Will you please cut open a few of the fruits with the hardened sap like substance and take some photos of what you see inside. Do you see any larva? What does the pit look like? Also, do you see any other symptoms on the leaves, flowers, twigs or branches of the tree? If you could do a little more investigating and send some photos, that would be helpful.

Hello,

I have opened some fruits with hardened sap and didn't find any larva (last photo) and the pit has the normal form and size. I found some fruits shrunken (first photo). The leaves are curling up (second photo), despite that I water once a week, 20-25 gallons of water.

Thanks, Milla

The plum moth (Cydia funebrana) can cause the hardened sap-like substance you see on the outside of the fruit, but typically you would find a small red larva inside the fruit, and this pest has not been found in Oregon. This is going to be difficult to diagnose without being able to inspect the fruit and I am down in Lane County. I suggest you reach out to the Washington County Master Gardeners (<personal data hidden>) to see if their plant clinic is operating and if they are accepting samples. They would need several samples of fruit to inspect, and some stems and leaves. If they are unable to help you, please reach out to the OSU plant clinic (https://bpp.oregonstate.edu/plant-clinic).