Hello. We have three adjacent pyracanthas at rear of house (northeast US),
that have been very large and hardy for at least 7 years. Grew like weeds.
Some branches 15 feet high. Now a few major limbs are dying. All three
plants. No apparent scale, no evidence of borers on bark or in soil
beneath. The only possible factor I can think of is that we did some
aggressive pruning last spring without "sealing" the cuts. Garden supply
guy says that should not have caused it - not necessary to seal pruning
cuts.

Garden supply guy says to prune dead limbs, sterilizing shears in alcohol
between cuts (is that neessary?), then use a growth supplement for soil that
supposedly strengthens the plants defenses against fungus, etc. (The
product is called Messenger. Active ingredient listed as "Harpin Protein".)

I'd appreciate any other insights.

Thanks, Ron