This could be caused by what is called
sooty mold.
It is caused by several fungi that grow on honeydew, a sugar excretion of aphids, scale, mealybugs and other insects. It appears as a dark brown or black coating or spots on leaves, stems and other plant parts.
However, sooty mold does not attack the plant directly. It grows on the honeydew produced by insects that are feeding on the plant.The fungal coating can block light absorption by the leaf and limit photosynthesis. In severe cases, leaves may turn yellow and drop.
The most effective way to control sooty mold is to control the insects producing honeydew. An application of insecticidal soap will help to control these insects and also wash off some of the sooty mold fungus.
If there no insects, it could also be due to excessive moisture on the plant.
Here are the description of this trouble:
http://www.heyne.com.au/gardencentr...us+Diseases.htm
http://plantpathology.tamu.edu/Texlab/multi6.html