Fortunately not something I have a lot of experience with, Canker can affect any horse at any time. 

An infectious process that results in the development of a chronic hypertrophy of the horn-producing tissues, Canker generally originates in the frog.  Anaerobic, it may be mistaken for thrush in the early stages, though it will easily bleed when abraded.  While thrush is a necrotic process, canker is a hypertrophic pododermatitis.  If left untreated, the disease will become diffuse and involve the frog, bars, sole and the stratum medium of the hoof wall in the palmar/plantar aspect of the foot.  It causes abnormal keratin production.

This grand old lady went from left to right in 16 days at our installations, after I assisted the vet with the removal of the canker and corrected her hoof shape along the way (well, got her hooves on the right road.  Not completely corrected in that time!), whilst treating the wound area in order to maintain it clean and dry, definitely dry (!) to assure the most rapid healing possible.