White Sapote should have deep green leaves, and i think you've hit the problem on the head--nutrient.

Now, without looking it up and trying to recall my word association to remember stuff, I believe yellowing on old leaves (interveins, etc) means either the iron or magnesium deficiency.

Either way, use a citrus fertilizer as your main slow release to prevent the deficiencies on the fruit tree; a mild (diluted) water soluble may help diminish the yellowing, as a foliar application and in the soil. Dont overwater tho in the process, it's winter.

if you don't live an area where citrus fertilizer is sold, would azalea fert like Hollytone work well? or something else?

Scott

I"m not familiar with the NPK ratio of Hollytone, nor if it carries any micronutrients.

YOu've 2 options:

1) feed you container tree TWICE monthly with a 20-20-20 water soluble with micros (this is most important thing you need)

or

2) get any slow release (perhaps Hollytone is this) that is about a 6-6-6 with micronutrients. Do'nt use ferts that are much higher than that, certainly not higher than 10-10-10. Some fruit tree spikes (6-6-6) may work if htey exist--you'll have to crush them down to a powder.