Bonsai Repairing
Black pines make amazing bonsai trees. However, the worst thing that can happen after years of training is for your black pine to die on you. Typically this can be caused by fungus or spider mites. However the most common issue with black pines is root rot.

Essentially, this means that your tree's roots have retained too much moisture and the roots have slowly decayed leaving your tree to slowly slip away. To prevent this, it does not mean that you should stop watering. Your tree still needs to have its daily intake of water. It does mean that you need to repot your tree and eliminate all the dead roots. If it's the middle of summer, you might want to not do this but you may be forced to if the roots have gotten very bad.

The best way to avoid any of this issue is using a good draining soil mix. With a fast draining soil mix, the tree will get its hydration without the fear of root rot. For other issues like spider mites or fungus, you can treat it with some pesticides or fungicides. However these can be resolved much easier than a problem with the roots, so make sure you use a good soil mix when you work with black pines.