Sooty Mold

 How do I treat black, sticky stuff on the leaves and bark of a tree? (Sooty Mold)

Symptom(s):

A sticky mold appears on the plant. 

Cause(s):

Insects that secrete honeydew lead to the mold (sticky stuff).  The black substance is a mold that grows on "honeydew", the sugary excrement of aphids, scales, mealy bugs and whiteflies.  Since the mold does not penetrate bark or tissue, most plants are not harmed by sooty mold, unless the infestation is massive enough to restrict photosynthesis. 

Control(s):

Reduce aphid numbers by allowing beneficial insects such as lady beetles to inhabit the plant. Aphids can sometimes be washed from the plant with a strong stream of water. Although not necessary, sooty mold can be washed from the leaves by drenching with detergent (4 ounces per gallon of water), waiting three to four minutes, then washing with a strong stream of water.

Pruning out dead and dying branches helps prevent infection by secondary pathogens.  Prevention or reduction of high insect populations is the only recommended control for sooty mold. Other causes might be environmental conditions such as lack of water or sun. 

R:  http://hgic.clemson.edu/factsheets/hgic2002.htm

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