Apple tree leaves curling

Planted one year old root stocks of honey crisp and empire apples in April and the trees have been doing great until now one of the honey crisp leaves has been curling and turning a bit brown. Can't tell if due to bugs or disease? Or am I overreacting and it looks normal? been watering all summer so I know it's not an underwatering issue. Trees were planted in a mixture of topsoil and compost.

There are lot of different causes of leaf curl.

Your tree looks like several of these:

Fungal Diseases
Pests
Nutrient deficiencies
Environmental stress
Herbicide damage
High temperatures
Insects

I would start with the fungal disease.  Here are several recipes for a non toxic fungicide