These do not appear to be disease symptoms or damage due to a pest. Additionally, fungicides are preventative measures only, not curative for existing infections, and very few products are labeled for indoor use (and probably fewer still for plants that produce an edible harvest). Leaf spots are quite common on senescing leaves (those shed by the plant due to individual leaf age or whole-plant stress), because they are breaking down as the plant cuts off their life support in preparation for dropping them.

Plants that spend the summer outdoors (which citrus do prefer) routinely drop some leaves as they readjust to indoor conditions. Compared to outside, indoor air is much drier (especially in winter), and the light levels are much lower. (Grow lights help to compensate for that by providing supplemental light, but if the citrus is only receiving natural light from a window, the lower intensity and reduced hours of daylight have an impact.) Citrus also prefer to stay comparatively cool during winter indoors, and many homes are a bit warm for their needs. Our Growing Dwarf Citrus page has more information.

You can try several approaches to see if the plant improves and regrows new leaves faster (and to see if if the plant fares better next autumn when it comes in again). Many environmental factors can contribute to leaf drop, including over-watering, over-and under- fertilizing, being too warm or cold, not having enough light, and low humidity. Stress can also cause fruit/flower drop.

• Monitor watering carefully, because citrus want to get somewhat dry between waterings (depending on pot size, mostly dry to the touch at least an inch or two deep into the pot). If the pot sits on a saucer to catch drips, empty it promptly so any collected water doesn't seep back into the drain holes and keep roots too wet.
• Put the pot directly in front of a sunny window, or try using plant grow lights to supplement (or replace) natural light. With more light, the plant might keep more leaves, though dropping the oldest of them is still normal, especially with a change in conditions. If using grow lights, having them run on an outlet timer for roughly 12 hours a day should suffice.
• Try using a room humidifier if the ambient humidity is low; don't mist foliage with water, as that's not effective in raising humidity for long enough.
• If you have been fertilizing, stop for winter, though if you didn't fertilize much or at all last summer, the plant might be a bit nitrogen-starved, in which case a modest dose of any general-purpose fertilizer should be fine.
• If it's been a year or two since the plant was repotted, the soil might be compressing with age and retaining fertilizer residues (and tap water mineral residues) and draining more poorly over time. When repotting, remove what old potting mix you can and replace it with fresh, even if the plant doesn't need to move into a larger pot. In fact, make sure the plant isn't over-potted (growing in a pot much larger than its root system) since this can make it easier to accidentally over-water.
• Keep the pot out of the path of heat vents or radiators, and rooms with a bit of a chilly draft might actually benefit them, as it reduces heat stress. (They don't want to be cold, but cool is ideal, and also makes it easier to maintain higher humidity levels compared to warmer air.)

Miri