Figuring out fairy rings
When we see circles of mushrooms growing in a lawn, why do we refer to mushrooms growing in turfgrass as “fairy ring”? Does it make you think of mischievous fairies or gnomes? A popular belief during the Middle Ages was that a ring of mushrooms appeared in a pasture after fairies danced at that spot the night before. In Scotland, it was bad luck for a farmer to till land where rings of mushrooms appeared, but in England it was considered good fortune to build a house on land where fairy rings were observed.
Today, we know that fairy ring is caused by more than 50 species of soil-inhabiting fungi belonging to the basidiomycete or “mushroom” group of fungi. We have a better understanding of the nature of fairy ring, and we also have some options for controlling fairy ring in turfgrass. Although fairy ring fungi do not attack turfgrass plants directly, they do affect turf negatively, as well as create aesthetic concerns in high-quality turf.
Fairy ring biology
Fairy ring occurs worldwide in all cultivated turfgrasses and is frequently observed on lawns, golf courses and park and recreation areas. In turfgrass ecosystems, fairy ring fungi colonize the thatch and organic matter in soil, and disease symptoms are described in one of three ways or types. On the surface, fairy ring symptoms can include rings or arcs of dead or unhealthy turf (Type I), circular bands of dark green stimulated and actively growing turf (Type II), or rings of mushrooms, “puff-balls” or “toadstools” (Type III). Below the surface, the fungal mycelium grows outward in a roughly circular pattern through the soil, breaking down organic matter and releasing nitrogen in the form of ammonia. Soil microorganisms process the ammonia into nitrates, which are available to turfgrass roots. The visible rings of lush, green turfgrass are the result of this nitrogen release and turfgrass root uptake in the soil.
Turfgrass injury due to fairy ring can occur at any time during the year, but often happens during periods of hot, dry weather. For example, in Florida it is common to observe fairy ring symptoms during the prolonged low rainfall period of late winter through early spring. In many other parts of the United States, fairy ring symptoms are observed during the hot, dry summer months and sometimes into the fall. During dry periods, mushrooms can appear in a lawn within a day after a heavy rain. No method exists to predict the occurrence of fairy ring, and an appearance at one site during one year does not automatically ensure a reappearance the following year. Fairy rings have been observed in areas with soil pH ranging from 5 to 8 and, therefore, can occur under any soil condition that will support turfgrass growth.
How does fairy ring damage or kill turf? Plant pathologists agree that turf injury occurs as an indirect result of the mycelium colonizing the thatch and soil. Large amounts of mycelium can accumulate and will eventually coat sand and soil particles. As a result, the soil becomes hydrophobic, or water-repellent. Further, turfgrass roots are less able to acquire water and nutrients due to physical competition with the mycelium in the soil. For these reasons, turfgrass plants are injured or killed due to drought and moisture stress. Keep in mind that localized dry spots and hydrophobic soil conditions are commonly attributed to many other factors besides fairy ring fungi.
In addition, some fairy ring fungi produce hydrogen cyanide that is toxic to plant roots. Therefore, fairy ring fungi can injure or kill turfgrass from a complex combination of hydrophobic thatch or soil, the release of compounds toxic to turfgrass roots, and competition for available water, nitrogen and other nutrients needed for plant growth.
Fairy ring is further categorized as either edaphic and lectophilic. Edaphic fairy rings are produced by fungi that primarily colonize the soil, while lectophilic fairy rings are produced by fungi that primarily colonize the thatch and leaf litter. Fungi that cause edaphic fairy rings can extend mycelium growth to a depth of 2 to 3 feet in the soil profile. Edaphic fairy rings are more common on lawns and pastures, while lectophilic fairy rings are more likely to develop on golf course putting greens and other closely mowed, high-maintenance turf. Visible circles and arcs in turf areas, whether caused by edaphic or lectophilic fungi, can range in size from a few inches to several feet in diameter.
Any decision regarding a fairy ring control strategy depends on whether the fungus is edaphic (soil-inhabiting) or lectophilic (thatch-inhabiting), the level of turf maintenance (lawn, park, putting green or fairway) and the degree to which the symptoms (Type I, II or III) are expressed. Fairy ring fungi are nearly impossible to actually eliminate because of their soil- or thatch-inhabiting nature. However, their symptoms can be alleviated.
Fairy ring control
- Suppression methods
Fairy ring symptoms can be suppressed or “masked” through the use of core cultivation, irrigation, wetting agents and fertilizers. Core-cultivate the affected turf on 2- to 4-inch centers and include at least 2 feet beyond the symptom area. Remove the soil cores, and irrigate to a depth of at least 6 inches. Using a soil wetting agent in conjunction with cultivation should improve water infiltration. Sometimes simply irrigating an affected area is enough to enhance turfgrass growth and vigor and, therefore, mask or hide fairy ring symptoms.
How much nitrogen (N) or iron should be applied to make the surrounding turf match the color of the fairy-ring-affected turf? I conducted a field study recently that showed that 0.5 to up to 2 pounds of N per 1,000 square feet was required. However, keep in mind that with these suppression methods, fairy ring symptoms are often only temporarily alleviated — the fairy ring fungus is still viable in the thatch or soil.
The use of a fungicide plus a soil wetting agent can be effective at reducing fairy ring symptoms, especially those associated with hydrophobic soil conditions. Recent field studies have shown that flutolanil fungicide (Aventis' ProStar 70WP) tank-mixed with Primer soil wetting agent reduced fairy ring symptoms, especially when used in conjunction with irrigation. The soil wetting agent helps to alleviate the hydrophobic soil condition and thereby allow water to move more easily through the thatch and soil profile; the irrigation helps the fungicide penetrate into the thatch and soil and reach the fungus.
Remember that a fungicide alone is only effective on fairy ring fungi and will not alleviate hydrophobic soil conditions; only a soil wetting agent can do that. Only a few fungicides are labeled for fairy ring. In addition to products containing flutolanil (such as ProStar and Regal's Systar), azoxystrobin (Syngenta's Heritage) also has labeling for fairy ring (see our “Fungicide Update,” in this issue, for other fungicide controls). Always consult the label directions when using fungicides and soil wetting agents.
The use of a fungicide to control edaphic fairy ring is difficult because a fungicide can become tied-up by the turf and thatch, never reaching the fungus in the soil below. However, research at Texas A&M University has shown positive benefits from subsurface injection to control fairy ring. Using a Cushman Enviroject, a tank-mix of ProStar 70WP and Primer were delivered via injection several inches into the thatch and soil profile. Fairy ring symptoms were alleviated within a few days, the turf began to quickly recover, and no further turf injury was observed.
- Antagonism methods
Another way to control fairy ring is to exploit the fungi's own antagonistic nature. Typically, two fairy rings will not cross one another because the fungi produce compounds that inhibit growth of other fairy ring fungi. This phenomenon is apparent on slopes, where the bottom of the ring is open, creating an arc rather than a ring. This occurs because the downslope movement of those self-inhibiting compounds prevents fungal growth in turf on the lower side of the ring.
A way to exploit this for control purposes is to remove the turf in the area affected by fairy ring, then till and mix the underlying soil in several directions. Replace the turf or re-seed or sod the area. By mixing the soil, you will distribute the compounds that cause the natural antagonism.
- Eradication methods
Fumigation and excavation are two aggressive methods that may work in a limited area.
For fumigation, remove sod in the affected area, mix the soil to a depth of 6 inches or more, fumigate (i.e. with methyl bromide, metam sodium, dazomet) and then re-seed or sod after a safe post-fumigation interval has passed.
With excavation, dig-up all infested soil in the ring area and 2 feet beyond, remove the soil to a 12 inch depth, replace with uncontaminated soil, then re-seed or sod.
The methods described here for antagonism and eradication are costly, labor intensive and not always successful, so be prudent when deciding whether to use one of these options.
Dr. Mike Fidanza is an assistant professor of horticulture (turfgrass ecology) at the Berks-Lehigh Valley College, Pennsylvania State University (Reading, Pa.). He can be reached at email@example.com.
DON'T EAT FAIRY RING MUSHROOMS!
Some fairy ring mushrooms growing in turf are considered edible, but many are poisonous.
Question: How can you tell the difference between an edible mushroom and one that is poisonous?
Answer: Only an expert can, so don't chance it!
While some poisonous mushrooms cause only an upset stomach and abdominal pain, others can be fatal. Small children are particularly at risk because they are more likely to come into contact with fairy ring mushrooms during their play outdoors. Even a small amount of a poisonous mushroom ingested could cause serious health problems to a young child. Any mushrooms that appear in a lawn should be removed — a task easily accomplished with a rotary lawn mower. For anyone wishing to eat mushrooms with their dinner, I suggest visiting the produce section at your local grocery store for safe, farm-grown edible mushrooms. Bon appetite!
GLOSSARY OF FAIRY RING TERMS
Type I symptoms: Rings or arcs of unhealthy or dying turf.
Type II symptoms: Circular bands of darker green turf.
Type III symptoms: Rings of mushrooms, toadstools or “puffballs” in turf.
Edaphic: Describing fairy ring fungi that mainly inhabit soil.
Lectophilic: Describing fairy ring fungi that mainly inhabit thatch and leaf litter.
Mycelium: The “vegetative” body of a fungus (as opposed to the reproductive or fruiting bodies of a fungus, e.g. mushrooms). Not normally visible in the case of fairy ring fungi.
FAIRY RING CONTROL STRATEGIES
Suppression: Apply supplemental irrigation and fertility; also consider wetting agents and fungicides.
Antagonism: Till, mix and spread soil from affected area.
Eradication: Till and fumigate affected area, or excavate and replace affected soil.
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