Thank you for writing.

I see the problem. It has been damp this year.  You appear to have a lot of mulch which may be keeping the soil damp--conducive to fungi.  https://www.rhubarb-central.com/rhubarb-diseases.html  I, like this author, have grown rhubarb for many years and never done anything to treat it.  It does like sun.

One final thought.  Your lawn appears quite clean. Garden plants are susceptible to herbicide drift.  If this problem emerged a week after some kind of lawn treatment, that could be the problem.

Discard the affected stalks by pulling them as you would for any other rhubarb stalk.  Stalks should be pulled not cut.  https://www.tasteofhome.com/article/how-to-harvest-rhubarb/

Cut back on herbicides.  Dandelions, violets are early food for emerging pollinators.  Bluegrass lawns that are cut short are not sustainable.  Grass should grow to 3 inches, the taller the grass, the deeper the root and the less susceptible it is to invaders.  Consider overseeding with some fine fescue.

https://extension.umn.edu/lawns-and-landscapes/lawn-care