Slime molds and sooty molds sound alike; so what is the difference? These two problems can be confusing to landscapers, consultants, master gardeners, and even specialists, so let’s look at the similarities and differences.
Sooty molds are black growths of mycelium of true fungi that appear on leaves or stems. Most sooty mold fungi are Ascomycetes in the Capnodiales family. They are superficial, meaning you can completely wipe them off the plant tissue with a paper towel or just your finger. They may appear black and fluffy in warm, humid weather; but they dry down to a thin, paperly layer in drought. This crusty layer can be peeled off the foliage.
Although sooty molds do not cause significant loss of plant health, they can reduce photosynthesis by covering up the foliage. More importantly, they grow on insect excrement or “honeydew.” This sugary substance is ideal for sooty mold growth. Therefore, if you see sooty mold on a plant, look for an insect problem. If you control the insect, you control the sooty mold.
Slime molds are not true fungi. They are classified as myxomycetes because they have some qualities of plants and animals. They are known as fungal-like members of the Kingdom Protozoa. Slime molds are slimy masses of naked protoplasm with many nuclei and no definite cell walls. They grow in moist dark places and creep over the surface of their substrate and engulf their food. They are not parasitic, however, and feed only on dead organic matter. This may sound like a creature from the deep lagoon, but the movement is too slow for us to watch.
Most slime molds are white, tan, or bright colors like orange and yellow. They have a slimy appearance in warm weather after a rain or irrigation. They have resting spores and flagellated zoospores that need water to survive. Like sooty molds, slime molds will not harm plants other than to block photosynthesis. Use a heavy stream of water to wash these off plants. Rake and stir mulches to help them dry out more quickly.