Back when I had 'unimproved', Aphid-prone Crepe Myrtles, their leaves developed that black crust I think you're referring to. The worst part was that the Aphid sap wreaked havoc on the Hybrid Musk roses below the myrtles. So I sprayed with 'Safer Soap'. I never noticed any harm, when I sprayed in hot weather, and it was quite effective, until the trees got too tall for me to easily reach.

And a 'Safer Soap' will be far less harmful to YOU, than 'conventional' pesticides.

If you planted really small trees, and don't have much money tied up in them, you may want research the specific cultivar you planted to evaluate whether you want to keep them, or replace them with one of the newer, less problematic varieties.

The problem with my old Myrtles was that they were just something a little nursery had propagated themselves...no name, and a definitely inferior variety...sickly and pitiful, regardless of how much I doted over them, until we sold that house. As with Roses, the specific cultivar is of crucial importance.

When we were back in Madison last month, we ogled some 'Pink Velour' Myrtles at Madison Home & Garden, that were to die for. (OK, the name summons some really downmarket imagery, definitely not worthy of the plant). The pink blooms are surrounded by new leaves, which are maroon. I like the general effect of that plant, for a small Myrtle. Catawba is a brilliant purple, and a small tree, too. Tuscarora is a medium-sized tree, mildly fragrant, and that happy, Watermelon pink we associate with Crepe Myrtles. For the big, vigorous, fool-proof plants, Natchez is the best white, and Muskogee is great, too, in a color somewhere between pale mauve and orchid pink.

Aphids are very fragile creatures. If nothing else, set the hose nozzle to a concentrated stream, and blast them.

Thanks for the info. I did not know they had the new crepe myrtle. I will try the safe soap. Had two beautiful "dynamite" crepe myrtles, but lost them the first year.