Slime Mold on Bark Mulch

Summer is a time of beauty with flower gardens exhibiting all shades of color and texture. Even the various bark mulches used to shade the soil and retain moisture add greatly to this beauty. Redwood bark, reddish brown when first scattered over the ground, turns a brownish gray. Aspen bark, while gray when applied, turns black as it ages.

Occasionally during the summer gardeners notice a growth on their bark mulch which closely resembles - should I say it - Dog Vomit. Yes, our office even has had gardeners call after taking their dogs to the vet because their dogs `must have eaten something bad' causing them to get sick. Other gardeners blame this problem on a neighbor because `they don't keep their dogs in at night'. I've always been curious to know how many veterinarians prescribe medication to correct this condition and how many neighborhood problems have escalated due to `sick' dogs. Gardeners calling our office in an attempt to discover what type of plants could be causing their dog's stomach problem are often surprised when told this colorful, vomit-looking mass is a unique and fascinating organism.

History:

Known as slime mold, they were once considered to be animals due to their creeping phase. DeBary, one of the founders of mycology, called them Mycetozoa, from the Greek words myketes (fungi), and zoon (animals). With DeBary's first use of Mycetozoa in 1887, the name continued to be used until the 1970's. Some previous researchers classified the slime molds in the phylum Protozoa of the animal kingdom.

Mycologists (those who study fungi) previously considered these strange organisms to belong to a class called Myxomycetes; myxa (slime) and myketes (fungi). Myxomycetes was the name first used by Macbride in his 1899 monograph of the slime molds. The use of the word was based on work done in 1833 by Link who considered these organisms as fungi. In Whitakers 'Five Kingdom' system Slime Molds are not in the Kingdom Fungi but are placed in the Protoctistans. See http://www.uoguelph.ca/~gbarron/Myxos/myxappen.htm for more information.

Habitat:

The slime molds live in cool, shady, moist places on decaying wood, leaves or other organic matter retaining abundant moisture. Bark mulch in a flower garden or shrub bed certainly fits that description. The same type of organism is often seen in the woods on decaying logs. Over 700 species are reported as existing.

Slime molds feed on decaying organic matter, bacteria, protozoa, and other minute organism which it engulfs and digests. In rare instances the slime molds have been known to creep over ornamentals, causing suffocation.

Life Cycle - A Simplified Version:

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.

Slime Molds of Turf:

The colonies of slime mold living on logs and bark mulch can be strikingly colorful in yellow, orange or red. Some slime molds produce cream-colored masses of cells along grass blades. While most often found in moist climates, several of these grass-inhabiting slime molds are found in Western Colorado. Slime molds often appear in the same area of the lawn year after year in four to six inch patches in various shades of purple, gray, white or cream.

Control:

Preventive chemical treatments tried over the years have been found ineffective. Slime molds are more a curiosity or nuisance than a threat to gardens or lawns. Once a colony starts to form, allowing mulch to dry out, or using a garden or leaf rake in the affected area helps break up the colony and provides some control. Washing the grass down with a forceful stream of water breaks up colonies. Mowing also is an effective way to remove slime mold from turf. Like nature's other organisms, slime molds should be looked at for their beauty and enjoyed as one enjoys a mass planting of dianthus or snapdragons.

Agrios, G.N. 1988. Plant Pathology. Academic Press, Inc.
Alexopoulos, C.J. & Mims, C.W. 1979. Introductory Mycology: Third Edition. John Wiley & Sons.
Stephenson, S.L. & Stempen, H. 1994. Myxomycetes: A Handbook of Slime Molds. Timber Press, Portland, Oregon