Last year I battled applescab in our crabapple tree all summer.

My own conclusion on this problem is that, for the homeowner, it is not
worthwhile to continue trying to spray a susceptible tree. You can't win.
Personally, I would cut down the tree and plant a disease resistant cultivar.
Read Father Fiala's book on crabapples to find out which varieties to ask for.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)