Hello,

It sounds like your plum tree might be dealing with a couple of issues. Let's break it down:

Lichens or Fungal Disease: Lichens are generally harmless and form a symbiotic relationship with the tree, but excessive growth can block sunlight and air circulation. If it's a fungal disease, it could be something like black knot or brown rot, which is common in plum trees. It might even be Phytophthora Crown Rot.

Black Knot at this link: Black knot - Integrated Pest Management

Black Rot at this link: Black rot - Integrated Pest Management

Phytophthora Crown Rot at this link: Phytophthora root, crown, and collar rot - Integrated Pest Management

Clear Substance: The clear substance oozing from the base of the trunk could be gummosis, which is often a sign of stress or infection in the tree. This can be caused by bacterial or fungal infections or even physical damage to the tree3.

Treatment Steps:

Prune and Clean: Prune any infected branches, making sure to cut at least 3-4 inches below the visible infection. Clean your pruning tools with a 10% bleach solution between cuts to prevent disease spreading.

Fungicide: Apply a copper-based fungicide to help control fungal infections. Be sure to follow the instructions carefully.

Improve Tree Health: Ensure your tree is well-watered (but not overwatered) and provide proper nutrients to help it recover. 

Monitor and Maintain: Keep an eye on the tree for any new signs of infection and maintain good air circulation by thinning the canopy if needed.

It's always a good idea to consult the MSU Plant and Pest Diagnostic Lab for a precise diagnosis and treatment plan. They can provide tailored advice based on your specific situation.

For a confirmed and precise identification of the problem, I highly recommend you send all your images by e-mail to the MSU Plant & Pest Diagnostics Lab.

Their email is <personal data hidden> - Please include your first and last name.

The experts there can also provide you with the best treatment options.

The lab will let you know if they need a sample sent.

The lab may also ask for additional images. So be prepared.

Their website is at this link:

Plant & Pest Diagnostics (msu.edu)

Their Contact Information is at this link:

Contact Us - Plant & Pest Diagnostics (msu.edu)

This link will explain the details of submitting a sample to MSU Plant & Pest Diagnostics Lab:

Submit Samples - Plant & Pest Diagnostics (msu.edu)

I hope this helps!