Just to be safe, do you have access to a microscope? I suspect you may have a Tomato Russet Mite infection just starting. Yellowing is first, then bronzing, then brown dried up hollow stems. You need at least 14X magnification to see them. If you have an extension office nearby, they usually have microscopes.
TRMs are well treated with 1 tablespoon Neem oil at 70% or higher concentration, and 1 Tablespoon dish soap, both mixed per 1 gallon water. Must be done every 4 days for 3 times to safely get the hatching adults.
Neem is antifungal just in case the yellowing is from fungus, but I suspect TRMs in starting stage.
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