Deformed Fronds Can Be Caused By Overfeeding
If they are in soil that is moderately fertile with lots of moisture and nutritious humus, ferns do not require a lot of fertilizer. A small sprinkling once a year is adequate for most situations.
Failure To Thrive Is Often Because Of Overcrowding
If growing conditions are correct and ferns fail to thrive, chances are they are overcrowded. When plants grow too close together, air cannot circulate easily among them. This is often a problem in shaded areas, especially in the sultry summer, when the humidity hangs over the plants and causes mildew problems. Dig up and divide very large clumps as soon as the weather cools somewhat. During the heat of summer prune the outside fronds to improve air circulation.
Rows Of Black Dots Under Leaves Are Spores For Reproduction
Certain fronds on a fern plant will develop regular rows of black dots on the undersides of their leaves. These are the spores that will drop to the earth eventually and generate special structures that produce the reproductive cells of the fern plant. Their presence is perfectly normal.
Leaves Yellowed and Curled Caused By Aphids
Look for aphids on the undersides of fern leaves. These insects are brown, black or grey and the size of the head of a pin with soft, pear-shaped bodies. Because they tend to cluster to feed, they can be distinguished from the black spore cases on the fern leaf undersides which are in regular rows. Aphids suck the juices from the leaves and stems of ferns, causing them to turn yellow, curl, and pucker. As a whole, the plant loses its vigor, its fronds drooping. Light aphid infestations are easily controlled by spraying plants vigorously with water to wash them off. Do this early in the morning, three times, once every other day. For medium to heavy infestations, spray visible aphids with insecticidal soap every two to three days until they are gone. For more information see the files on Controlling Aphids
Reddish Brown or Blackish Spots on Fronds Are Caused By Nematodes
Ferns are susceptible to leaf nematodes. Nematodes are not insects, but are slender, unsegmented roundworms. Most are soil-dwellers and are invisible to the unaided eye. Infested plants have reddish brown or blackish bands or spots on their fronds. In the bird's nest fern, the base of the frond is affected first; then brown discoloration extends upward to cover more than half the leaf. If all the fronds in a cluster are attacked by nematodes, the plant usually dies. To control them, add lots of compost, especially leaf mold, if it is available, to the soil. Not only will the ferns get a boost of nutrients, but the compost encourages certain fungi that attack nematodes. If there is no compost available, pour dilute liquid fish emulsion on the soil around affected ferns. This drench should repel the nematodes. For more information see the file on Controlling Nematodes
Rotten Spots on Fronds; Odor Present – Caused By Bacterial Soft Rot
Small water-soaked lesions on fern fronds are a sign of soft rot caused by a bacterium. This disease enters ferns through wounds. Small brown lesions appear on fronds near the stems. These enlarge and become soft and mushy. The diseased tissue usually gives off a foul odor. Plants become stunted, wilt, and die. Soft rot cannot be cured. Dig up and destroy infected fern plants.
Gray Spots on Leaves Are Caused By Leaf Blight
A leaf blight or leaf blotch called Botrytis blight is caused by a fungus. It appears on ferns as ashy gray spots. The fronds of infected plants may die back several inches. Remove and destroy any infected fronds or shoots, and dig up and discard any severely diseased plants. Put them in the trash to avoid spreading the fungal spores around the property. Spray all remaining ferns with a sulfur-based fungicide, making two applications five days apart. For more information see the file on Controlling Fungal Disease