What is causing the Necrotic Ring Spot problem?

Necrotic Ring Spot is a soil borne lawn fungus, which is identified by circular to irregular sunken straw colored or gray patches approximately 4-16 inches in diameter. Necrotic ring spot fungus is caused by human interference and is cured by stopping the poor cultural practices of humans.

Necrotic ring spot is a symptom of a problem and if we fix the cause of the problem we should expect it to go away. Therefore, the first step is to fix the cause of the lawn problem which is severe over watering of the lawn for a very long period of time (months). Necrotic ring spot is a common lawn fungus worldwide but it is most common in states like Colorado where the majority of lawns have sprinkler systems.

The over watering of the lawn is the cause of necrotic ring spot fungus for two reasons. First, over watering stimulates the Necrotic Ring Spot fungus spores and secondly over watering the lawn kills the beneficial microbial activity in the soil, causing a problem we call "dead soils".

Dead soils lack beneficial microbes which stabilize the soil, and the absense of beneficial microbes means the necrotic ring spot fungi can live without competition. So without the beneficial microbes to fight the fungus and the excessive moisture to stimulate the spores, the necrotic ring spot fungus can spread quickly.

When an lawn fungus outbreak does occur, the Necrotic Ring Spot is difficult to control and will take about 9 months to cure. The reason for the extended length of time for treatment is because it takes 6 months to 1 year to rebuild a dead soil. Until the beneficial microbes have been reestablished then the necrotic ring spot fungus will continue to be a problem.

Necrotic ring spot damage

Why you should not use a fungicide to control Necrotic Ring Spot Fungus:

Organo-Lawn never recommends treating Necrotic Ring Spot with a lawn fungicide because the fungicide will kill and prevent the beneficial microbes from living in the soil. Also, the reason the beneficial microbes died in the first place was because too much water replaces air in the soil and without air the microbes die. The cure for necrotic ring spot is to rebuild the soil and the beneficial microbes will kill the necrotic ring spot fungus spores.

Important Note: The Necrotic Ring Spot fungus spores are present in all soils and are not spread via core aeration, lawn mowing or other lawn equipment. The fungus spores are everywhere; the only time they become a problem is when they are stimulated with too much water.

Before and After Photos of Necrotic Ring Spot

Click on an image below to enlarge the picture.

How to cure Necrotic Ring Spot fungus:

  1. The most important thing to do is to fix the improper over watering of the lawn. Necrotic Ring Spot is only found in severely moist or wet areas with poor drainage. Make sure to water deeply and infrequently giving enough time between lawn watering so the soil has time to dry out. Also, it is important to water early in the morning as opposed to late at night. If the over watering is not corrected then the lawn fungus problem will never recover! Organo-Lawn will not guarantee curative results for your Boulder or Fort Collins area lawn unless you have a sprinkler startup / audit performed by an Organo-Lawn employee.
  2. It is very important to completely rebuild the beneficial microbial populations in the soil. The fungus will continue to be a problem until the beneficial microbial activity is restored. This takes time.
  3. Perform 2-3 aerations, one in early spring, one in late spring, and one in the fall to alleviate soil compaction and introduce air into the soil so the microbes can breathe. Remember without air the living beneficial microbes in the soil cannot breathe.
  4. Manually remove the dead grass in the necrotic ring spot circles. You can do this using a heavy metal rake.
  5. Stimulate beneficial microbial activity by applying Humate soil conditioner 2 times per year. Humate is recommended because it will stimulate beneficial microbial activity. Never fertilize the lawn with fast release or inorganic nitrogen sources. Fast release nitrogen is typically white in color and high in salt, which kills beneficial microbes.
  6. Change all fertilization to organic fertilizers. Chemical fertilizers are high in salts and kill microbes. A very good organic fertilizer for curing necrotic ring spot is Synergy with Corn Gluten Meal.
  7. Never apply synthetic chemical herbicides (weed control) or chemical fungicides to the area. Chemical herbicides and fungicides can hinder or even kill essential beneficial microbial activity.
  8. Never mow the grass when it is wet, and always wait 24 hours after a mowing to water the grass. After a mowing the grass blades have been damaged. It takes 24 hours for a grass to repair the abrasion. If water is added to the lawn before the lesion has time to heal, the fungus can use the water as a transportation mechanism and infect new and otherwise healthy blades of grass.
  9. Until the necrotic ring spot fungus is eliminated, bag the grass in the infected area, increase the mowing height to 3” or 3.5”, and increase the mowing frequency so no more than 1/3 of the blade of grass is removed per mowing.
  10. If the lawn does not respond to steps 1-9 and it has been 1 year since treatment began on the disease it is because the soil is still dead or absent of beneficial microbial activity. Microbes are just like everything else they need air, heat, water (moist but not suffocated with too much water), and food (organic fertilizers). If the lawn has not recovered it is because one of the following is still not being achieved: Air, water, heat, and food. Remember too much water replaces air in the soil and if the lawn is being over watered the microbes cannot breathe.
  11. Notice that we DO NOT recommend the use of a fungicide. The reason for this is that fungicides kill beneficial microbes and we are trying to rebuild the beneficial microbial activity in the soil. We have found that the lawn will recover much faster if a fungicide is NOT applied because the fungicide can kill beneficial microbes for up to 3 months after the initial application. DO NOT APPLY A FUNGICIDE AS IT IS COUNTER PRODUCTIVE.

Lawn watering Boulder

Treatment of necrotic ring spot fungus seems simple; however, it can be complicated. Our recommendation is to contact Organo-Lawn in Boulder (303) 499-2000 or Fort Collins (970) 225-9425 and schedule a disease diagnosis with one of our professional lawn care technicians. Our state licensed lawn care technician will come up with a plan to correct the problems that are causing this fungus.

Photos of Necrotic Ring Spot Fungus

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Mild necrotic ring spot in a lawn in Boulder, Colorado.

Severe necrotic ring spot in a lawn in Fort Collins, Colorado.

Necrotic ring spot is always found in areas of the lawn that are severely over watered.

Necrotic ring spot is a clear sign of a dead soil.

This is a very severe case of necrotic ring spot fungus that was found in a lawn in Lafayette, Colorado.

This is one of the most extreme case of necrotic ring spot that has ever been seen and was found in a lawn in Longmont, Colorado.

Necrotic ring spot takes anywhere from 1-2 years to cure.