Sap on black walnut tree leaves
My two 50+year old black walnut trees leaves are covered with sap, which is covering(and killing) nearby plants. Never saw this before.
The sap makes the tree branches heavy, but it has leaves out anchoring walnuts. The other dying plants are !ilacs( have been near over 50years) forsythia, and old-growth would yellow roses.
Thank you for contacting Ask an Expert. It sounds like it's possible your have an infestation of some type of sucking or piercing insect such as aphids which is causing sap to appear. It's likely the "sap" is honeydew which is a sweet sticky liquid secreted by the aphids. If the plants below are covered with a black substance this is probably sooty mold which is a mold growing on the honeydew. This can sometimes be washed off. Sooty mold does not kill the plants but weekends them because the plant receives less sun onto their leaves. Generally aphids will not kill your trees. Here is a link about them. https://extension.colostate.edu/topic-areas/insects/aphids-on-shade-trees-and-ornamentals-5-511/