Question
My mother has a weeping pussy willow tree that has a black substance on the leaves. The entire ground under the tree in covered with a black oily substance that is dripping from the tree. What causes this and what can we do to fix it.
She lives in Hickman, KY.  zip code 42050

Answer
If the dripping sap is sticky then this is a sign of an insect either aphids or scales--more than likely aphids. These insects suck the plant juices from the tree and excrete a substance called honeydew (containing high concentrations of sugar)these are sticky and usually a black sooty mold will grow on the honeydew. Control the aphids with an insecticide and the sap will stop.

Numerous contact insecticides are registered for aphid control. Since aphids are often placed under considerable pesticide pressure in field crops and greenhouses, they may be resistant to certain categories of insecticides. Therefore, if you do not obtain reasonable control, consider rotation to another insecticide. Contact insecticides currently registered for aphid control include: acephate (Orthene),  bifenthrin (Talstar), chlorpyrifos (Dursban),  diazinon,  malathion, nicotine sulfate, pyrethrum, rotenone, resmethrin, and tetramethrin + sumithrin.
These will need to be sprayed on the foliage. I would try the orthene first it seems to give a better treatment than the others.

There is an insecticide that can be applied to the soil around the tree and the roots will transport it o the leaves and twigs. It is used for larger trees that are difficult to spray. It will give a long lasting effect (usually for ht season) This is called Bayer Advanced Tree and Shrub Insect Control. Here is a web link to more information on this product. Since it is late in the year I would try the Orthene spray now and next spring apply the Bayer product. Check with you local nursery for these products.
http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html