Ornamental Plum Trees with Scale

A couple of plum trees on a relative's property, each maybe 15 feet tall, are infested with many dark, small, beetle-like "shells." They are very dense on (the bark only of) some limbs. The leaves are not infested, and appeared normal on first inspection. I showed a sample twig to a garden supply fellow, and he id'd it as scale and said malathion would take care of it. I sprayed, following directions on the Ortho container. Three days later, much of the scale appears to be exuding a clear, thick liquid. If I try to scrape away some scale, it comes off as a mushy mess, and the outer bark itself appears to be compromised. Some limbs now are showing signs of leaf shriveling as well. Would appreciate advice on what's happening, and whether this approach will indeed eliminate the scale and restore the trees to good health.
I searched the groups. There are a few relevant posts, but opinion appears mixed as to the efficacy of malathion, and there's little explicit focus on plum trees. Some folks recommended spraying with an oil, and I did see a relevant product at the store, but the guy said that's only for winter when limbs are bare.
Thanks for any help, Ron

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