In some cases, fresh sooty mold may be washed off of
plants, but unless the causal insects are controlled, it
may reappear. To prevent sooty mold, you need to
manage the insects. The insects involved are small and
may be present in large numbers before the black
strands of sooty mold appear. Trees and shrubs
should be observed frequently during the growing
season for honeydew and insects. Remember -- look
for insects not only on the affected plants--but on
over-story plants as well for evidence of an infestation
when sooty mold appears.