What is this yellow fungus on my lawn?

A patron came in and described something that had appeared on their lawn overnight. At first, they’d thought a dog had been sick in their yard, but on closer inspection it seemed to be an ugly yellow fungus. The patron reported that when it rained, the mass released a smoky-looking cloud of spores. It also appeared to bleed.

We had noticed something similar growing in the mulch in front of the library a few weeks earlier. It had eventually disappeared on its own, and we’d never tried to identify it, but we knew what our patron was describing. Because we thought the yellow mass might be a fungus, we checked The Audubon Society Field Guide to North American Mushrooms, David Arora’s Mushrooms Demystified, and Peterson’s Field Guide to Mushrooms. However, we couldn’t find what we were looking for.

Searching on Google for an image of yellow fungus led us to an article by Susan Mahr on the Wisconsin Master Gardener Program website. There were several pictures that our patron recognized as the organism growing on their law, which the website identified as the “dog vomit slime mold.” Technically, it’s not a fungus at all. While scientists used to place slime molds in Kingdom Fungi, they were moved to Kingdom Protista, which serves as a catch-all for eukaryotic organisms that are not plants, fungi, or animals.  

According the Mahr, the dog vomit slime mold begins its life as a spore. These spores develop into cells, which join up to form the slimy-looking yellowish-brown plasmodium. The plasmodium creeps across the land, consuming the bacteria and other organisms that thrive in decay. When it runs out of food or gets too dry, the plasmodium transforms into an aethelium, or a “fruiting body” – the puffy yellow mass that will release spores and begin the cycle anew.

The aethelium will continue to develop, hardening and darkening in color. It may start to liquefy in spots, which can make it appear to bleed, as our patron noticed. Once the hard crust is broken, it will release its clouds of spores. This may have been the “smoking” that our patron noticed.

Despite its unappealing name and appearance, the dog vomit slime mold is almost entirely harmless. (The spores may aggravate asthma or allergies.) It will eventually dissolve on its own after a few days, though it can also be broken up. A slime mold needs moisture, so they’re usually spotted in hot, humid weather. Watering less, or raking up damp mulch, can be another way to be rid of them.