For those that think this is normal new growth, I'm sorry but how do you see that?
There is clear purpling of the midrib and slight chlorosis from midrib>margin. That is not the slight yellowing that is associated with new growth. He's barely been feeding her, why is it crazy that it would show signs of a deficiency?
OP, below is an early Xmas gift(if you celebrate it) from me to you. It's your new best friend in situations like this. Although it may not give you the answer you need, it is an excellent starting point.
As for the chlorosis thing, chlorosis=lack of chlorophyl. Which is where the "chloro" prefix comes from. Necrosis(necro=death) is the browning/dying of leaves.
Look into mobile vs immobile nutrients. Mobile nutrient deficiencies show up in new growth so start with those. Keep us posted though, it may not be bad.
Also if anyone else disagrees with what I have said, please support yourself with references. I have mine if you wish. If you cant see that discoloration you need an eye exam though