Question
This is the second year of this problem. Our trees are at least 60-80' tall.  Need to know if there is any product to coat or clean underlying plants, rocks, etc. that are still black from last year, since the trees themselves are impossible to treat.  Thank you.

Answer
This sounds like Tulip poplar scale. The tuliptree scale is one of the largest soft scale insects in the United States. The tuliptree scale is a key pest of yellow poplar or tuliptree, magnolia, and occasionally linden. This soft scale insect is so prolific that it quite often covers twigs and branches. The insect lives under the shell like structure and sucks the plant juices from the twig.


Large numbers of these soft scales may give an infested twig a warty appearance. One of the first indications of an infestation of this pest is the abundance of honeydew (sticky, sugar-rich material) secreted by developing soft scales during the growing season. Ant and wasp populations that seek the honeydew are often found in association with this soft scale insect. These ants may need to be managed since they protect this scale insect from predators and parasitoids. The honeydew is high in sugar content and  on which black sooty mold grows. The sooty mold may turn the leaves, twigs, and other surfaces beneath an infestation black. Feeding by this pest may weaken young trees by removing plant fluid. In some instances this species may be so prolific that it covers all of the twigs and branches. This could result in a rapid decline of an infested tree.


Systemic insecticides can be sprayed on the foliage and branches BUT with a large tree like yours this is not possible. There is an insecticide that is applied to the soil around the tree and the roots will carry it to the twigs and foliage and when the scale insects suck the plant juices they are killed. Once the scale insects are killed the sticky honeydew and the sooty mold will go away. This is called Bayer Advanced Tree and Shrub Insect Control. This product will control and prevent scale insects for the whole season--up to 12 months. Here is a web link to this product--check with your local garden type store for it.
http://www.bayeradvanced.com/tree-shrub-care/products/12-month-tree-shrub-insect