Can someone kindly advise me.

The leaves at the apex has turned pale yellow with some brown spots on it. The stalk has some white colouring on it Seems like fungus. I tried to remove it by scrubbing it with water. But it was of no use. Someone kindly advise me. This is a mango sapling 6 months old. This variety is called Reliance Jumbo. I use fermented goat droppings and also spray Fish Amino Acid @ 5ml/ ltr of water to all my plants regularly. Hence possibility of iron defeciency would not be the reason. Someone plz help me.