Garden Pests and Diseases: Black gunk covering leaves of my giant bird of paradise, 1 by valtal

valtal wrote:
I recently noticed that the giant bird of paradise plants in my front yard along a fence and two lanes from the freeway has black gunk covering the outer most leaves. It does not rinse off. I tried vinegar wash, fruit and vegetable wash, and a harsher kitchen spray. None of these caused the black gunk to wash off. I tried rubbing some of the leaves with the vinegar and water and some of it comes off, but it is very caked on and would take more rubbing than I intend to do. Any similar situations out there?