks Bamboo.

I will just have to leave the fuchsia as it is. If I remove all affected leaves, I will need to remove every leaf.

The plant would be better being left, so can absorb more Sun,than a bare plant.

Agree, but suggest you collect fallen leaves (from the fuchsia) from now to end of season to lessen the rust spores around the plant for next year.

3 in 1 action Rose Clear Ultra
After trying seems to work ok