most likly caause is drought (give it more water) fertilization may be a secondary cause....

Do you have it properly mulched? That could go a long way towards helping the moisture issue and, in the long run, any fertility problems. Remember to mulch wide, not deep, and don't place any mulch up against the trunk(s).

are you saying not enough fertlizer, or to much, clarify please

Could be either. Have you added fertilizer? Have you had your soil tested? The only way to know for sure is with a soil test. Your local extension agent should be familiar with local soils and might be able to give you general guidance concerning local soil fertility.

Fertility wouldn't be on the top of my list of suspects. Unless you have a reason to think your soil is very poor and deficient in some nutrient, I'd pursue other possibilities. Adding a little compost certainly wouldn't hurt, but avoid using fertilizer unless it's use is determined to be needed.