Question
I have several mature crape myrtles in my front yard (morning to noon sunlight) which have a black soot-like film which appears on the both the leaves and the limbs.  You can actually wipe it off most of the leaves.  Is this a fungus and, if so, how can i treat it?  thanks.

Answer
It sounds like sooty mold, which is a fungus that grow on honeydew. Honeydew is high on sugar content. The problem is not the sooty mold but insects that suck the plant juices. There are two insect types that will do this--aphids and scales. I would guess that is is aphids since they are common on crept myrtle. Both of these insect suck the plant juices and secrete a substance called Honeydew and the sooty mild grows on it. Control the insects and the sooty mold will go away. Next spring as the tree starts to bud out treat the tree with an insecticide called Bayer Advanced Tree and Shrub Insect control.  This is applied to the soil around the tree and the roots will transport it to the leaves and twigs and when the insects suck the plant juices they are killed. The reason to wit until the spring is that during the Fall and winter months there is little movement of water and nutrients from the roots to the upper parts of the tree and the insecticide will not be moved. In the spring the plant needs more water etc and it absorbs these through the roots and move them quickly to the leaves. This product should least all season to control and prevent these insects. Check with your local nursery for this product. Here is a web link to more information on this product. http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html#