It was 78 degrees this week in North GA! Spring is in the air (I've gone through a box of Puffs Plus already)

I'm a new home owner of a home on a new construction site. We built about 3 years ago and planted a fescue lawn after the home was complete. When do you guys recommend to start fertilizing lawns? I still several spots that have poor growth (mostly due to terrible soil in north Ga)

Also, we have 3 yr old fruit trees. Should we start fertilizing them as well?

Thanks!