Why are my Gunnera leaves going yellow?

At first glance, this reminds me of cucumber mosaic virus, though you'd normally expect to see twisting or torsion of the leaves as well. Bad news if it is a virus, not much you can do. However, if the yellow marks can be moved, or rubbed off, i.e., as in rust, it might be a fungal infection.

Thanks for your reply. Sadly it sounds like cucumber mosaic virus - the marks are within the leaf. The newly emerging leaves are unaffected at present so I'll see how they do. I'm reluctant to remove the plant as it has particular sentimental value and it's been growing really well but I wouldn't want it to spread elsewhere in the garden. The leaves themselves look fine - other than the yellowing - no shrivelling or puckering.

Just remember that when they start to brown and die when the weather gets colder to bin or burn the leaves and not compost or use them to cover the crown of the plant. This will avaoid spreading any disease etc.