These are known to require high humidity. Do you know what the humidity is in the surrounding area of your Calathea?

Planto

If it is spider mites, then I think the key to getting rid of them is to be consistent in spraying it down every few days so you catch the mites as they hatch.

Some people claim that high humidity will discourage mites though it isn't clear if that is really true. In either case, calatheas usually do appreciate extra humidity.

If this was my plant, I'd probably mist the foliage every day to discourage the mites in general and spray with something to specifically combat the mites (I like to use a Neem Oil solution) every 3 days. If you can put a humidifier near it that might help too.

This post was edited by summersunshine on Sat, Oct 18, 14 at 11:17

Oh those are so pretty! If the pot has peaty potting soil, not letting it dry out may be causing the roots to rot. The pot looks quite small for the size of the plant, and if it's staying moist for a week, that's concerning. If the roots have filled the pot, and/or made a solid pancake of roots at the bottom of the pot, that will negatively affect the foliage as well.

What a pretty plant! It looks like someone painted leaves on the leaves :-)

quote:
-But ever since these brown spots have been developing. I am keeping the soil relatively moist, watering every 1-2 weeks and preventing it from drying out.

so before that you allowed it to dry out? that could definitely cause the drying on leaf edges.
calathea will suffer if allowed to dry out. and it's normally recommended to keep it pot-bound - and then only repot when it is dormant, like in february. it just hates roots disturbance.
when in growth it takes a good amount of water and it likes to be very warm and humid. when temps drop below 68F it slows down growth and starts taking very little water: so water it very sparingly then.
it's perfectly ok to grow it in peat mix (soilles mix peat-based), just like african violets or tuberous begonias. you just need to make sure you do not soak it too much too often. and normally may be 30% perlite is added to aid aeration/drainage.
do not water it unless the top of soil starts feeling somewhat dry. the trick you can use: put down some long fiber sphagnum moss (used for orchids) on the surface, like loose mulch of sorts. it'll wick water from the soil. when you see that it's dried up but is not crisp yet, then you can water.
overall the plant looks very good - i see only a couple of leaves with spots. it is also quite normal for older leaves to show signs of decline in fall-winter.
the following link is very good: it says fluoride in water can cause spots on tip/leaf edges. notice also the recommended fertilizer ratio of 1/0.4/1.8

Here is a link that might be useful: calathea culture

Calathea lanciifolia, does not need tons of humidity like some other Calathea cultivars because it has tougher leaves, as shown in the photo. I believe your spray mix has suddenly affected the leaves.

Really, Teen? Interesting. Was unaware that there were any Calathea that did not require high humidity levels. Good to know.

Not to say extra humidity is not appreciated, and if your air was bone dry(below 30%) humidity, they might start to brown but I think most houses dont drop that below that very often, and when it does, watering should be enought to avoid it.

on the contrary, avg 35% RH in winter heated houses is common and that's too low for this plant. some people have even below that. depending where you are that can be for 4-6 months. watering the plant will not raise humidity much around it and if you have below 68F - the plant won't be taking much water/growing; so watering it will only help to rot it, even in low humidity. i have stromanthe, the care is very similar, except it can take cooler temps better. but at 65F it just totally stops taking any water at all and my humidity never drops below 65%. i don't let it dry up either, but leaf edges slowly brown by spring and old leaves yellow one by one and are totally replaced by new growth. but it's slow to start, it waits until my sunny windows heat up to 75f-80f (it's in bright moving semi-shade/dappled sun, so it gets warmed up).
the link i posted recommends min 60%.
Ebokai,
i am curious how long have you had the plant and if it went thru winter with you before?

Thank you petrushka, your comments are very insightful.

This is the Calathea's first year in my home. It is located about a meter from a east-facing window that gets most but almost no direct sunlight early in the morning. The rest of the day it is in mid-shade. Temperatures in my house are currently around 64F and once I turn on the heating in the winter I keep it around 66-68, so I should probably switch to winter-season watering schedules, ie watering less but preventing dry soil.

Since my first post I have washed the leaves to get rid of any soap that might have remained from spraying the leaves. I also put a pot filled with water next to the calathea to help with local humidity. I've checked the leaves for any signs of pests but I don't really see anything out of the ordinary.

I also have another pot with water on the radiator so it humifies the room once I start heating.

Another thing I've noticed is that when I first got the plant, it closed its leaves at night and opened them during midday, but it isn't doing this anymore.

I will keep an eye on the calathea the coming weeks and see if the brown spots continue spreading.

This post was edited by Ebokai on Tue, Oct 21, 14 at 12:43

in one of the links i pulled up they actually pointed out that this particular kind of calathea is the least likely to develop brown leaf edges (most of them do over winter).
one thing that comes to mind is the low air temps. IF you were to keep the roots warm by putting it on a heating mat/electric pad from pharmacy on low-med setting (which i do for some of my tropicals) - you probably can fool it ;) and it will not go dormant. warm roots are more important then warm air. but that entails careful watching to make sure it does not dry up. another thing is quality of light. IF you can maintain warmer roots and bright light - it should continue to grow. but that is easier said then done.
but i did maintain a good growth on my thai aglaonema thru winter/spring by keeping it on a heating pad. it did not grow as much as in summer, but it was growing.
unfortunately calatheas are not effortless plants. they look spectacular, but unless you have the right conditions those great looks can go fast...