Worm leachate is the fluid draining from a vermicompost system. Depending on what the worms have been fed, it could have very high levels of potassium or nitrogen. I would expect some enzyme metabolites, perhaps some soluble forms of sulfur, and likely high levels of yeast or other decomposer fungi. I have not experienced fungal damage to plants from these fungi.

I would strongly caution you against using full-strength vermicomposting leachate as a foliar spray. I am not aware that it can confer disease resistance to plants when used as a foliar application. Rather, the high levels of salts may contribute to tissue burning. In this context, I do not mean sodium when I say salts, but rather ionic forms of nitrogen and potassium that would dissociate in water. Plants vary in their capacity to take up N and K through their leaves.

I would limit the use of vermicomposting leachate to a soil drench, and when used in that way, I always dilute the material - one part of leachate to 9 parts of water. This would be as a side-dressed source of plant nutrients. It's well-established that increased salts can damage plant roots, making the dilution critical

The leachate has a very strong and disagreeable odor, even when diluted. For that reason, I strongly recommend that it only be applied to plants which are outdoors. Never use it on houseplants unless they have retired to the porch for the summer. You don't have to make that experiment, as I have already made it for you. :)

I suspect that the disease resistant actions of compost tea are the result of beneficial micro-organisms. These could occupy sites on the leaves that might otherwise be colonized by disease organisms. In natural systems, organisms of any size, and especially micro-organisms, complete for habitat, in this case sites on the leaves.

Thank you for this very interesting question. Please write again if you have additional questions.