I had the same problem this week in Northern NJ Morris county I had to pull my Purple Cherokees just 2 plants, but I saved over 40 tomatoes some over a pound that where just starting to ripen, anyway I had the same symptoms and I think you are right.

Rain sometimes splashes up, or carries in it's ground hugging mist, herbicides which will make branches look like that, they can shrivel up pretty fast too.

Unfortunately, because i won't learn my lesson with herbicide application, it has happened to me again this year.

I have no idea if this is what happened to your plant, but like you i wouldn't take a chance, I'd snip the offending branch and begin spraying the proper fungicide.

If it's wilt fungicide won't help, but I will spray tomorrow when it dries out a little (I hope) in the AM. I did pull the whole branch.

I don't use herbicides, neither do my immediate neighbors, neither does the farmer I got my mulch hay from and it's only this one plant, 1 sucker (so far) so I am stumped. I really hope it's not going to spread to my entire crop (have about 80 tomato plants and I don't remember how many peppers planted in this soil, more tomatoes planted 30 ft away in another area using same manure and hay, and more veggies - mostly curcubits - planted near the house with again same manure and mulch hay).