Sounds bad, got a lot of people stumped.
What about weather, has it been peculiar for say the last 2 to 3 years?
Nothing works on it, all fungicides fail, maybe if you guys can have a few in different locations try some different techniques.
Over here our usual tough as nails mango trees have been looking pretty ordinary, seems the tips affected by fungal problems. I tried phosphite and I tried macozeb, the fungus is anthracnose. Now the instructions said phosphite wont work on anthracnose but that mango tree did better than the one I used macozeb on.
But there exists another problem hard to control, possums eating off the juvenile foliage keeping the tree defoliated, then pests move in. So I treated with confidor, got rid of the bugs but the possums were still a problem.
I found this interesting article, in this article you can see it wiped the indigenous tree population out and a new resistant species is being introduced.
ScienceDaily: Chestnut Trees To Spread Across Landscape Again, Says Purdue Scientist