Dark Brown Spots on Underside of Fern Leaf

Outside Leaf Edges

• Adiantums, or maidenhair ferns, don’t always look like ferns but the spores on the underside of the leaves are one of several identifying factors that place it in the fern family. The spore sacs of the common maidenhair fern form at small indentations along the edge of the leaflet, and are often U-shaped. The southern maidenhair fern has delicate multi-lobed leaflets, and spore sacs concentrate at the tips of the lobes. Pteris, or brake ferns, also have many species that form spore sacs along the outside of the leaflets of the fertile fronds but not the sterile fronds.

Spore Lines

• The dark spots along the underside of the leaves of these ferns are more linear and sometimes even look like small wooly insects. The Hart’s tongue fern (Asplenium scolopendrium) has flat leaves with no divisions, and dark brown spore lines that are thick and somewhat horizontal. Asplenium hemionitis, or spleenwort, has an elongated maple-like leaf with small symmetrical dark brown spore lines all along the backside of the leaf.

Thickly Covered Leaflets

• Some ferns have fronds that are thickly carpeted with dark-colored spore sacs along the underside of the leaflets. Cyrtomium caryotideum, or holly fern, had thick leaflets where the underside is virtually covered with small dark brown spore sacs. Cyrtomium macrophyllum, or big-leaf holly fern, is also covered with a dark fuzz composed of small spore sacs. King fern, or Todea barabara, is a large fern from tropical climates that forms thick brown spore sacs on leaflets close to where they connect with the stem.

Spots

• Some ferns just form spots along the underside of the leaves like polka-dots. Arachniodes standishii, or spider fern, is a frilly fern covered with brown, somewhat random, spore sacs along each leaflet. Phlebodium aureum, or golden polypody, have spots that tend to form near the edge of the leaflets. Dryopteris cycadina, or shaggy shield fern, is a temperate-zone fern that has has nearly black spots that run in symmetrical rows along the length of the leaflet.