Staghorn ferns are native to the Philippines, Southeast Asia, Indonesia, Australia, Madagascar, Africa and the warmer climates of America. In their native environments, they thrive as epiphytes, growing on tree trunks, roots and branches. Staghorn ferns are so-called because their foliage resembles the antlers of a stag. When the ferns begin to reproduce, the fern's foliar fronds (fertile leaves) develop a layer of fuzzy-looking brown sporangia on their undersides, containing new spores.
Rhizoctonia is a fungus brought on by overwatering. Look for black spots on the basal fronds (the thick round leaves). The spots spread rapidly if left untreated and can kill the entire plant. Treat Rhizoctonia by withholding water and humidity.
Pseudomonas, or leaf spots and blights, are small and water-soaked and capable of enlarging rapidly to over an inch in diameter. The lesions are dark brown to black in color with concentric light and dark rings that may be encircled by a bright yellow halo. Lesions are very common at the leaf margin but may spread beyond that. Under very damp conditions, leaves may drop off.
Mealybugs and Scales
Mealybugs and scale insects suck fluids from the leaves, stems and sometimes the roots. Unfortunately, symptoms aren't obvious until the infestation has progressed to the point of leaf yellowing or defoliation (leaf dropping). Scales are tricky to detect and hide easily in bark crevices or and leaf axils. The University of Florida Extension recommends using a 10x magnifying glass to identify them.
Soft scales and mealybugs excrete large amounts of something called honeydew, which provides the ideal medium for the development of a black mold called sooty mold. Sooty mold is very unattractive and interferes with photosynthesis. Curb scale growth by increasing airflow to your plants and giving each plant more breathing space.