Another possibility I've heard about. When a horse's hoof goes from wet to dry on a consistent basis such as daily then it can generate cracks on the hoof wall. Once those cracks begin to get to a certain depth they can harbor bacteria and such.
It's just something I've heard about and if you find it not to be other causes, you could look into it being the wet to dry conditions as a possibility or contributer.
I think I also remember that you wouldn't wanna take a knife and try to dig the cracks out to get to the bacteria. Doing said such things would only cause it to become worse. If I recall the best thing to do is soaks with acv to kill the bacteria. But to avoid the cracks would take quite a bit of effort. You would need to keep her hooves dry at all times. That is if it is from the consistent wet to dry conditions. I'm watching my dvd's, "under the horse", and pete mentions it in there. When I come across that part I'll remember to post what he says about it. It may be a few days though cus I just started over and I'm on dvd two.
Just a possibility to consider, cus I don't have the experience to say, "yup that's what it is". But it could be a multitude of things too, so I suppose it's good to know about all possibilities.
And I sppose only for a curious nature I have; has that crack on the left cracked all the way through the hoof wall at the toe?