Bumps on Fern Leaves Vol. 9, No. 20
“The bottoms of our fern leaves are covered with little brown bumps. Are these scale insects? If so, how do we control them?”
Ferns are attacked by several scale insects, both soft scales and armored scales. There is even one armored scale species named fern scale, Pinnaspis aspidistrae. But scale insects are never neatly arranged and spaced like this.
The bumps on the bottom of fern fronds are called sori, and they contain the reproductive structures of the plant. Unlike higher plants, ferns do not bear flowers and produce seed, they produce spores, more like fungi. The spores are produced in groups or packages called sporangia, and sporangia are produced in special structures on the bottom of the leaf called sori. Look closely and you can see some of the sporangia associated with each sorus in the photo. Some ferns produce sori on most all their fronds, while other species produce sori only on special fronds, called fertile fronds.
Each sorus contains lots of sporangia and each sporangium contains lots of spores. Once mature the spores are released to be dispersed by wind. What are the odds of any one of these spores landing in the type of moist, protected spot they need to germinate and survive? Very small. But ferns produce a lot of spores, so it happens.
As with many insects, fern biology is not as straightforward as you might think. These spores do not just germinate and grow into new fern plants. Fern spores are haploid, meaning they only have half as many chromosomes as they need to make a complete plant. Spores that land in a suitable spot germinate into tiny haploid plants with a single two-lobed, thumbnail-sized leaf. These little ferns have male and female reproductive structures on the leaves. The male structures produce mobile “sperm” that must travel through a film of water on the leaves to the female structure to fertilize the egg. This may occur on the same leaf, resulting in self-fertilization, or given adequate water, the sperm may end up fertilizing an egg on a nearby leaf, resulting in cross-fertilization. In either case, the resulting embryo, which is now diploid, grows into a new fern.
Of course, ferns reproduce asexually as well, and this is how ferns are usually propagated in the nursery industry. See the US Forest Service website on reproduction in ferns for a more complete explanation of fern biology.
Sori are also sometimes mistaken for spore pustules of rust fungi, and on grasses, rust pustules are sometimes arranged in rows because of the parallel vein structure of the leaves, a bit like the rows of sori on fern fronds.
See the Bug's Eye View on Yellow Wood Sorrel Seed for an example of another plant reproductive structure that is sometimes mistaken for an insect pest.
Ferns are not affected by many insect pests because they produce some very effective defensive chemicals, including chemicals that mimic insect growth hormones and interfere with proper growth of caterpillars and other insects. But the Florida fern caterpillar is able to cope with these defenses and can cause serious defoliation of Boston ferns and other fern species.
Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
The information given here is for educational purposes only. Always read and follow current label directions. Specific commercial products are mentioned as examples only and reference to specific products or trade names is made with the understanding that no discrimination is intended to other products that may also be suitable and appropriately labeled.
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