Why Is There a Yellow Fungus on Grass?

Grass can fall prey to many different diseases, especially in the wet, warm spring. These diseases can make otherwise well-cared-for lawns unsightly. Since there are so many types of disease, lawn conditions are often identified based on the shape and size of the problem area. Color is also a useful indication.

Fungi are the most common cause of lawn diseases. These are small organisms spread by spores that float in the air or move through runoff water. These fungi typically require at least some sunlight, a reliable source of moisture and nutrients, which they receive from the grass tissues themselves. The fungus then spreads new spores and creates new colonies throughout the lawn.

Melting out is a condition in which fungi creep forward in a splash pattern, killing grass as it goes. This usually means that the grass is dying and thinning out. Thinned grass can expose lower, whiter root section, and dying grass dries out and fades away--both factors can give grass a yellowish appearance. This is most commonly caused by the Drechslera poae fungus.

Frog eye patterns are caused when a fungus spreads in a circular or crescent pattern across the grass, often leaving a spot of healthy grass in the middle. If this pattern is yellow in color, chances are high it is the Rhizoctonia yellow patch, which grows in cool and moist climates and is yellow throughout without any black strands. Aeration will prevent many problems with this fungus.

Dollar spots and patches are small, roundish areas of the lawn that have become infected. These yellowish spots are typically caused by Lanzia or Moellerodiscus fungi, and can be controlled by a fungicide when they start to develop. Well-fertilized lawns can often resist these types of fungus.

Healthy and well-managed lawns will be able to fight off a fungus infection. Lawns should be planted with grass varieties that are best suited for the area, especially if the climate is a wet one. Lawns that are fertilized at least once year will grow stronger and be able to recover from a fungus condition quickly. Different grasses have different mowing requirements, but they should all be kept mown down to the proper level. The soil the lawn grows in should also be aerated and drained properly so that the soil can breathe and the grass can dry easily. Fungicides are also used to kill fungi problems already present.