Most fungal diseases in trees require an early spring spraying plus additional sprayings during the year, but, having said this, you must know exactly what you are spraying for.
We are like the Wal Mart of fungus when it rains a lot. I am spraying systemics (agri-fos) so I see that early spraying as allowing time for uptake.
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.
Absolutely garlic works. It is a contact killer though, like Thyme oil. Both have numerous petri dish studies. Thyme oil even fights back fusarium. I am sure the layer upon the soil is useful in stopping the arrival of windborne spores. In this cause I am fighting Phytophthora ramorum an oomycete, water molds that get nasty by adding in on top of all the other advantageous fungi out there when it is wet. It was by chance that I learned potassium salts of phosphonic acid stopped root fungus attacking other plants like my Sugi trees and Winter Daphne shrubs. I lost a Japanese maple but saved others, even one small root of a winter daphne I thought dead but is now regrowing.
At least in my part of the state we don't have Phytophthora ramorum or Sudden Oak Death on anything.....yet. But we do have Oak Wilt, Bretziella fagacerum which is still fatal but just takes a little longer and is transmitted by an insect on an open wound on the tree.
I don't think the higher limbs are as important as the trunk and lower limbs on fruit trees but still important. I use a hose end sprayer to spray my fruit trees now. It uses more spray product by using a hose end sprayer but old age has a way of evening everything out. I too am beginning to struggle with tall fruit trees and I am thinking about just topping the entire tree thus reducing their size to something I don't need to climb a ladder for. This will also reduce production but the older I get the less I eat.
Looking like its getting close to that time again. My fruit trees are already starting to show bud swell with this recent spell of very warm weather.
