I have a mature peace lily plant that is developing a sticky sap on the leaves.. What could be the cause and cure?
The sap is dripping onto the surrounding floor.
The dripping is likely to be "honeydew" excretions from a sucking insect pest, probably aphids.
Inspect the leaves and especially the under sides. You can wipe the pests off with a rag or wet sponge to minimize the infestation and nuisance.
You can use neem oil to treat the plant.