Honeydew is a sweet, clear, sticky substance secreted
by insects such as aphids, mealybugs, scale, and
whiteflies. The honeydew drops from the insects to
the leaves and twigs. Wind-blown sooty mold spores
that stick to the honeydew then have a suitable
medium for growth. When spores germinate, they
send out black fungal strands (mycelium) that covers
the plant tissue and causes the discoloration. A heavy
layer of black mold may build up on needles (Fig. 1)
and on twigs (Fig. 2) over more than one growing
season. On leaves, this coat of mold screens out light
and reduces the plant's capacity to produce food. On
some trees no obvious damage may be apparent.