By Ann Hazelrigg
Extension Plant Pathologist
University of Vermont Plant Diagnostic Clinic

Have you ever noticed something bright yellow or orange on your bark mulch after a rain? You may notice that this blob actually has moved across the mulch over the course of a few days! 

This is a unique organism called a slime mold. Slime molds are no longer classified as fungi, but instead as a member of the Kingdom Protista (Protoctista). Slime molds are not harmful to adults or animals, and you can break them up with a rake if desired. 

There are hundreds of different slime molds with a range of colors and forms, but the bright yellow one we see after rainy weather is in a group called the 'plasmodial' slime molds. This blob or plasmodium, often mistaken for dog vomit, consists of one giant cell that can actually move across the mulch, albeit very slowly. 

As it moves, it ingests bacteria and fungi, growing up to a foot or more in diameter. If the weather stays favorable and there is plenty to ingest, the slime mold will produce a reproductive structure that is stalked and filled with spores. As it breaks apart and dries, the spores are released. 

The thick-walled spores are resistant to adverse conditions and can remain dormant for several years. Once the environmental conditions are just right, the spores germinate and produce a new slime mold. 

Another major group of slime molds is the 'cellular' slime molds. These are single-celled organisms, each with its own nucleus. The cells move through the environment at about one millimeter per hour, ingesting food as they go. When conditions are favorable, these cells aggregate and form a 'pseudoplasmodium' or fake plasmodium. 

Scientists have figured out that the individual slime cells release a complex system of chemical trails, and aggregation occurs as a result of each cell detecting these chemical signals. Research has shown slime molds can figure out mazes due to these chemical trails. 

Another group of scientists placed food sources on a map of the major cities of Japan and introduced a slime mold. The organism branched out in several pathways to find the food, and then died away on the routes that were not as direct. 

This resulted in the living slime mold following the shortest most efficient path to the food. The researchers also found these slime mold pathways mimicked the same routes the Japanese railway system used to design the most efficient route between cities.


Ann L. Hazelrigg