I asked the color thinking it could be sun burn, but on a bay it is highly unlikely.
Have you seen the area that is infected (or losing hair)? Is it gummy? Dry and scaley? Does it have an ordor to it? Is the rest of the mane & tail healthy?
The area that the horse grazes in, is it normally wet or dry during the summer months? Is the horse on pasture 24/7 or is it brought in at night?
Is the horse pastured with a buddy? Some horses will chew on another horse's mane & tail.
Does the owner use any type of fly sheet in the summer?
Hondo Von Dopplet L Bauernhof "Hondo"- GSD
Lilie's Tug McGraw "Tug" - Golden Retriever
Maggie - Mini Dachshund (Rescue)
Lonestar Pivo - Texas Blue Lacy
Ashe - Barn Cat
Katie / APHA