The lower leaves on my key lime tree have this black film on them, it will smear and come off if I rub it. I have a vague memory of this happening before but I'm not sure, is it a residue from aphids? Or something else?
The tree is still young and won't bear fruit this year so would a systemic get rid of it? I don't mind using it now since it will be at least a year before it fruits.
