My pony seems to have developed a patch of this on her back for the first time ever today, despite only being turned out in the daytime

Apart from dilute hibiscrub, does anyone have any treatment/management suggestions?
She hasn't been rugged in the past normally as she tends to be overweight and has had one lami episode so we try to keep her slim. She is well groomed and the ground is free draining, she doesn't have any mud fever, so the problem is just from above.
Its a bit frustrating as we have just paid 000s (literally) for a super duper field shelter with the idea of the horses living out more....

...it's coming in a weeks time!