The foliage on my Clematis began to look like this. I researched the possible problems, but found no decisive answer. No pesticides were sprayed near this plant, so that is ruled out. Fearing that it may have been a virus, I went ahead and removed the plant 🙁. Comments from anyone who can address the issue would be welcome.

The reason you haven't found a decisive answer is because there are many factors that need to be considered, such as: location, and how much sun it gets, and watering. Many clematis are susceptible to "clematis wilt." From the photo, I can only speculate that's what it is, but I cannot tell for certain. More information is needed. Good luck to you.

Your response is appreciated, Sassy Bloomer. Whatever was wrong with the plant, my other clematises are doing well.

Not clematis wilt, which is distinct by a sudden stem collapse and dark brown to black leaf spots and then wilting and die back.
It does look like some kind of leafspot virus, there are a few that can hit clematis but it could also be a nutrient deficiency, like magnesium.

None of the stems and leaves were turning brown. I did quite a bit of research and it looked more like a virus than a nutrient deficiency. The plants was thriving the previous year and began to develop the leaf spot the following year. Perhaps I jumped the gun, but I removed the plant to avoid the possibility of the virus spreading, if indeed it was a virus. I do appreciate your responses. Hope no other clematis lovers have to deal with this. Having to remove a plant you love is heartbreaking.