It might be due to soil conditions, since Maranta (prayer plants) are sensitive to water quality, like municipal tap water additives (fluoride) or any water source that is high in minerals ("hard" water) or sodium salt (like the small amounts in some bottled water or household water softeners). Using purified water can help, as can regular repotting (perhaps once year...it would depend on how fast soil residues are building up). If plants are over-fertilized, this can also contribute to "salts" that build up in the soil over time, which can stress or damage roots. If you aren't sure when the plant was last repotted, it might be helpful to repot it this spring. (Doing so in winter would be fine, but if its stress seems minimal, it might be easier to wait until it's in a stage of more active growth.)

Lower ambient humidity indoors can stress a variety of houseplants, including Maranta, though how much this is contributing to any leaf yellowing or shedding is hard to determine. Are you misting the plant's foliage? If so, discontinue, since wetness on leaves can invite disease by allowing fungal or bacterial spores to more easily infect a leaf. At this point, in case you have been fertilizing recently, you can weaken your doses or stop them entirely until spring, using only plain water during winter since the plant will naturally slow growth in response to shorter days.

If any leaf yellowing or brown spots are a result of infection, there isn't much you can do about it except to cut off the affected leaf if the damage spreads to encompass most of the leaf. (The small amount of yellowing in the photo is not worrisome.) There aren't many fungicides labeled for use indoors, and even those that are labeled for such use might not be very effective, nor might they have much impact on a bacterial pathogen.

It's hard to tell from the photos which leaves are/have yellowed in terms of leaf age. If they are the oldest leaves, which will be those furthest from the center of the clump (new leaves emerge from the center of the cluster of stems), then their shedding tends to be normal, both as an adjustment by the plant for the change in growing conditions, and as routine leaf shedding of a plant's oldest foliage as it continues to put out new growth. Accelerated leaf shedding may suggest a plant is stressed, but it would not be unexpected for a Maranta to lose a few old leaves over the course of a growing season if it's been producing new growth normally.

For now, just keep an eye on the plant's symptoms and make sure that it's watered only when needed (feeling the top inch of soil and watering only when it's becoming somewhat dry to the touch at that depth), and that the pot isn't sitting on a saucer that has drained water in it since we don't want this soaking back into the drain hole and keeping roots soggy. (After watering a plant sitting on a saucer, empty the saucer promptly so it doesn't puddle water under the pot.) If the leaves touch the window itself, cooler nights might be causing the leaf to yellow where it was in contact with the glass, or moisture condensation on the glass from a cool evening allowed an opportunistic leaf infection to set in. Overall, the plant looks to be in great shape for now, so hopefully the yellowing is just minor stress or only related to some normal leaf loss as it adjusts.

Miri

Hello Cathy,

Our apologies for the slow reply...a system glitch in Ask Extension caused us to receive client replies very late.

A Brita filter might not remove all the tap water components that might stress a Calathea over time, but it shouldn't hurt to try. You can see what the filter removes on the product label, or inquire with the company if it's not clear. Fluoride is one element it might not remove, for example, which could contribute to leaf tip damage on fluoride-sensitive plants. Minerals that help with taste, like calcium and/or magnesium, also might remain in filtered tap water which, over time, could build up in potting mix and stress roots, though repotting yearly is one way to mitigate that.

Miri