Marks on a fern are not all signs of disease. Before treating brown dots, photograph the underside of the frond and look at how they are arranged.
Fern reproductive structures can appear in organised patterns, while pests, damage and other problems require a different assessment. Identify the fern and compare the structures with reliable information for that plant.
Look at a whole frond
Include healthy-looking fronds in the photographs. Note whether the marks repeat in similar positions and whether tissue around them is changing. A single close-up without scale or context makes comparison difficult.
Do not scrape every dot away before asking what it is. You may remove the evidence needed for a useful identification or damage sound foliage.
If the plant is declining, check the growing medium, light, humidity and any visible pests as separate observations. Normal reproductive structures would not explain every other symptom automatically. Keep the plant name, recent care and dated images together. The task is to distinguish a normal feature from a problem before choosing an intervention, rather than treating the colour brown as a diagnosis.
The US National Park Service fern guide explains that small brown or black dots on frond undersides may be sori associated with spore production. Compare the pattern and the identified fern before treating a normal reproductive feature as a pest.
Related reading: Brake Fern Care: Light, Humidity, Watering and Division.