It looks like it may be rain rot, although rain rot is usually on the back and hindquarters. I used Pine O' Pine in a 5 gallon bucket of water and a sponge to wash my horse down with it. I just soaked him good every day, the pine oil in the Pine O' Pine killed the bacteria and his hair grew back in.
Is there something she is rubbing on? Most of the missing hair is on bony areas, so if something is making her itch (like biting flies), she may be scratching the itch and rubbing the hair off. There is a product called M-T-G (for mane, tail, groom) that has been around for donkey's years; it's basically sulfur powder in mineral oil. Some horse folks swear by it. It needs to be put on in the evening, because it can contribute to sunburn if the horse is exposed to direct sunlight, and you want to wear gloves because the sulfur smell sticks around for ages if you get it on your hands, but it does help to grow hair on itchy horses.