Question
My Son has a lot of Gardenias and they have a Black podery substance coating the leaves like they have been painted it is an old house and have been here many years. He has sprayed for insects. and if you hose and rub the black comes off. Some leaves are yellow but most still green. The stuff is also on branhes what can we do for them.

Answer
Sounds like a scale insect problem.

Various scale insects feed on gardenias, including the soft scales ?Japanese wax scale (Ceroplastes ceriferus) and cottony cushion scale (Icerya purchasi); and the armored scales ?tea (Fiorinia theae), greedy (Hemiberlesia rapax) and oleander scales (Aspidiotus nerii).

Scales are unusual insects in appearance, and as a result are sometimes misidentified by gardeners either as parts of the plant itself or as disease organisms rather than insects. Adult female scales are small and immobile, with no visible legs. They secrete a waxy coat that varies significantly in shape and color depending on the species. Adult males tend to be very small and have wings which allow them to fly so they can locate females.

Scales feed on plants by piercing the leaf, stem or branch with their mouthparts and sucking sap. Their feeding can weaken or kill branches. Heavily infested gardenias are often stunted with small flowers and leaves. Leaves may yellow and drop early. Like aphids, soft scales excrete honeydew.  The growth of the sooty mold fungus on honeydew results in leaves that are covered in dark fungal growth.
This is called Sooty mold.
Control the scale insects and the sooty mold will go away. Treat the shrub with an insecticides called Bayer Advanced Tree and Shrub Insect Control. This is applied to the soil around the shrub and the roots will carry it to the leaves and twigs killing the scale when they suck the plant juices. Check with your local garden type store for this product. Here is a web link to information on this product.

http://www.bayeradvanced.com/tree-shrub-care/products/12-month-tree-shrub-insect...