Bella: Some of my Red Brandwines have the same problem and I just keep cutting the dead yellow branches off. Mine also is at the bottom of my plants. And I also have flowers and fruits on mine. I haven't posted the problem here because I have so many other, seemingly more serious problems, with my other tomato plants and did not want to overwhelm this site with different photos of different problems. I will be interested if someone replies to your post, and maybe I can solve my problem too. And we have had real wet weather here in Georgia, all spring. Have not had to water but twice since the beginning of spring.
Good Luck,
Sheila