Yellowing leaves?

[dtw]

On my biggest plant, there is a main branch which is about 15ft long, which is growing very fast
and appears to be normal.
There is another branch off this which is about 5ft long, but the leaves are a lot smaller and
have gone yellow.
What is the reason for this?
Should I cut it off so it doesn't spread to the main plant?

I'll try and post a picture tomorrow (afternoon).

[dtw]

[legendaryone]

The yellow leaves i have on mine i remove, the plant doesn't seem to mine, it's what i have done to my tomatos for years.