Sad looking lemon tree

[Localher0]

My potted lemon tree is looking very sad. It has a white fungus on it which despite repeated spraying I can't completely get rid of. There is new growth but it definitely doesn't look as good as it did. Any suggestions for nursing it back to health and keeping it going?

Lemons are hungry plants. Have you been feeding it? Lift it out of it’s pot, knock off the loose soil, and replant into a bigger pot with fresh compost mixed with composted manure. Then keep well watered.

Thanks @VenusClapTrap I have been feeding it citrus food. It's in a 45cm x 37cm pot now. Does it need an even bigger one?

Well lemons are trees, so you can always go bigger with pots!

It might be mealy bug or scale insect if there are white fluffy patches. I blast them off with a hose but I don't know if you can get rid of them altogether.