Milky sap is a naturally occurring rubber, also known as latex, secreted mainly from cuts in the fruiting bodies. Several species secrete this fluid when damaged, but none can ‘leak’ as much milky sap as the weeping milk cap. The fluid stains easily and gives the fungus an undesirable off-flavour, so pickers usually want to avoid damaging the fruiting body.
Commercially, natural latex is derived from the rubber tree Hevea brasiliensis, but the liquid secreted by fungi has the same consistency and properties. So latex is the name of the milky sap and the natural rubber material we make from the sap.
Sources
Photographer – Oliver Karlöf 
Wikipedia – Lactarius – Milky Sap
Wikipedia – Latex
April 2023, TÄNKOM | Revised February 2024 RETHINK
