Yellowing of new growth: Tomato bushes

 My back yard tomato bushes all have a yellowing of the new growth starting
at the stem and radiating outwards. The yellowing does not reach the
extremity of the leaf .
At this point it does not appear to be having any effect on the plant but
will it do so later on?
What is the cause and does it need treatment?

Don

 Thank you Jonno for your answer.  I'll follow it up.

Don