White line disease “eats away” and deteriorates the inner section of the hoof wall causing hoof wall separation. Because the hoof wall supports most of the horses’ weight, any structural weakening in the hoof can cause lameness, even leading to laminitis or founder, a potentially crippling condition.White line disease can initially appear as a small spot where the sole of the foot meets the white line. For more developed cases, it can be several inches long going from one side of the hoof to the other. It has a white, powdery look to it. Scraping it with a nail produces a chalky residue. Continued scraping can find holes and cavities running parallel with the inside hoof wall way up towards the coronet band.

The white line is the thin "whitish" area between the outside hoof wall and where it meets the sole. It can be seen by picking up the foot, looking at the sole of the hoof without the shoe, around an 1/8" thickness. To farriers, it's the area nails are driven to hold a shoe on. 

How does a horse get white line disease?
Fungi or bacteria invade the white line within the hoof wall (laminae) causing it to break down. If left untreated, white line infection is a potentially devastating disease that can spread rapidly around the hoof, inside the hoof wall, making the horse extremely lame. It can spread to other healthy feet quickly.