Black fungus is a common problem on flower leaves and other parts of plants. More often called "sooty mold" for its sootlike appearance and texture, black fungus does not hurt the plant, according to the University of California. Still, it does take away from a plant's aesthetic value.
Black fungus can be a single fungus or a combination of several different types of fungi. Fungi in the Capnodium genus are the most common, followed closely by the Fumago genus and Scorias. Three others--Antennariella, Limacinula and Aureobasidium--are also seen but are less common.
Black fungus grows on and receives its nutrients from honeydew, a sticky substance secreted from the bodies of insect pests. The stickiness of the honeydew allows airborne fungal spores to easily stick to it, according to Cornell University. Once stuck, the spores begin to grow and multiply. The insects that secrete honeydew are those that commonly infest plants, such as scale, aphids and mites. The larger the insect infestation, the more honeydew and, consequently, the possibility of a large black fungus problem.
Sooty mold can affect almost any type of plant, from flowers to large evergreen trees. Black fungus can grow anywhere honeydew lands--even if it drips from a plant onto the sidewalk. Any part of a plant can be covered with the black, slippery fungus, according to the University of Washington, including flowers, leaves, twigs and even the fruit.
Prevent black fungus spores from landing on your flower's leaves by keeping an eye on the insect population on your plant. If you notice signs of an infestation, spray off the bugs with a strong stream of water, or in extreme cases, use an insecticidal oil or spray. Getting rid of the insects means you are getting rid of the honeydew that attracts the fungal spores.
Sometimes, plants are already covered with the sooty mold by the time you notice the insects. First, get rid of the insects, then treat the plant to a nice washing with a lot of soapy water. While the black fungus is unattractive, it is actually quite easy to remove in most cases, according to the University of California. Another option is to simply let it wear away. Without the honeydew provided by the insects, the fungus will gradually decline.