Identifying and Preventing Sooty Mold

Sooty mold is a collective term that refers to a conglomerate of ascomycete fungi. These fungi can alternate genera, but the more commonly occurring genera are Cladosporium and Alternaria.

At first glance, sooty mold may be mistaken for black mold, causing some people to confuse the two. However, they are distinct entities. Despite its name, black mold (Stachybotrys chartarum) typically presents as dark green and fuzzy and is caused by elevated moisture levels. Sooty mold, a plant mold, may have a similar appearance but is triggered by 'honeydew' rather than moisture.

Sooty mold on California laurel (Joseph O'Brien, USDA Forest Service, Bugwood.org)

Black mold is notoriously toxic to humans and can cause a variety of medical issues. On the other hand, sooty mold isn’t toxic to plants or humans. It doesn’t infect plants, it merely covers them; even upon human contact, there is no effect, unless of course there is an allergy. This means you don’t have to throw out affected fruits and vegetables! They are perfectly fine and safe to eat when washed with mild soapy water.

Honeydew-producing insects, like aphids, scales, and whiteflies are the culprits when it comes to sooty mold. The honeydew acts as the growth medium for the fungi to grow and will cover anywhere the honeydew has had surface contact; whether that be plant leaves and stems or furniture, fencing, rocks, and even vehicles.

Sooty mold, although considered a disease, merely covers the foliage. As mentioned, there is no toxicity but causes indirect damage. When covering leaves, it prohibits photosynthesis which causes stunted plant growth and foliage that prematurely ages and dies. The mold is an excellent indicator that honeydew-producing insects are present, especially in new plant growth.

Comparison of sooty mold with healthy leaves (Sarah Vanek, Bugwood.org)

To eliminate sooty mold, you must first eliminate the pests that produce honeydew. The fungi itself can be easily rinsed off with a strong stream of water or mild soapy water, but if the insects are still present they will continue to produce the honeydew, thus more mold. Sometimes, resorting to insecticides or the natural predators of pests is necessary, although a forceful water stream can also be effective.

You may also notice that there is a population of ants on or near the mold-infested plants. Ants feed on the honeydew and will often try to protect the insects that excrete it to save their food source. When utilizing an insecticide, there is less concern about this issue. However, if opting for organic methods with natural predators, it is necessary to strategically position baits or sticky substances at the plant's base to discourage ants and eliminate the colony before the predators can efficiently manage the pest population. To further prevent ants from accessing trees and shrubs, the sticky substance can be put on the trunks and the limbs that have contact with roofs or other access points can be trimmed.

Sooty mold can be a nuisance in gardens and landscapes, affecting the health and appearance of plants. It is important to address the underlying issues to manage and prevent the growth of sooty mold. By employing effective prevention methods and timely interventions, you can uphold the health of plants and reduce the effects of sooty plant mold.

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