Fern frond with sori underneath. Photo: Lise Roy
Question: There’s something wrong with my fern. There’s a large cluster of strange growths under a leaf and I’m afraid it might spread. What do I have to do to cure it?
Answer: Actually, there is nothing wrong with your fern. The bumps are not caused by an insect or disease. Rather, your fern is pregnant … in a manner of speaking.
The bumps are sori (singular: sorus) and produce spores. Spores are equivalent of seeds in higher plants and, when released, are carried away by the wind and produce baby ferns when they fall onto moist soil in the right conditions.
Since producing sori is simply normal for a fern, obviously, no treatment is necessary and indeed, sori add interest to your plant, although they won’t be particularly visible from above. Your fern—a holly fern (Cyrtomium falcatum)—will likely produce more and more fertile fronds over time, as it has clearly reached maturity.
Sori or Scale Insects?
Note that it’s possible to confuse sori with scale insects, insects pests also often found under the fronds of ferns and as well as on the leaves and stems of other plants. As they too produce brown bumps, there could be some confusion.
Fortunately, the two are easy to tell apart. Just give one of the bumps a little flick with your fingernail. That will knock a scale insect loose, but not a sorus: it’s an integral part of the frond and you can’t detach it without serious damage.
To learn how to use these sori to produce new ferns, read Grow Your Own Ferns… From Spores!.