Bacteria and fungi live in the soil and on skin. It's not uncommon to have overgrowth of bacteria and fungi in warm, damp conditions - fetlocks and pasterns are perfect places for this to take hold. In a recent EQUUS magazine there was an excellent article with pictures, showing different kinds of skin conditions ranging from rain rot/scratches to ringworm. All have different causes and treatments. In general, if your round spots are around the size of a dime in diameter, and hairless underneath (with maybe a little pus), you might try an antibacterial like Vetericyn. You'll know it's working if 1) the spots dry up; 2) the skin heals over; 3) The spots don't get bigger; and 4) the spot don't spread.
It's really, really important not to pick at the scabs/spots if they're stuck down; that just damages the skin further and causes it to spread. Pick off the loose ones, treat with the antibacterial, and keep the horse in a dry stall or lot. It's important to clip the feathers on the back of the legs with scissors or clippers *(but not too close, don't irritate the skin)* so that the legs can stay dry.