Crepe myrtles (and yews) are very susceptible to aphid & whitefly infestations. The black substance is called "sooty mold" which is a fungus that grows on the excretions (or honeydew) of the aphids or whiteflies. The mold will especially thrive on crepes that get less sun.
A dilligent regimen of insecticidal soap and strong jets of water to dislodge the aphids and clean the leaves (the mold can be washed off) may be the only way to go if you can't provide more sun to that mytle.
[ September 15, 2002: Message edited by: gardenzgal ]
