Euphorbias are relatively untroubled by pests and diseases but may be affected by a rust.

There are chemical treatments available for rust. Also reducing nitrogenous fertilisers has been shown to reduce the susceptibility to rusts diseases.
I note you cut yours back in the autumn!
This is what the RHS say:
Cultivation
Grow in moist, humus-rich soil in light dappled shade; can be invasive on light soils
After the plant has finished flowering in June prune back the flowering stems as close to the base as possible to encourage plenty of new shoots for flowering the following year.
Euphorbias are relatively untroubled by pests and diseases but may be affected by a rust.
All parts are highly toxic by ingestion; sap may irritate skin and eyes.
Hope this helps.

Thank you for this information Drc, it seems I have been cutting it back at the wrong time of year, but I've been doing this for many years and its never suffered rust before. I have now cut it back and if it makes some new growth I'll spray with a suitable chemical, and I've made a note to check for any signs of rust when growth appears next year.

Lets hope the new growth looks better JK.