Spots on leaves of Staghorn Ferns can be caused by several types of fungal infections, from blight and leaf-spot to mold or mildew. These infections are the result of over-watering or letting water splash on the fronds. You can generally control nearly any type of leaf-spotting problem by removing the affected leaves, providing the plant with better air circulation, and making sure that the area where you're keeping the plant is not too humid.