Root rot seems to be the automatic culprit far any bonsai that is not looking well.
In my experience root rot is actually quite rare.
Moving trees from shaded, sheltered to more sunny position will often result in brown leaves just like this. Just like our skin the leaves produce sun protection to match where they are. When suddenly shifted into sun they get sunburned. I find it is better not to shift trees but if that is required make the move gradually over a couple of weeks so the leaves can react to the increased sun and produce the necessary UV protection.
When leaves start to look sad it is much more likely to be dehydration.
Anyone who is worried about root rot is also likely to be reluctant to water properly and with current dry conditions in many areas a tiny tree like this can dry out in just a few hours. When potting mix gets dry it is hard for water to penetrate. Most of the water that is applied just runs down the sides and out the drain holes so every day the root ball gets a little drier until the tree just cannot make it through the day.
Change of seasons is a classic time for dehydration to affect bonsai. Not only are we starting to reduce watering in response to cooler days but the pot will now be getting quite full of roots making it increasingly harder for water to get into the soil. I have lost many trees in both autumn and spring due to changing seasons and not enough water.
The tree does not look like it is wired into the pot so you should be able to just slip the whole root ball out of the pot intact and check the sate of the roots. Being out of the pot for a few minutes, even up to an hour, won't hurt the tree. You can also check to see if it is wet or dry down there while you have it out. After checking just slide it back into the pot and press down. If the soil threatens to slide off the roots as you lift the tree out stop and press it back in. If that happens you have a root problem but will need time and preparation before dealing with it so just push it back in and put the tree back where it has been.
We should also be aware that, assuming you are actually in Australia, autumn is here. My tridents have already responded with the start of autumn color so it won't be long before the leaves start to drop. this browning of leaves does not look like autumn change but could be related.
As you can see there are a number of possibilities that could fit the info given but
this is simply sunburn. There is no cure for the affected leaves. They may drop off if too badly affected but as the leaves will soon drop anyway I would just ignore them until that happens. In the meantime those leaves will remind you of a mistake and you are less likely to do it again. If they really upset you the affected leaves can be snipped off. It won't hurt the tree either way.
Hard to tell if this tree is trident or A. ginala as there is not scale in the photos so almost impossible to guess the real size of the leaves or the whole tree or pot. Trident is more likely for a commercial mallsia like this I think.
Not having and idea of the location also makes it hard to guess at environmental factors that may be affecting the tree or to offer specific advice for the location. That's why we encourage all members to put a general location in the profile. It will give us good insight and will save you from remembering to type a location each time you post.
