Grass and Mulch Molds - What's that Funk and Will it Do Something Funky to Me?

Published: April 1, 2011

The time for spring mowing and mulching is almost upon us, and with that our labor and attention starts leaning heavily towards the outdoors and the myriad of organisms that inhabit our backyards. The arrival of flowers, honeybees, and butterflies may be comforting and welcome sights, but we may also find unfamiliar organisms that can startle or frighten.

Fungi are the most commonly encountered organisms that call our grass and mulch beds home. They are everywhere in your landscape (and your home), mostly living as saprophobes (eating dead material and organic matter). By doing this, the fungi serve a critical role in our ecosystem and if it weren't for them we would be walking on logs and tree limbs. Other fungi can be plant pathogens, causing diseases in our gardens, trees, ornamentals, and (my discipline) turfgrass. Some fungi can also cause human disease, but they are rare and normally afflict immuno-compromised individuals or those with severe allergies. For this reason, if you smell or see fungal molds in your garden mulches there is not much need for alarm. They are just enjoying the buffet you put out for them!

The most familiar fungi produce mushrooms, of which some are edible and commonly adorn our pizzas, pastas, and other dishes. Mushrooms found in your backyards, however, can be poisonous and should never be ingested unless identified by an expert. One of the more common mushrooms found in landscapes during the mid to late summer is Chlorophyllum molybdites, which has green spores and usually sends the ingester to the hospital with considerable gastrointestinal upset. If that phrase doesn't stop you from eating unidentified mushrooms, nothing will. If you have mushrooms in your lawn, and particularly have children or pets that could venture a nibble, simply remove them with disposable gloves or immediately wash your hands afterwards.

One of the more striking inhabitants of our grass and mulch beds may be a member of what are termed "slime molds". The slime molds are not related in the least bit to fungi, which we characterize as having mycelium with cell walls made of chitin (like in a lobster shell). The slime molds have no cell walls, which makes them more like us than plants or fungi. However, a slime mold is a multinucleate mass, called a plasmodium, that is not divided into cells. It moves like an amoeba, with internal protoplasm streaming internally from one direction to another. Slime molds are actually taxonomically placed most closely to protists, a group containing amoebas.

A select few slime molds can be plant pathogens, the most famous (to us plant pathologists) being Plasmodiophora brassicae, which causes a disease called "clubroot of cabbage." For the most part though, they simply are predators of other microbes that they absorb through their cell membrane. Interestingly, they also reproduce by spores and some slime molds can produce fantastic spore-bearing structures that can mimic fungal structures. Slime molds that frequently sporulate on turf do no harm to the grass, and no control measure is necessary (Figure A).

The most common slime mold that is brought to our attention by alarmed homeowners is Fuligo septica, the "dog vomit" slime mold (Figures B & C). Some call this a fungus, which is incorrect. It is a common inhabitant of bark mulch where bacteria and moisture are in ample supply, and normally first appears after frequent rains in spring or early summer. In its early growing stage, F. septica can be a brilliant yellow color. In Mexico, it is gathered at night when the plasmodium is active and eaten like scrambled eggs! However, this mass shortly develops into a hardened structure called an aethalium, which holds large amounts of dusty spores. At this stage, it appears as if the neighbor dog lost his lunch near your petunias.

There is absolutely nothing that could or should be done to chemically control slime molds (or mushrooms) in the landscape. If the sight of "the blob" offends you on turf, simply water the organism off the turf leaves. Removing or eradicating slime molds from your mulch is an arduous task, so just cover it up with surrounding mulch. Or learn to appreciate them, because there is certainly no need to be as afraid of them as Steve McQueen was.