Slime molds do not cause disease, but their sudden appearance as white, yellow, gray, pink, purple, or brown patches on grasses may cause alarm.

Causal agents

Slime molds, which are taxonomically classified as myxomycetes, are the causal agents of slime mold occurrences on turfgrasses. The genera Mucilago, Didymium, Physarum, and Fuligo are commonly associated with slime molds affecting turf. These organisms are not fungi but rather protists.

Primary Hosts

Cool, wet, humid weather favors the growth of slime molds. 

Disease Cycle

The life cycle of slime molds involves several distinct stages. It begins with the presence of spores, which are microscopic structures that serve as the reproductive units of the organism. These spores can be dispersed through various means, such as wind, water, or human activities. When favorable conditions are met, the spores germinate and give rise to small, mobile cells known as myxamoebae. These myxamoebae can move and feed on bacteria and other microorganisms present in the environment. They continue to grow and multiply through cell division.

Under certain conditions, such as high humidity or the presence of a food source, the myxamoebae aggregate together to form a larger, multicellular structure called a plasmodium. The plasmodium is a gelatinous mass that can be slimy in texture and appears as a mucilaginous mass on the surface of grasses. The plasmodium stage is the most visible and active phase of the slime mold’s life cycle. It moves across the turfgrass, feeding on organic matter and microorganisms present on the leaves and stems. During this stage, the plasmodium may produce sporangia, which are small structures that contain spores. These sporangia are often seen as pinhead-sized fruiting bodies on the grass blades or stems.

The spores within the sporangia are eventually released and can be dispersed to new areas, either by wind, water, or other means. Once the spores settle in a suitable environment, they can germinate and begin the cycle anew.

Epidemiology

The epidemiology of slime molds is influenced by weather conditions and the presence of thatch. Cool, wet, and humid weather conditions promote the discharge of swarm cells. Warm, wet, and overcast weather conditions favor the movement of plasmodia onto grass sheaths and leaves. Thatch supports the growth of slime molds by providing a source of food and higher microbial populations. Slime molds can be spread to new locations through sporangia transported with grass seed from seed production fields.

Symptoms

Slime molds do not cause disease in turfgrass, but their sudden appearance may cause concern due to their unique symptoms. Symptoms of slime molds on grasses include the sudden emergence of white, yellow, gray, pink, purple, or brown patches on the turf surface. These patches can vary in size, ranging from circular to irregularly shaped, with diameters ranging from 2 to 60 cm.

Affected grass does not typically die or turn yellow as a result of slime molds, and the symptoms usually disappear within 1 to 2 weeks. Slime molds may not cause significant harm to the turf, but they can create a cosmetic issue, affecting the overall aesthetic appearance of the grass.

Credit: John Kaminski/Penn State University

Figure 15.5.1: (a-d) Slime mold symptoms appear as black or brightly colored patches of turfgrass.

Signs

The patches consist of pinhead-sized sporangia, which are the fruiting bodies of the slime molds. The sporangia can be found on grass blades and stems, forming mucilaginous masses. The sporangia themselves can have various colors, such as white, yellow, gray, or purplish brown. They may also appear cushion-shaped or cover a larger area, especially when growing in organically rich soil or on wood and bark mulch.

Credit: John Kaminski/Penn State University

Management

Management of slime mold is generally not necessary, as symptoms are cosmetic and do not damage the affected turfgrass. Fungicides are never recommended.

Cultural

Cultural control options for managing slime molds in turfgrass involve implementing various practices to create an environment that is less conducive to their development and minimize their visibility. While slime molds are generally harmless to the grass, their sudden appearance can be visually concerning. One of the simplest methods of managing slime molds is by physically removing the spore masses from the grass foliage. This can be done by gently washing the leaves with a stream of water or using a rake or brush to lightly agitate and remove the spore masses.

Regular mowing of the turfgrass can also help in managing slime molds as it cuts off the spore masses on the leaves, reducing their visibility. Additionally, implementing practices that reduce thatch accumulation, such as core aeration and mechanical dethatching, can help create conditions less suitable for slime mold development.

Chemical

Since the symptoms are cosmetic, there is no need for fungicides to suppress slime mold.