Slime Mold

Slime MoldA bright yellow slimy blob is commonly seen in the summer on mulched flower beds. It is not pretty, unless you like yellow, and it soon gets uglier. The yellow blob turns gray, becomes hard, then breaks down into a brown powder. People complain that the yellow blob looks like dog vomit and that the brown powder stains sidewalks.

Plasmodial slime molds, like Physarum shown here, are basically enormous single cells with thousands of nuclei. They are formed when individual flagellated cells swarm together and fuse. The result is one large bag of cytoplasm with many diploid nuclei. These “giant cells” have been extremely useful in studies of cytoplasmic streaming (the movement of cell contents) because it is possible to see this happening even under relatively low magnification. In addition, the large size of the slime mold “cell” makes them easier to manipulate than most cells.

A second group, the cellular slime molds, spend most of their lives as separate single-celled amoeboid protists, but upon the release of a chemical signal, the individual cells aggregate into a great swarm. Cellular slime molds are thus of great interest to cell and developmental biologists, because they provide a comparatively simple and easily manipulated system for understanding how cells interact to generate a multicellular organism. There are two groups of cellular slime molds, the Dictyostelida and the Acrasida, which may not be closely related t each other.A third group, the Labyrinthulomycota or slime nets, are also called “slime molds”, but appear to be more closely related to the Chromista, and not relatives of the other “slime mold” groups.

Life Cycle :The vegetative body of the slime mold is a plasmodium, an amoeboid mass of protoplasm which has many nuclei and no definite cell wall. Under Western Colorado conditions, the creeping phase the common bark-inhabiting slime molds dries into hardened structures producing dark masses of spore-like bodies and clouds of dust-like particles when body breaks apart. Some slime molds are known to move into drier, more exposed locations in order to accomplish this life cycle change.The spores, capable of surviving unfavorable weather, are spread by wind, water, mowers, or other equipment.

Under cool, humid conditions, the spores absorb water, crack open and release a single motile spore. Each motile (swarm) spore feeds like the plasmodium undergoing several changes before uniting with another spore to produce an amoeboid zygote. The zygote enlarges, becomes multinucleate and forms a plasmodium.Some species produce a stalk of hardened cells which other cells climb to create a fruiting structure from which spores are produced. This starts the cycle over again.For a more detailed discussion of slime molds and their life cycles, see the book by Stephenson and Stempen listed as a reference.

Damage: There are various species of slime molds, each resulting in a discolored, irregular patch ranging from several inches to several feet in diameter. Discoloration is due to extensive sporulation by the fungus. In general, small capsule-like spore masses, each about the size of a pinhead, grow perpendicularto the surface of the leaves. These fruiting bodies are typically grayish-white to blue-gray or ash colored and contain purple spores. Some slime molds appear as thin, white, yellow, or gray layers of slimy pastelike material that covers the leaf blades. These masses later dry to form bluish-gray, black, or white powdery growths on the leaves. At this stage, the grass has the appearance of having been dusted with soot. In the case of heavy spore production, some yellowing or chlorosis of the leaves may be observed due to shading of the turf causing reduced photosynthesis.

Control:The recommendation for areas experiencing light to moderate slime mold infestations is to simply let nature take its course. Heavy infestations can be removed via mechanical means such as mowing,raking, or using a forceful spray from a garden hose. Washing the leaves with a stream of water should be attempted only after the onset of dry weather to avoid further development or spread of the fungus. Chemical management is not typically recommended.

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