Black spot is a fungal disease which affects the leaves.
On a bonsai this usually shows as small dark coloured spots on the leaves.
It is easy to treat but you will need to spray the foliage with a fungicide (available from garden centres).
It is important to repeat spray according to the instructions on the bottle.
If your bonsai does have black spot you will probably notice spots on all the leaves (new and old). If the spots are only on the old leaves please read on!
Old leaves (ie not the new growing tips) frequently show small spots. These leaves will drop and new ones will replace them. This is just a sign of aging. If there are no spots on newer/younger foliage them I would suspect that this is the cause.
Red Spider Mite
Red Spider Mite is a very tiny pest, which is just visible with the naked eye but can go un-notiiced. It is the symptoms which normally become apparant in the first instance. Red Spider thrives in warm, dry conditions so rarely affects outdoor bonsai.
Bonsai affected by red spider mite tend to display a pale mottling on the leaves. If you look closely fine silk-like webbing and hanging leaves can also be seen.
Eventually a bonsai with red spider mite will start dropping leaves.
To treat for red spider mite we recommend that you spray twice weekly with plant invigorator. Spray to run off so that the leaves, branches, trunk and under the leaves are dripping. Repeat twice a week for 4 weeks and then return to using weekly.