Why do fern leaves have brown spots on their undersides?

The brown spots often seen on the undersides of fern leaves are not signs of disease or insect infestation; they are clusters of sporangia known as sori (singular: sorus). These organs are essential for the plant’s reproduction, as they produce and house the spores necessary for the fern's life cycle.12

Depending on the species, these sori may appear naked or be protected by a specialized scale-like structure called an indusium, or by the rolled-over margin of the leaf. Because they develop over time, they are frequently mistaken for pests or fungal growth by those unfamiliar with fern biology. These structures are also a primary characteristic used by botanists for fern classification, as their arrangement and shape vary significantly across different genera.12