Slime mold on grass are usually not harmful as they do not kill or damage the plants. The foamy white, black, grey or yellow stuff on the grass blades or mulch beds is a primitive organism that usually feed on decaying matter and other fungal spores in the turf.
Slime mold can appear in turf suddenly or overnight following a prolonged grass wetness. Areas with poor drainage and heavy thatch buildup may as well enhance their development. They are usually prevalent during warm wet weather and eventually disintegrate and disappear on their own when the moisture recedes.
Unlike pink snow mold, slime mold do not attach grass but they use the blades for structural support necessary for flowering and producing spores. Since they obtain their food by engulfing bacteria, fungal spores and other tiny pieces of organic material found in the turf, they are an important part of the ecosystem.
Slime molds appear as thin, white, yellow, or gray layers of slimy paste like substance covering leaf blades. A thick layer of slime mold may affect the rate of photosynthesis, and you may notice yellowing of the plant leaves.
When these masses dry out, they form a black or white powdery covering on the leaves making your grass to appear as if dusted with soot. Slime mold such as the dog vomit may affect the aesthetic appeal of a lawn and you may feel the urge to get rid of it.
How to Get Rid of Slime Mold on Grass
While slime does not cause any serious harm to your garden, most homeowners are not pleased looking at them especially the dog vomit type. Although letting them disappear on their own would be the best approach, a large colony may require you to take an action. Mold can cause a health concern when inhaled.
If your lawn or mulch bed is experiencing low to moderate infestation where grass blades are not fully covered, there is no need of taking any control measure. Heavy infestations can be controlled through mechanical means such as lawn mowing, raking. These methods helps in enhancing air flow into the turf thus driving away moisture.
A heavy downpour or washing the slime with a spray from a garden hose will obliterates mold although it is likely to help in spreading spores and providing wetness which is essential for their survival. That means you will see more of the slime mold in the next couple of hours if you use water to get rid of them.
Fix drainage issues and remove any standing water in the yard. You may also reconsider your watering patterns to induce partial drought in your garden or lawn. The slime mold will lack moisture to thrive on and after a few days mold will begin to dry and die. Resume watering after all the mold is gone.
Prune leaves and branches in the affected area to allow more sunlight and air circulation. Mold and fungi such as lawn mushrooms thrives in dark areas. Clearing the area to allow more sunlight will automatically drive them away.
Preventing thatch buildup or dethatching will also help in limiting the amount of decaying materials in the tuff. This will help in keeping mold and other fungi at bay. Dethatching also comes with more benefits of allowing air and water to freely reach plant roots.
Employing a chemical control method such as fungicides can do more harm than good thus not recommended. Fungicides and pesticides usually kill other important microorganism in the soil. The chemicals can also mix with water thus threatening aquatic life, pets and any other beings.
While slime mold is regarded harmless to your grass or plants, the prevailing conditions may favor other harmful fungal infestations that may need a further control response.
Ohio State University Extension: Slime Molds on Turfgrass
UNIVERSITY OF MARYLAND: Slime Mold – Lawns