Question
Northern Illinois 60 mi from Missippi., Rock Falls Ill.  Sandy soil. Question:  I have had an arc of almost looks like fertilized around a crab apple tree for about 3 years.  I spread grub killer last year, but the arc still appears. This arc or semicircle is about 20 feet across.  This year, there are about 3 more smaller ones appearing around the yard in the close proximity.  

Answer
Hi Ken H,

Your 'arc phenomenon ' is puzzling.  The fact that the affected area is extra-green by comparison to surroundings generally indicates a higher level of nitrate in the soil.  This can be caused by decaying plant material under the soil surface. Perhaps old decaying roots are implicated. It may even be caused by bird droppings from above adding excess nitrate to areas.   

A fungus technically called "Marasmius oreades"  causes an infection of turf grasses which grow to form 'Fairy-Rings" in lawns.  The disease can cause bright green spots and circular areas which expand and leave behind brown-dead areas and/or a general decline of the grass health and an associated appearance problem.  A circle of mushrooms usually develops around the edges of the infected area.


 These are usually entire circles of darker green vegetation that eventually end up as dead areas. I would guess they can also make arcs if trees are in the way,...I just don't know and have never heard of such,...they are usually in the open areas of lawns and make full circles.

The fungus can live under the ground for many years in its spider-web-like mycelial stage.  Every so often there will be the vegetative 'MUSHROOM' stage that pops-up where the ring is.  For a while there will be a ring of mushrooms that can make microscopic spores to spread the colony far and wide.  Perhaps you have not yet had the vegetative phase yet to develop or have not noticed it. The mushrooms are very visible in the landscape when they arise.  It is important to harvest the mushrooms and dispose of them BEFORE they can produce spores to be rid of the fungus in the area.

The fungus can kill the grass and affect large areas over time.
There may now be fungicides on the market that work on this type of fungus.  I have not yet up-dated my fungicide articles,...so it may be worth shopping around and researching what's new on the market.  Fungicides that target mushroom-like fungi have not been common at garden centers, but I think there should be some new products around. Most fungicides just target the more economically important fungi like the smuts, rusts, molds and mildews.


One of the surest ways to be rid of the 'Fairy-ring' Fungus  is to actually work to dig it out of the ground.  You will have to dispose of the affected soil or sterilize it and replace it with better soil. This can be a rather labor intensive project, however and may not give good results unless all of the nearly invisible mycelium is removed.

Acidifying the top-soil and spraying the areas with a solution of iron sulphate may help to prevent fungi and mushroom out breaks.

Improving soil drainage, sun-light availability and ventilation will also help to prevent fungal problems.

With very dry conditions, sometimes the mycelium will naturally die-out and the fungus will become history due to droughts and dry spells it cannot survive.  

Is summary, your arcs are not something  that can be well diagnosed here without a closer examination.  But the fungus or sub-soil nitrate scenarios seems plausible causes.

Good Luck!
__Jim Gibbs,
  Microbiologist