I don't know a lot about using TM but being a beekeeper i have an
opinion. If you have been keeping bees very long you have spores in your
hives and the only reason your bees don't have foulbrood is that your
bees are good housekeepers and just in case you do have a out break you
medicate them with TM 2 times a year. The spores are there all the time.
They get there because you can not control your bees getting honey from
dead wild hives or dead hives from other beekeepers. If you don't have
any spores  you would not have to  doctor for it. For beekeepers that
make a living at it the patties were God sent.  The patties, and the
mites killing all the wild bees, have greatly help beekeepers that have
a lot of hives control foulbrood. The % of bees that get the correct
amount of TM for the correct amount of time increaced greatly  because
of the patties, but never forget you never kill the spores and you never
know if your bees ever have had it or not because a good house keeper
will clean it up befor you can find it but they do not clean the spores
up they are there to stay.
 
BUD