Sooty mold

Scale and sooty mold on a Eucalyptus tree

Sooty mold is a common name for several types of fungus that grow on the honeydew excreted by insects such as aphids, scale insects, and whiteflies. These molds appear as a black or dark gray coating on the leaves, stems, and sometimes the fruit of plants. While sooty mold does not directly infect the plant tissue, it can indirectly harm the plant by covering the surface of the leaves and reducing photosynthesis, which can lead to a weakened condition and reduced vigor.

Causes and Development

Sooty mold develops when the fungal spores, which are present in the air, land on the honeydew deposits left on the plant surfaces by sap-sucking insects. The honeydew serves as a nutrient-rich substrate that allows the mold to grow and proliferate. The presence of sooty mold is often an indicator of an underlying pest problem that needs to be addressed to control the mold growth.

Impact on Plants

The primary impact of sooty mold on plants is aesthetic, as the black coating can be unsightly and reduce the ornamental value of the affected plant. However, in severe cases, the mold can cover a significant portion of the leaf surface, interfering with photosynthesis and transpiration. This can stress the plant, leading to reduced growth, weakened condition, and in extreme cases, death if the plant is unable to produce enough energy for its needs.

Management and Control

Management of sooty mold involves addressing the underlying insect infestation that is producing the honeydew on which the mold grows. This can be achieved through various means:

• Integrated Pest Management (IPM) strategies that include monitoring for pest populations, using biological control agents, and applying insecticides when necessary.
• Washing the honeydew and sooty mold off the plant surfaces with a strong stream of water or a mild soap solution can help reduce the mold growth and improve the plant's appearance.
• Maintaining plant health through proper fertilization, watering, and pruning practices can help the plant resist pests and diseases, including sooty mold.

Prevention

Preventing sooty mold primarily involves preventing the insect infestations that lead to honeydew production. Regular inspection of plants for signs of aphids, scale insects, and whiteflies, and taking prompt action to control these pests, can help prevent sooty mold from becoming a problem. Additionally, avoiding the use of excessive nitrogen fertilizers, which can promote lush, tender growth that is more attractive to sap-sucking insects, is also beneficial.