Can fermentation combat Phytophthora Blight?

Fermentation itself is not a “cure” in natural farming. Fermentation is used as a means of converting biomass into more plant available compounds.

That being said, you can very likely cure your flight through the use of natural farming. Good fungi beat it had fungi, to just have to get t right fungi.  I recommend finding a location that is thriving with similar soil and moisture content at a higher elevation.  IMO the snot out of it and apply it to your land.

I have all but completely cured Fabraea Leaf Spot, aka black leaf spot, on my pear tree.

When we moved into our house, the pear tree was half rotted and dead.  The filling year it barely had any foliage, and all foliage had Fabraea.  I was told by the previous occupants that in the 12 years they were here, they had never seen a single flower.  I removed all dead and damaged branches, then began applying Natural Farming Techniques.

Our soil is 100% clay, so I began adding organic matter to the top of the clay.  Mostly grass clippings that were chock full of deer droppings.  Every 2 weeks I brewed up the soil treatment with some imo 2 and let it bubble for 24-72 hours depending on the temperature.  I sprayed so surfaces of the tree, troy whatever was left on the ground inside the root zone.

Immediately that spring, the tree came to life with a renewed vigor.  Covered in green foliage, it looked 100 times better than I had ever seen it.  Still no flowers.  But!, there was only maybe 10% Fabraea compared to the year before.

This is currently year 2, remember trees take time.  Only about 5% Fabraea and I got flowers, about 30 of them!  None produced any fruit, but we are making progress.