Phornium problem

Seems like I am having a few problems this year (what's new as a gardener you might say) - the latest is my Phornium which is developing large patches of what looks like brown rust within the leaves.

Does anyone have any suggestions what it may be and if so how to treat it please.

 

It does look like rust, Dont actually have any phormiums so cant really help other than cutting off the affected leaves as low as you can.

 

Thank you Pete - have just done that and moved the snail colony that decided to move in too.