Hello!

I recently acquired a Japanese Maple, and since I bought it, I've noticed black spots on it. I assumed it was sunburn, but I want to rule out any kind of fungus or pest. Do any of you recognize these symptoms? By the way, the new shoots don't have these spots.

On the other hand, I've been watering my bonsai daily at night, but I've noticed some of its leaves have turned yellow and are falling off, which makes me think I'm overwatering it. The tree's substrate is muddy, and it's been raining daily in my city for the past two months, with cloudy skies and average temperatures of 22°C / 73°F. I'm considering spacing out watering to three days. Do you recommend doing this?

Thanks for your help.

I would water in the morning. That way there would be the most moisture when the sun is shining. 

Also, if you are in a place with significant insect problems, watering at night can attract insects to the house.

 

It is not sunburn. As we cannot see the whole tree it is not easy to know what might be wrong if anything. Watering in the morning is better, and looking at the base of the trunk the moss needs to be removed from the trunk. You must not water to a schedule, watering is done when needed, if the plat is not drying out then don't water. I have until recently had to water every day, and sometimes twice a day if it is hot.

You say the "substrate is muddy"? and that you are watering daily, in spite of the rain? Well, there's your problem...you're over watering, pure & simple. How do I know? I've been through similar situations & usually if you let the plant dry out for a couple of days, you will notice it recovering..  
 

It is not overwatered. As I said it is normal and nothing to worry about. The leaves will be abscising soon anyway so it is not a problem. Unless there is persistent rain for hours I wait for it to stop and then water especially at this time of the year with the heat we have had which dries the trees out rapidly when it stops raining. I have only missed one days watering in the last 4 months which was in the past few days. Obviously if the soil is damp then there is no need to water.