Red rust on rhubarb
Thank you for writing.
There are two possibilities.
There is also Downy Mildew that I have this year. https://www.smartgardener.com/plants/5705-chard-rhubarb/diseases/780-downy-mildew
It is early in the season. Aggressively remove diseased leaves. Never use a knife or clipper to harvest rhubarb leaves. Grab the stem firmly, pull slightly downward and jerk. https://www.wikihow.com/Harvest-Rhubarb