Could have more than one possible cause, none discernible using these photos only. Otherwise a probably more generally significant involved aspect is that you are in fact trying to grow a cold climate adapted deciduous hardwood as if it were a tropical house plant.

How to get an apple tree out of dormancy (easy tips) – Knowhowgarden

Is there a problem

Yes: apple trees aren't house plants.

You might talk to the people who do bonsai apple trees, as they do keep apples as house plants. It will drop it's leaves in the fall no matter what you do, and would probably appreciate a period of cold.

" You might talk to the people who do bonsai apple trees, as they do keep apples as house plants. "

Sorry but no, they don't. They will bring them indoors to a cool or unheated place over winter (only because they are grown in containers and therefore subject to root cold damage) but the rest of the year the are outside. Same as the rest of the hardy woody plants they grow as bonsai. That's a BIG difference from growing them as houseplants!

Woody plants - trees and shrubs - that are hardy and that grow outdoors across most of the US rarely make viable houseplants. Indoor growing conditions just do not provide the requirements these plants need to thrive - seasonal temperature swings, a period of dormancy, vernalization/changes in day length, direct sunlight (and not through a window!), humidity, insects for pollination, etc.

Apple trees need an annual, obligate winter rest (predictive dormancy) in order to maintain long term viability. You cannot expect the plant to live for more than a season or possibly two w/o that winter chill. Decreasing day length (actually, it's the increasing length of the dark period) causes the tree to 'set' buds for next year's growth and moves it toward the dormant state. Increasingly cold temperatures seal the deal, forcing deep dormancy. The tree needs chilling to 'release' it from its dormant state with the amount of chill required varying by plant variety with some cultivars needing as little as 300 chill units or as much as 1200. Roughly, a 'chill unit' is 'earned' by the tree being exposed to temperatures between 32 - 40*F for an hour. Temperatures below 32* do not count toward accumulation of chill units, and the tree being exposed to temperatures above 42-44"F can detract from the number of already accumulated chill units (voids some of them is a good way to think of it), thus adding to the amount of chilling time ultimately required.

Trees which have had their chill requirements met pass into a state of quiescence, during which they are fully capable of growing, but growth is suppressed by cold temperatures. When temperatures rise above about 45* for a number of consecutive days, it will force onset of what the tree 'thinks' is spring growth. The tree must be protected from frost after onset of spring growth (leaves are opening).

Decreasing day length (actually, it's the length of the dark period) causes the tree to 'set' buds for next year's growth and moves it toward the dormant state. Increasingly cold temperatures seal the deal, forcing deep dormancy.

If kept indoors, your plant might survive until next spring or summer, but it WILL at some point go dormant. Very seldom can plants deprived of dormancy be pulled back from the brink but should that occur, the only thing which will bring it out of dormancy is an appropriate cold rest (dormancy).

The affected leaves could be culturally related (as in too much water) or the first phase of leaf abscission in preparation for dormancy. This first phase is called resorption, during which the plant collects mobile nutrients and other biocompounds from the leaf prior to a layer (abscission layer) forming where the leaf attaches to the branch. Once started, formation of the abscission layer is irreversible.

It's best to plan to acclimate the plant to cold now, but do not thrust it out into temps below freezing. Put it in a shady spot for several days when nights won't be below freezing if possible. After that, allow it to get knocked back by frost in temps around freezing. You can over-winter it by burying the plant with it's pot in a garden or bed. It's important, because the entire root system is within the confines of the pot, that you throw some snow on it regularly, perhaps weekly. If you cannot do this, remove the tree from the pot, plant it in a garden, bed, or against the N wall of a heated building. Alternately, the tree can be over-wintered in an unheated garage or shed as long as you see to its need for water.

Al

Shows what I know about bonsai! (eg almost nothing) I've only ever seen them indoors; I assumed that that's where they lived much of the time, save for a chilling period in winter.

The little apple tree seems to have stabilized. Thanks to all that replied.

Do you mean it is still in leaf and you did not give it a chill rest period as advised?

It's still in leaf, but hasn't made any new ones lately. It's in cool place in the house where it can get some afternoon sun.

Apple trees cannot live indoors. They are NOT houseplants and should not be treated as such! Lack of a dormancy period and winter chill hours will shorten its lifespan significantly. It will never be productive provided it lives long enough so what's the point of keeping it inside?

I plan on putting it outside in the spring.

An updated photo taken on Feb 8, 23.