Check the roots. Root-rot is very common.
If any of the braided trunks are still alive, separate them and
put them into new soil that has very, very little peat moss in it.
Good luck! You might want to keep an eye out for a new
Pachira in the meantime.

Josh

Nancy, what do the black spots look like? Have you ever seen scale bug? Can you ID scale? They're mostly brown but come in white too. They're bumpy lumps that can be scraped off with a fingernail. Scale secrete honeydew which is sticky, which makes me think scale. Honeydew attracts ants in later stages.
How big is your Pachira/Money Tree?
Can you carry to the sink/shower, and hose off? If so, wash leaves thoroughly using liquid dish soap, (not Dawn). Inspect plant. What about spider mites? Toni

It's likely being watered much too often, hence the root rot/fungus problem.

Thanks for the input. A friend of mine suggested scale, but these black spots cannot be rubbed or flicked off the leaf. I have also washed the leaves and am now hiding & watching. The trunk appears to be healthy, not spungy. I guess there is the possibility of too much water. It's a small plant (bonsai) & I've watered it about once a week. The soil felt dry to the touch but I don't know what the roots are doing. Should I pull it out & look?

Nancy, I've had better luck caring for Money Trees the same way I do most succulents. Underpotted, water when dry, even less in winter, well-draining soil and no fertlizer, whatsoever during short days. Did you fertilize?
As long as the trunk is firm, your Pachira is alive. Do you have a picture?
LOL, you're hiding and watching. That's funny.
If you know what to look for by unpotting then do it. What size is the bonsai pot? Check roots to see how much room is left between roots and pot. Some bonsai pots are large, and depending on space makes a difference. If you water once a week, (religiously) and soil is wet/moist, your Pachira will develop problems, including bugs. The good news, if your MT was dead, the trunk would be mushy/soft.
How many leaves are on your plant, how many yellow?
You mentioned a pest, used oil. Did you literally see bugs on your plant? Did the problem start after using oil? Toni

Yes, Nancy, check the roots. That was my first piece of advice.
The soil/roots should smell like soil or vegetation, not like sulphur
or a swamp.

Pachira like to be watered, fairly well flooded actually,
and then allowed to dry. Drying is key. Soil should be fast-draining,
which will allow oxygen down into the root-zone. Another thing: Pachira
doesn't need as much water in the winter months. A weekly watering, year-round,
is *most likely* too much (unless you live in a very dry region or have a
plant with so much foliage that it is transpirating large amounts of water).

Good luck!

Josh

I have a pachira for three years now, it is also braided, about 6 ft tall. It looks like it secretes some kind of sticky substance, at first I was looking for scales and such, but looks like this dew comes from new leaves. So next time look for offenders before spraying.
I second other opinions, if trunks are firm, it will come back, yellowing leaves may be due to spraying. Keep it on dry side, in smallest pot possible.