hi Ross, you mean the dreaded black knot, yes? it's been decimating the plums & cherries around here as well.
from what i've read, the only way to mitigate the spread of it is to bury the infected
wood is it travels by air. don't burn it!
i didn't know that it spread to apples, but what i've been using on the apples for the cedar
apple rust is a trick learned from Michael Phelps,
nettle tea foliar spray.
it seems to be working so far to improve the trees' vigor, but this year will be the tell.
i'm going to try it on the cherry sucker from the tree i had to cut down this month due to black knot.
got
nettles? if so, stick some in a 5 gal
bucket and drop some sugar/molasses in it with
water, seal the lid and let her ferment for a bit. i've been using a paint strainer to strain the brew. i also spray on the ground around the trunk.
as always, try a small section first to make sure the brew's not harmful instead of helpful.
hope this helps.