The staghorn fern is a tropical epiphyte sometimes planted in the home garden as a leafy ornamental. Although it is an interesting specimen, this fern is susceptible to a number of fungi and diseases, one of which is the fungal disease Rhizoctonia sp. Infected staghorn ferns develop black spots on their fronds that eventually spread and kill the plant. The fungus moves quickly, but prompt attention may save the staghorn.
Stop watering. The primary cause for rhizoctonia fungal infection is overwatering. Allow the staghorn fern to dry out for a month or so or until the symptoms subside or disappear.
Spray the entire staghorn fern with a plant fungicide; add water and mix per manufacturer's instructions. Coat all parts of the staghorn, not just those with signs of infection. Pay special attention to crevices. Stop spraying just before the point of runoff. Time the application so there are at least 12 hours of dry weather after you spray.
Retreat the staghorn fern every seven to 10 days or at the interval prescribed by the manufacturer. Again, time the application so that there is no rain for the longest possible time after each application. If it rains after an application, respray the plant even if the proper time interval has not elapsed.
Resume proper watering practice once the staghorn fern's fronds begin to look wilted. Then water to thoroughly moisten the rooting medium. Wait to water again until the staghorn fern's leaves begin to look wilted again. Use this leaf wilt as a watering guide. Once you have a gauge of when the staghorn fern's fronds begins to wilt, water a few days before on a regular schedule. Staghorn ferns generally require water once or twice per week during warm sunny weather. During cool, wet winter and fall, water at about half that rate. Always check the staghorn fern's rooting media before you water, especially if it has access to rain. If it is wet or water logged, do not water. Even if the edges dry out, the bottom and interior of the fern still may be waterlogged.