Bluestone(copper sulfate) is good. Doesn't work for every infection but helps most IME. It's strong & toxic & best not get it on your hands, or any skin of the horse. Straight salt is pretty good too. Chlorine dioxide is apparently good too, but afraid I haven't used it & can't tell you how to mix. I'm sure if you google... I'd scrape/cut out the infection as much as necessary, then, depending the environment & size of hole(it's generally best left open to air, but...), you could either plug it up - you can use clay(I use Tuff Rock clay poultice) or beeswax & mix it with CS or such, or you could make a paste to put in the hole, or soak the foot in a solution, then leave it open.
Infections like this(& thrush - same beasts) are from soil born organisms. They are classed as 'opportunistic', meaning they tend to invade compromised tissue. Once in though, they can eat further into healthy wall material. It can be insidious & unseen, inside the hoof capsule, so I believe any infection is best scraped/cut out as necessary, as well as treated topically. They're anaerobic - reason to have it open to air if poss - and like warmth & damp. Inside the wall material can be damp enough even if the environment's dry, so mud & crud is not strictly necessary - just that that compromises the hoof more & so gives the 'opportunistic' infection more chances to thrive.