Many different types of scales infest ferns. They lay their eggs on the fronds, and in spring to midsummer the young scales, called crawlers, settle down to feed. These small (1/10 inch) soft-bodied young feed by sucking sap from the plant. The legs usually atrophy, and a hard crusty or waxy shell develops over the body. Mature female scales lay their eggs underneath their shells. Some species of scales are unable to digest fully all the sugar in the sap, and they excrete the excess in a fluid called honeydew, which coats the fronds.

Aphelenchoides fragariae This disease is caused by a nematode that infests the leaves. Nematodes are microscopic, clear worms that usually infect the roots of plants. This nematode, however, infects the foliage of a large number of plants, including bird''s-nest fern. The nematodes enter the leaves through the breathing pores. Inside the leaves, they feed on the tissues, killing them, then spread into healthy tissue, extending the damage. The veins in the leaves act as barriers, resulting in the parallel bands. The nematodes reproduce rapidly in the infected leaves. They spread outside the leaves when the leaves are wet by swimming through the film of water. They may be splashed to other leaves or plants. When the condition is severe, leaves or plants may be killed.

Cottony cushion scales and mealybugs infest many plants in the garden. Mealybugs are usually found outdoors only in warmer climates, but they may be found on indoor plants anyplace. The visual similarities between these insects make separate identification difficult. They are conspicuous in late spring and summer because the females are covered with a white, cottony egg sac, containing up to 2,500 eggs. Females lay their egg masses on leaves and stems. The young insects that hatch from these eggs are yellowish brown to green. They feed throughout the summer, causing damage by withdrawing plant sap from the ferns. Some species of scales and mealybugs are unable to digest fully all the sugar in the plant sap, and they excrete the excess in a fluid called honeydew.

Soil-dwelling fungi (Pythium species), also known as water molds, cause this plant disease. These fungi attack and rot the roots of many plants, including ferns. The fungi may invade only the smaller rootlets, stunting the plant, or they may invade the main root system and cause severe rotting. Fern roots are dark, so inspecting them may be difficult. Infected roots are unable to pick up enough moisture and nutrients to support the plant. The leaves of many ferns are so stiff that they wilt only slightly. Plants in soil that is too wet are more susceptible.

Ferns are sensitive to excess salts. Soluble salts are picked up by the roots and accumulate in the leaf margins, where concentrations may become high enough to kill the tissues. Salts accumulate from water or from the use of fertilizers, or they may be present in the soil used in potting. Salts accumulate more rapidly and do more harm if the plant is not watered thoroughly each time.

Several different types of scale insects attack ferns. The young, called crawlers, are small (about 1/10 inch) and soft bodied and move about on the plant and onto other plants. After moving about for a short time, they insert their mouthparts into the plant, feeding on the sap. The legs disappear, and the scales remain in the same place for the rest of their lives. Some develop a soft covering, others a hard covering. Some species of scales are unable to digest all the sugar in the plant sap, and they excrete the excess in a fluid called honeydew.