Question
a  large oak in our back yard is weeping a sap from the leaves we have lived here 13 years and not see this before is this a serious condition as we would not like to loose this tree as it is very old. We live in southern Manitoba Canada

Answer
Check and see if the sap dripping is sticky. If so this is a secretion from an insect called aphids.
They suck the plant juices from the leaf and secrete this Honeydew which is high in sugar content and usually a sooty mold will grow on the sap and make the leaves look black. The foliage will have to be sprayed to kill the aphids.. On large tree this maybe very hard to do and may take a professional with a power sprayer to get the insecticide up into the foliage. These insects will not kill the tree but the sap will get on everything under the tree.

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, lindane, malathion, nicotine sulfate, pyrethrum, rotenone, resmethrin, and tetramethrin + sumithrin.


Systemic Insecticides Several systemic insecticides are useful in aphid control. Aphids have sucking mouthparts and are thus very susceptible to pesticides located in the plant vascular system. Some of the systemic insecticides also have contact activity. Systemics injected or applied to the ground are less harmful to beneficial insects. Systemic insecticides include: acephate (Orthene),  and a newer insecticide called Bayer advanced. These are applied to the soil and the tree takes the insecticide to the leaves and kills the aphids as they suck the juices. These are fairly expensive. Check with your local nursery/garden type store for these.