This could be fungal (Rhizoctonia) or bacterial in nature. It appears that the lesions only affect a small amount of the frond, so that's a good thing. If it were my staghorn, I would use a sterile sharp knife and cut away the damaged lesion, extending the cut into healthy green tissue. I would then dab the cut borders with hydrogen peroxide and repeat the hydrogen peroxide application the following day. I would then sprinkle those cut-out areas with cinnamon. Be sure your staghorn gets bright indirect light and air movement. Also, you want that/those staghorns to dry out well before re-watering. Over-watering is the cause for most staghorn problems.
If this has only occurred on one frond, and the above treatment/change of conditions seems to have worked, leave the frond alone. IF the lesions form again, I would remove that frond. You don't want the rest of the plant to become infected.
