I need help with my Tree, I think its dieing! :(?

I have a Thunderchild Flowering Crab Tree I planted it Mid-Late May- Used Maricle Grow Trees and Shrubs soil. The tree was doing well til about 3 days ago. All the leaves are falling off little by little, They fruit looks fine and the tree isnt dead, yet. If someone can give me some tip or something to do so my tree doesnt die! Id greatly appreciate it!