Question
We have several different types of trees (oak, maple, redbud, crepe myrtle, lemon) that are all developing this thin 'skin' of black growth on the leaves #really all over#.  It has almost killed the lemon tree.  It's a real thin black film that flakes off with a fingernail.  The leaves are a healthy green underneath, but they are covered in the black film.  We thought it was aphids, but the leaves show no indication of any infestation.  Should we try the soapy water mixture or do you have another suggestion?  These trees are all different ages, but all less than 10 years old.  The crepe myrtle showed signs for the last several years #started with a green/gray mossy growth#, but it is now totally black.  If it matters they are all on the side of our property that runs along dense, wooded, unclear acreage.  Any thoughts will be greatly appreciated, THANK YOU!

Answer
I could be scale insects--they like aphids will suck the plant juices from the plant and secrete a substance called honeydew which is high ion sugar content and sticky. A black sooty mold will grow on the honeydew causing the black appearance on the leaves and branches. The leaves should be sticky to the feel. The scales look like small turtle shell attached to the branches and twigs. The insect lives under this shell. The best way to treat the scales is use an insecticide called Bayer Advanced Tree an Shrub Insect Control. This is applied to the soil around the tree and the root will carry it to the leaves and twigs and when the insects sucks the plant juices they are killed--it also works for aphids and a number of leaf eating insects. Here is a web link to this product, check with your local garden type store for it.

The timing maybe a problem--soon the trees will start to shut down for the winter in most places so the root activity moving water up will slow down. IF the hardwoods have not begun to turn their leaves you can go ahead and treat now. It will take a couple of weeks for the insecticide to reach the top of large trees. OR you can treat early next spring and get results all season long.

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