ISSUE 2   May 22, 2008


From May 14 to May 21, 2008, the NDSU Plant Diagnostic Lab has received 31 samples. The table below summarizes the types of samples processed during this period:

Sample Category

Number of Samples

Seed Health and Phytosanitary


Woody ornamental Evergreen












Have you ever seen green rings of grass in your lawn that seem to appear out of nowhere? Have you noticed a ring of mushrooms that might show up as if overnight? If so, a type of fungus that causes a whimsically named fairy ring might be living on the organic matter in your lawn. If you submit a turf sample to the NDSU Plant Diagnostic Lab, and fairy ring is the suspected problem, a version of the following standard report is what you might receive back in the mail:

"The turfgrass sample has been examined. The reported symptoms were turf with yellow rings, about 1" thick, in patches ranging from 1 to 2 feet in diameter. The peat-backed sod is reportedly three years old.

No foliar symptoms (other than the yellowing) were detected, so foliar diseases do not appear to be seriously impacting the sample. Although the roots were thickly formed within the peat layer, they also penetrated to the underlying clay soil surprisingly well, which is desirable and beneficial for the health of the lawn. Roots appeared healthy, with no discoloration; an occasional lesion not consistent with known root diseases was detected, but these were rare and did not seem to be associated with a pathogenic fungus. Disease does not appear to be the primary problem on the sample.

The sample had a thick thatch layer (primarily the peat backing of the sod). Thick thatch (anything over 1/2" thick) can create environments favorable for pathogenic fungi (those that can cause disease on grass) as well as saprophytic fungi (those that feed on dead organic matter within the thatch layer and don't cause disease). In the sample you submitted, I did not detect signs of fungi that cause serious root diseases. As a result, I suspect that the main problem may be due to purely saprophytic fungi in the basidiomycete group of fungi, known as fairy ring fungi..



Many of the fairy ring fungi fall in the order Agaricales. Many (but not all) basidiomycete fungi produce mushrooms, and several of the fairy ring mushrooms are poisonous. If fairy ring is occuring, mushrooms (also known as basidiocarps) will sometimes, but not always, grow along the perimeter of the ring. The fungus is growing on dead organic matter, rather than directly infecting the grass. Symptoms vary depending on the type of fairy ring, but in general, if plant death or stress is obvious, it may be due to water stress resulting from the creation of a hydrophobic soil (due to dense mycelia, or vegetative growth, of the fungus). Other causes for the inhibited grass growth may be due to lethal concentrations of ammonia (a by-product of the fungus as it breaks down matter), nitrogen depletion, toxic levels of hydrogen cyanide or ethylene potentially released by the fungus, actual root infection by the fungus (a pathogenic process), creation of hydrophobic conditions by release (from the fungus) of a hydrophobic substance, and weakened plants killed by other stresses. It's not well-understood precisely what causes the symptoms.

Management is limited. Fungicides are generally not considered effective against fairy ring fungi. A wetting agent may be helpful if water stress is noted in the affected areas (wetting agents help break the surface tension of water so it can better overcome hydrophobic conditions and penetrate to the roots of the grass). Injecting water in the fairy ring areas can minimize water stress of the grass; the goal is to saturate the soil to prevent drought stress of the grass and to promote microbial activity that is antagonistic to the fairy ring fungus. This procedure requires repeated injection (such as with tree root watering wands, or deep holes punched into the soil surface), and when used in conjunction with wetting agents, the soil is more uniformly saturated. It may be difficult to achieve and maintain satisfactory levels of soil saturation, though.

A less practical approach is to remove infested sod and soil and replace with clean soil (overseeding or re-sodding would be needed). If this option is chosen, sod and underlying soil must be removed from about 20 inches beyond the advancing ring, to a depth of 8-30 inches, depending on severity. Ideally, the excavated area should be re-filled with clean topsoil similar in texture to the soil removed. Seeding is preferable to sodding, since seeding poses less risk to reintroducing fairy ring fungi. Another option is to cultivate the affected area repeatedly, and allow it to sit without seed for a period of time (such as over the winter), and then seed the following spring (when conditions favor growth of the grass, typically in May for our area).

Reducing further formation of thatch and increasing rate of thatch decomposition are key management strategies that can help minimize fairy ring. Power raking and core aeration can help initiate this process. These are discussed in a little more detail under General Turf Management Tips, below.

Masking the symptoms: Since complete control of fairy rings is probably not realistic, masking symptoms may be more reasonable. Symptoms can often be disguised with deep irrigation to the grass root zone (to reduce water stress) and with moderate nitrogen applications at the appropriate times of year. Core aeration and use of wetting agents can also help with water penetration to the root zones.

General turf management tips:

Conditions that can favor fairy ring are a thick thatch layer or soils high in organic matter. A thick thatch layer should be minimized, ideally in spring when grass is actively growing (for our area, the recommended 'rule of thumb' is to power rake after the lawn has been mowed three times). In some years, it may be too late to power rake this time of year, but this spring has been cool, so it might be okay to still perform this procedure if temperatures remain cool and favor the growth of the grass. Power-raking will help reduce the thatch layer, but it is stressful to the grass. Aggressive power raking at the wrong time of year may lead to a flush of weeds that may out-compete the stressed grass, so care should be taken when performing this procedure.

Core-aeration is another technique that will also help reduce the thatch layer, but more importantly, this procedure will further encourage roots to grow more deeply into the soil profile. Both power-raking and core-aeration may be beneficial for your lawn, especially if you have a thick thatch layer and compacted soils. Both procedures can be done now as long as temperatures remain cool. Core aeration is less stressful to the grass than power raking. Core aeration may need to be performed more than once in a given year to achieve desired results over a period of several years (it may not be a quick fix; every situation is different).

The ideal mowing height for lawns is about 2-1/2 inches to 3 inches. A short mowing height is very stressful to lawns in our areas, and this practice leads to poor root development which may predispose your lawn to drought stress."

Fairy ring image
Fairy ring in a sodded lawn in Fargo. 
A darker green ring of grass, with or
without basidiocarps (mushrooms)
and no ring of dead grass, is
characteristic of type 2 fairy rings.

[Source: Compendium of Turfgrass Diseases, third edition, APS Press, 2005; Edited by Richard W. Smiley, Peter H. Dernoeden, Bruce B. Clarke; 167 pp.]

Kasia Kinzer
NDSU Plant Diagnostician

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