Garden Pests and Diseases: Cuban Oregano dying, black spots?, 1 by SincereDeer

SincereDeer wrote:
My Cuban Oregano (Plectranthus amboinicus) has been in distress for about a year now. After I accidentally burnt it in the sun outside while trying to acclimate it for last summer, it just hasn't been as vigorous. Leaves have been dropping and none have grown to replace them. The only new leaves are at the tips. Under all of these leaves are tiny black specks that don't move, but seem suspect. Some portions of the stem are also darkened, but are not mushy or soft. I haven't inspected the roots yet, although I may soon if I decide to start from (cleaned) cuttings again.

Any ideas what might be up? It has been living on a bright window sill with afternoon sun. I'd very much like to prevent whatever this is from happening again!