Acer Palmatum possible fungal infection

I have a green leaf Japanese maple whose leaves have been curling. I originally thought it was underwatering, but I'm now afraid it might be an infection. I'm not sure if this is correct or something else.

Timeline:
- Mid-December - Bought plant. Watering schedule is once a week, in partial sun.
- Jan 1 - Last time plant was noticed fully healthy (similar to picture below)
- Jan 6 - Was going to water according to usual schedule. First noticed curled leaves (already curled same amount as in pictures below), thought it was just underwatering or heat stress. Bottom watered and moved to a shadier area that still has partial sun.
- Jan 7-9 - Monitoring. Leaves not curling more, but not recovering either. Still green, but slightly losing color over the days.
- Jan 9 - Quick (5-10 mins) of bottom watering.
- Jan 10 - Pictures taken below. Stems still green underneath (green circle), still alive. First noticed possible infection. Black spots on bark/death of cambium (red circles), not sure if these were here before. Powdery mildew on some leaves (orange circles) that were not here last Jan 7. Sprayed baking soda/soap/water mixture lightly on leaves.

Area: Philippines*
*only have wet/dry seasons, no winter. December/January is a cool dry season here.

Does this look like just underwatering + powdery mildew? Or is it bacterial/fungal infection? And do I just keep watering every few days, spraying baking soda/soap/water mixture once a week, keeping in a shadier area, and hope for the better? What else should I try - do I need to check the roots, remove the black bark, etc.?

I've been taking care of plants for a while now, but this is my first bonsai and my favorite plant. I've actually cried a couple of times worrying she might die. Please, how do I save her?

Have you taken care of temperate plants in your area or indoors before?

Are there any maples growing in your forests or in gardens around you?

Have you ever seen a long term healthy maple bonsai in your area?

Those questions is just so you can determine if it is even possible to grow the species in your area. Looking at the climate in the natural distribution of the species and usda recomendation (zone 5 to makes me think it is impossible. Growing indoors is absolutely not possible, so a slight chanse is to move it outdoors.

Japanese maples is decidious trees. When grown in temperate regions they loose their leafs in the cold part of the year. Since you are in your "cold" season, I would say do not bother about any issues with the leafs. Just remove them. Once that is done, please provide a photo on the dormant buds. Looking at them will make it possible to say if there is still life left or not.

Thank you so much!

I haven't taken raised temperate plants before. I've heard of fully grown maples in cooler parts of the country (average 27°C), and was told this was grown from a slightly cooler area as well. The area I'm in ranges from average ~27°C in cool seasons to average ~35°C in warm seasons, although the lack of winter is a problem. I'm not sure how long maples would survive here without winter dormancy. For now, I'm just trying to keep it alive for as long as possible. I'm also moving the tree outside to a spot with partial sun, thanks for the heads up.

I have attached pictures of the buds below. Is it safe to remove the leaves given that we don't have a winter period or a spring period for leaves to grow back? Additionally, if the leaves still have some green, should I wait to see if the leaves perk up after a few days/weeks outdoors or should I remove the leaves now?

I am not sure anyone here can help you. Bonsai is about long term growing and developing thriving trees into beautiful specimens. Not about keeping sick and weak trees, "alive as long as possible".

I see no problem with removing the leafs. They are already dead and are doing nothing. If it grow new ones or not when it gets warmer again does not depend on that.

I see some dried, dead buds and some that still looks good. My guess is that there are two possible outcomes. It grow back a few very weak leafs or it does not grow back at all.

Next time get yourself a species that can be grown good in your area. There are thousends of interesting species out there that thrives in your environment.