University of Manitoba - Faculty of Agricultural & Food Sciences - Sooty Mold
Sooty Mold

Problem type: Disease

Name of problem: Sooty mold

Plant name(s): 
All plants

Symptoms / Characteristics:
Dark grey to black sooty growth forms on the surface of leaves and stems. Heavily infested leaves often turn completely black. The patches of sooty growth are actually colonies of fungi that thrive on the sticky, sweet honeydew excreted by sap-sucking insects such as aphids, scales, mealybugs and whiteflies. The honeydew is essentially a nutritive medium ideal for supporting the growth and development of fungi. In fact, sooty mold fungi are not exclusive to plant material. They will flourish on almost any surface on which honeydew has collected, from patio furniture to rain gutters. Sooty mold fungi do not actually attack the plants and do not require a host plant for survival. However, the fungus does pose a threat to the plant by coating the leaf surface and blocking light penetration. In turn, photosynthesis is inhibited and infected plants begin to exhibit symptoms of light deficiency.

Control / Preventions:
Controlling the sap-sucking, honeydew-secreting insects is crucial. The insects are far more detrimental to plant health than the sooty mold fungus and the development of sooty mold usually indicates that the insect infestation is quite severe. Once the insects have been eliminated, ensure that the remaining honeydew is washed from the plant.

Wherever possible, sooty mold can be removed by gently washing infected plants with warm, soapy water. However, many sooty mold fungi adhere quite strongly to the plant surface, making removal difficult. They may survive a washing and persist for a period of time before withering away.