If you could post a photo, that would be helpful.
Meanwhile, my first thought-- and I'm NOT a tomato expert, they will be along shortly-- would be to clean things up as best you can as soon as you can. Get rid of the yellow/dead stuff. Do not compost the debris.
Without a photo, one other thing comes to mind. If you have 50 plants, my guess is that they are spaced closely, and that the lower stems and leaves are shaded by the
growing plant. Without a lot of light, these stems are naturally going to wither & die. The dying tissue is susceptible to a number of nasty things that dying tissue can't "fight off." Identifying the secondary problem is only useful if the entire plant is in ill-health (so to speak). If you are seeing new growth on the plant, and if the fruit that's set is in good shape, you might be just seeing the normal growth of a tomato.
Clean up the dead stuff. Can't hurt.