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

From Google,
Yellowing leaves can be caused by several situations. The following are
the most common: 1) Early Blight (fungus) - If this is the cause, you
should see small circular brown lesions on the leaves as well. The spots
become larger. Eventually bottom leaves fall off. 2) Shading - As the
plants mature, the bottom leaves get heavily shaded and naturally turn
yellow as they senesce (age). This is not a problem. Any yellowing
leaves are no longer productive and should be removed to improve air
circulation. 3) Low nitrogen or potassium - If the nitrogen or potassium
level is too low, yellowing will occur. Nitrogen deficiency shows up as
a general yellowing of the entire plant. Potassium deficiency will
appear as bright yellow leaf margins (edges) on otherwise green leaves.
4) Low Magnesium - Interveinal yellowing on older leaves is from low
magnesium. It this is suspected, spray 2 TBL per gallon of magnesium
sulfate (epsom salt) on the plants and see if they "green up" in a few
days. For all suspected nutrient problems, a tissue analysis is recommended.

Me, I suggest adding some trace elements to your plants as a start.
Spraying regularly with Bordeaux antifungal copper sulphate does help.
Just dont overdo it. Avoid watering the leaves, remove the lower leaves
and irrigate or grow tomatoes in shallow ---\____/--- dips in the ground
so water can be pooled, avoiding sprinklers. This alos prevents washing
of the copper sulphate. Repeat after rains.


From Wikipedia
As an herbicide, fungicide, pesticide

Copper sulfate pentahydrate is a fungicide. Mixed with lime it is called
Bordeaux mixture to control fungus on grapes and other berries[8],
another application is Cheshunt compound, a mixture of copper sulphate
and ammonium carbonate used in horticulture to prevent damping off in
seedlings. Most species of algae can be controlled with very low
concentrations of copper sulfate. Copper sulfate inhibits growth of
bacteria such as E. coli.

Don Smith wrote:
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