Asked November 21, 2021, 3:41 PM EST
Carroll County Maryland
We cannot make a diagnosis from your photo. We are reaching out to an Extension specialist for more information. We will contact you once we receive more information.
The Extension specialist we consulted thinks this is due to mites (perhaps rust mites, not necessarily spider mites) damaging the skin of the fruits as they feed. They will probably be too small (or nearly so) to see without magnification. The interior pulp/juice of the damaged fruits should be unaffected and safe to eat, and the plant overall will probably be fine unless the damage causes the plant to abort (drop off) the fruit due to stress. As with spider mites, you may be able to suppress populations by treating the plant with horticultural oil or insecticidal soap.
If you opt to treat the plant, a thorough coating of all surfaces (leaf tops and bottoms, fruit, stems) is essential for good efficacy, and treatments will probably need to be repeated several times to fully knock-down the population. Make sure citrus (or at least edible tree fruits) are included on the label so you know the product is intended for use on harvestable crops - while several products within or between brands will contain the same active ingredient, they may not all contain the same inert ingredients, so may not be interchangeably safe for this use. The label will also tell you how long of a gap you may need to leave between the final application and harvesting any fruit.
The only obvious sign that a pesticide treatment is working will be the lack of symptoms of damage spreading. That is, new growth - or in this case, new fruits forming - won't develop the same symptoms. Plant tissues that were already injured by the mites will not heal (and an insecticide application might happen to damage them further, though this isn't a concern), even if the mites themselves are removed.
As with most indoor plants, citrus require moderate to high humidity levels to thrive. If the air is fairly dry this time of year (often the case), consider using a room humidifier near your houseplants to boost ambient humidity. Misting won't do much as an alternative, nor will humidity trays if there's too much air exchange with the rest of the house. A humidity boost isn't necessary, but it might help the plant overcome or resist any pest attacks in the future. Here is additional information about growing citrus in our area, if useful going forward: https://extension.umd.edu/resource/growing-dwarf-citrus