I don't get near the rain you do but I do get fungal diseases on my fruit trees periodically, not very often, but, I do still get them. As you know I am a totally organic gardener and I do all sorts of things to stop diseases from ever getting a chance to start and the most successful thing that I do to stop fungal problems before they begin is to spray garlic. I spray it numerous times per season on everything, from my asparagus to zucchini and everything in between. Truthfully, I don't positively know for sure that it works but fungal problems are very rare. It is super cheap and easy. Garlic is a limited systemic pesticide and fungicide but as far as fungal diseases in tree roots I am clueless. But, like I said, I have very few to zero insect problems or fungal diseases on anything.
The only fungal disease I get on a regular basis is Early Blight and I take care of that in a totally different way but still spray the plants with garlic anyway.