Question
what is the gluelike sticky film on the leaf of my pistachio tree, and what can I do abaut it?
thank you
 Martina

Answer
This is usually an indication that there are insects such as aphids or scale on the leaves or stem.  The sticky substance is called honeydew and is the result of the insects feeding.  There may be a black mold present on the leaves as well.  It is just growing on the sugary honeydew and causes the plant no harm.  The insects need to be controlled.  You can use Safer's Insecticidal Soap which can be bought at garden centers.  Dishwashing liquid can be used but there is a possiblity of washing off the wax on the leaves, which may cause wilting.  Look on the undersides of leaves for small round bodied bugs--- these are aphids.  Scale will look like little oval shields closely appressed to the stem.  They do not move when feeding and can be scraped off with your fingernail.  There are more powerful insecticides that can also be used, but the safers soap is safe for you.  Let me know if you need further assistance.  Thanks.