Please send me a picture of the plants and a close up picture of a partly affected leaf.  There are two many possibilities from the information that you have given.

These include 1) rotting of leaves on the ground, 2) edema from too much rain, 3) herbicide drift and 4) various fungi.  

https://www.rhubarb-central.com/rhubarb-diseases.html

Steve, I'm sorry for the delay in responding. These are two pics I took today. I hope they help you in diagnosing the problem. Any help you can give us is greatly appreciated!

Thanks! Bev Jones

I did not get your pictures.  Most of the time, this is because the file is too large. Email them to me <personal data hidden>

Thank you for your pictures.  They were small but consistent with Round up injury  https://www.canr.msu.edu/news/rhubarb_weed_control_in_michigan
I would not eat any spears that showed signs of infection or damage. The principal issue here would be secondary invasion of the dyng stem by fnui or bacteria.

All in all, roundup is not a good spray to use around gardens.  It is completely indiscriminate and will take the good with the bad and ugly.  I can't comment on your situation because I do not know what you wer trying to do with the Round Up.

Steve,

  Thanks for your response and information. Our garden is fenced to keep the deer and other animals out of the garden. My husband sprays Round-up around the perimeter of the garden, both inside and outside of the fence to keep the weeds down. He sprays in the evening after the wind dies down. The rhubarb is located about 5 feet inside the fence, and normally he avoids that area for several feet either way of the rhubarb. He forgot about that this year when he sprayed, and this is the first time browning of the leaves happened to the rhubarb. It started several days after spraying, so we thought it was the Round-up, but wanted to check with someone more knowledgeable. Thanks for including the website, we'll definitely check it out.

  Thanks for you help, we appreciate it.

Bev Jones

There is no reason to regularly spray the perimeter of a garden or yard with roundup.  It plays no useful role in weed suppression.

It contributes to Roundup resistant weeds. https://extension.umn.edu/herbicide-resistance-management/herbicide-resistant-weeds#history-and-prevalence-928410  

It is directly toxic to pollinators https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2664.13867  and indirectly toxic by decreasing plant diversity upon which pollinators depend.