I just bought a mandarin, variety 'nova'.
I'm not sure if it's healthy or not, or if it's meant to be like this. It has plenty of new growth but all leaves including new growth are yellow, and the bottoms of the leaves have speckles on them. The tops have slight green in the middle that resembles some kind of deficiency. Other than this is seems to be fine, it's not dropping leaves or anything, so I'm not sure if it has a problem or not. I have a pomelo and it's green so wasn't sure if this mandarin is supposed to be like this or not.
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