its more likely to be magnesium deficiency. give it a dose of Epsom salts [magnesium sulphate] that should do the trick. give it plenty of water to ensure the minerals get absorbed by the roots.

Chlorosis can also be caused by a virus disease, if you follow seaburngirls advice and nothing changes, I would think it's a virus, and the only treatment that I know of , is to destroy the plant, sorry, Derek

Magnesium deficiency makes sense, thank you, I will try that. I would have thought that Holly tone would have provided some Magnesium but maybe it wasn't enough? Chlorosis is very common on US native Magnolias, especially when grown in the midwestern states because of the soil here so I am thinking it is likely lack of nutrients/high pH.

Perhaps the roots being dry would contribute, the same scenario can happen over here to shrubs/trees that have their roots in a too dry position - if winds, sun and/or drought are indicated that will also dry them out.

It turns out the soil pH was too low and I did not use enough of the Iron/Sulfur mix the first time. I treated it again a couple weeks ago and now the soil pH is 5.4 and the leaves are green and healthy looking again.