This plant is Dracaena marginata which is now an unlikely member of the Asparagaceae family. These plants are native to Africa and are usually trees which grow in full sun in dry conditions.
The picture shows a couple of sunken lesions which are typical of fungal/viral/bacterial action. This is not likely to be the cause of the issue, only the symptom. The other items I notice are:
- new growth looks healthy
- stems look firm, not dry and dead
- red margin is quite prominent indicating adequate light levels
- fresh soil with a high peat/organic matter content appears to have been added
The sunken lesions indicate the a soil that is too moist. This plant grows in full sun in hot environments. I suggest removing the top dressing of organic matter and reducing the amount of water. Observe the plant for the next month. If there is no improvement repot with a soil that drains faster. Sharp sand, perlite, vermiculite, small gravel ( .5 cm diameter) are all suitable things to add to an existing soil mix.
Follow these diagnostic checks to determine if the most common causes are present:
- check the underside of the leaves for what appears to be tiny grains of salt. These are spider mites and can be controlled with 5 ml dish soap in 1 litre of water applied to the underside of the leaves with a cloth three times at five to seven day intervals
- check the crown of the plant for pale growth. There is a less common spider mite that chooses to live in the apex rather than the leaves. Treatment is the same but not normally as successful.
- check for white cottony tufts on the underside of leaves or the crown which are mealy bug. On this plant they are very hard to get rid of. Soap and water as described above is a good start.
- remove the plant from it's pot and examine the roots. Healthy root are firm and white. Larger roots have a yellowish colour. Roots that are soft or black have root rot which is typically found in over watered plants.
- feel the stems of the plant. If they are soft or feel hollow then they are dead or dying. The most common cause is overwatering. This will cause slow leaf drop as the amount of nutrients reaching the leaves is inadequate.
- check for adequate light levels. When grown in high or moderate light this plant will normally have red margins on the leaves. In low light these are less prominent.