Our lime tree has black and white thick stuff on the underside of the leaves. The foliage is curling and turning yellow. The limes are almost ready to be picked. I have had the tree for four or five years and have had no pest trouble until now. Both insects secrete honeydew, a sticky substance on which sooty mold, a black fungus, grows.
Mealybugs are sap-feeding insects that form white, powdery colonies on leaf undersides and stems.
They can distort and yellow foliage, and stunt growth. Scale, a mealybug relative, can look more like a disease than an insect. These usually resemble fuzzy, waxy or hard bumps. There are several scale species in gray, yellow, white or reddish brown. All weaken or kill a plant by sucking sap.