Poorly crab apple

Allyblueeyes

Can someone please recommend the best ‘tonic’ for my crab apple tree.  It’s only been in just over a year and has done really well up until now. Today, it appears a bit sad and some of the leaves have become a bit wilted and they fall easily from the tree.  It’s watered regularly but  I’m guessing it’s in need of a good feed.  Any suggestions welcome.  Many thanks.