Slime Molds & Fairy Rings in Lawns

Lawn diseases usually involve fungi infecting grass plants, causing various symptoms, and often resulting in dead grass. However, two common visible lawn abnormalities caused by soil organisms that are not actually infecting lawn grasses are slime molds and fairy ring.

Slime molds, caused by a group of organisms called myxomycetes (closely related to amoebas), have been noticeable in many Wisconsin lawns recently. Exceedingly small, crusty, ball-like fruiting bodies will appear in mass on grass blades, usually whitish, grayish, or blackish in color. These masses of powdery spores will easily be wiped off the grass blades. The rest of their life cycle, slime molds will be unnoticeable in the soil or thatch layer of lawns.

Warm, moist weather, such as after recent heavy rains throughout much of the state, will favor slime mold development into the visible stage on grass plants. They usually disappear as fast as they appear, and no control is necessary. While heavy amounts may lead to some yellowing of the grass, slime molds are not considered a threat to lawn health. Sweeping or hosing them off with a stream of water will make them less visible.

Fairy rings are fungi breaking down organic matter in the soil, not actually infecting lawn grasses, visibly appearing in a variety of ways. The most common fairy rings are large circles or arcs of darker green, taller grass, due to nitrogen being released in the underlying soil. Mushrooms appearing in those arcs are the fruiting bodies of the fungus. In some cases, arcs of brown grass appear, especially on golf course turf, when the fungi mycelium get extremely dense, inhibiting water movement into the soil.

Why do fairy rings appear in a particular area? The “food” source is often old tree stumps, roots, and other substantial amounts of organic material in the soil. Once this material is depleted, the fairy ring will disappear, but it may take considerable time. Several fairy rings may appear close together on lawns growing on sites that were previously wooded areas.

Fairy ring appearing as green arcs in established lawns can be masked by increased watering and nitrogen fertilization, keeping within suggested lawn care guidelines. Lawns watered and fertilized on a regular basis tend to have fewer problems with fairy ring. Core aerating to reduce thatch may also reduce fairy ring fungi.

Prior to establishing new lawns, be sure to remove tree stumps, large roots, and any construction lumber buried in the area. When renovating lawns that previously had fairy ring, blending all soil in the area together is helpful, as different fairy ring fungi are antagonistic to each other.