The leaf symptoms shown in the above picture is a Manganese deficiency. This can be corrected by applying a manganese supplement to the tree. Manganese deficiency often shows up with new growth, which often corrects itself as the trees roots absorbs the element. In citrus this early symptom is rather frequently seen.

A lot of the older leaves have the yellowing signs of magnesium deficiency, so that could be one of it's problems.

I just took a closer look at the whole tree again. I see some spider webs, but we've had a ton of spiders here since the rains.

I'm seeing signs of some sort of insect movement on the leaves. Some look like snail slime, but not slimey(if that makes sense). But I don't see any insects or powdery stuff on it.

Here is closeup of the leaves:

Looks more like manganese deficiency than magnesium lack, but not familiar with local soil conditions.

Take some leaves/photos to a good local nursery for help. We’re in SoCal and have a (very old) lemon tree.

Also, scroll down to #7 in this link.

https://www.ruralsprout.com/fix-lemon-tree/

Gardergal48 My post said MANGANESE deficiency not Magnesium deficiency. In citrus the symptom of a manganese deficiency is .... Newest leaves with green veins WITH green borders to the sides of the veins on otherwise yellowish leaves of normal size. The reason the tree often corrects the manganese deficiency is the transient deficiency symptoms are due to a marginal supply of available Mn in the soil, which the tree roots temporarily are unable to extract enough Mn to supply the large rapidly growing new flush of growth .

The squiggly tunnels could be from citrus leaf miner. They grow in new leaves and as far as I can tell, the best treatment for those is to not prune at a time that would have the moth's egg-laying schedule coinciding with new growth.

(It's ugly and will be there a long time since the leaves are long-lived, but usually not significant enough to be harmful.)

“The squiggly tunnels could be from citrus leaf miner.”

Exactly what #7 in the link I posted shows.

Thank you all for your responses. It certainly seems like leaf miner damage to the newer leaves. I sent the pictures to our local county extension master gardener program and will post their response when I get it.

I need to figure out a fertilizer schedule for this tree.

@chloebud thank you for that link. Lots of good information there!

I see obvious leaf miner damage as well. Leaf miners usually don't affect fruit, I believe.

Have you tried a foliar feed yet?

I use this granular fertilizer:

Leaf miners

Depending on where you are in southern California, you should check for HLB. It can cause irregular mottling of the leaves. You also have leaf miner damage. https://ucanr.edu/sites/ACP/