Your image isn't showing here, so I saved it and enlarged it.

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It looks like either a rust, or Miscanthus Blight to me.

Not rust. Not blight.

Instead, the variegated cross bands are old, and possibly damaged by drought or excess water in the past.

If a few blades are affected, cut them out. If all are affected, consider cutting the plant to only several inches tall and let it regrow.

Thanks for attention!
Maybe the picture is too small to see clearly.Now I submit some others. Some teachers say it rust or blight,and some say it just turns old as you mention.
I really have no idea because I searched in the web and saw nobody's Miscanthus "Strict" appear this phenomenon.
and seek help here,:)
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I get that same thing on my zebra grass in the later months too. I thought it was bugs or something!

Rust or age ~ mine gets this every year. Some years are worse than others BUT .. it never seems to hurt the plant. Get use to it, you can tell people that it is normal for Miscanthus sinensis 'Strictus'
Since it never seems to spread beyond the 'white zones' I would 'guess' that it is not rust.
A.

I repeat: Not rust; not rust; not rust!