Question
We have a gardenia plant that we have planted outside.  its
leaves have a black discoloration on their leaves.  What is
it a and how do you treat it.

Answer
Sounds like sooty mold. This fungus grows on the secretion of sucking type insects. The insect suck the plant juices from the plant and secrete a substance called honeydew which is high in sugar content. The fungi live on this sugar. Control the aphids and scale or white flies and the sooty mold will go away. I would use a product called Bayer Advanced(TM) Tree & Shrub Insect Control. This product is applied to the soil around the shrub and the roots transport to the foliage and stems and when the insects suck the plant juices they are killed. Check with your local nursery for this product. Here is a web link to more information on Bayer Advanced.  

http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html