Why are my plant leaves sticky and covered in black mould?
Sticky leaves with a black film are a two-part symptom with one cause. A sap-sucking insect is excreting honeydew, and a harmless fungus is growing on the sugar. Treating the black film achieves nothing; the insect above it is the whole problem.
The one observation that decides it
Look directly ABOVE the sticky surface, because honeydew falls downward — the insect is on the leaf or branch overhead, often on a different plant entirely. Then identify it: dense colonies on soft new growth are aphids; clouds of tiny white insects that fly up when disturbed are whitefly; immobile brown or white bumps that do not rub off are scale; white cottony masses tucked into leaf axils are mealybugs. On pears specifically, curled blackened leaves with honeydew are pear psylla.
Start from the plant in front of you
6 things it could be
Ordered by how much they can cost you — if the first entry fits what you can see, rule it out before considering the milder ones.
Clusters of tiny green, black or grey insects on new growth and leaf undersides; curled, distorted leaves; sticky honeydew and black sooty mould; ants tending the colonies.
Conditions: 16–28°CFirst move: Treat, then re-check in 5–7 daysSeen on: Peppers, Lettuce, Cabbage, Beans, Spinach, Kale and 12 more
White cottony masses in leaf axils, on stems and roots; yellowing and stunted growth; sticky honeydew and black sooty mould; ants attending the colonies.
Conditions: 20–32°CFirst move: Treat, then re-check in 5–7 daysSticky honeydew on leaves and fruit followed by black sooty mould. Leaves curl and blacken, shoot growth is stunted, and fruit is russeted and small. Small orange or brown nymphs sit in the leaf axils.
Conditions: 10–30°CFirst move: Treat, then re-check in 5–7 daysSeen on: Pear trees
Small brown, tan or white bumps along stems and leaf veins that do not rub off easily; yellowing leaves, dieback, sticky honeydew and sooty mould.
Conditions: 18–34°CFirst move: Treat, then re-check in 5–7 daysClouds of tiny white moth-like insects under leaves; yellowing, wilting and weakened plants; sticky honeydew and black sooty mould on lower leaves.
Conditions: 21–33°CFirst move: Treat, then re-check in 5–7 daysA black, soot-like film coating upper leaf surfaces, stems and anything parked beneath the plant. It rubs off with a wet cloth, leaving healthy green tissue underneath. Heavy coverage shades the leaf enough to slow growth.
Conditions: 15–30°C · humidity ≥75%First move: Watch it before you act