Necrotic ring spot appearing on lawns

The hot weather and frequent rain are creating ideal conditions for fungus to thrive. 

Rings and crescents of dead grass are a telltale sign

 The most serious issue we see in the heat of summer is necrotic ring spot. Caused by the fungus Leptosphaeria korrae, in its advanced stage the damage takes the form of rings or crescents of dead grass.  It’s main host is unfortunately the most common grass on home lawns, Kentucky bluegrass. 

 At early stages, the damage appears as scattered patches that coalesce and pull out easily. The roots and crowns have been killed, so nothing holds the grass in the soil anymore. The fungus spreads outwards in an arc form by spores and mycelia. New grass can grow back in the region the fungus has abandoned, resulting in the characteristic ring shape. Cool wet weather promotes its growth in spring and fall, but the dry hot summer reveals its symptomatic arcs and rings more blatantly. Unchecked over years, the rings of dead grass can grow to over a couple feet in diameter. 

 There is no cure for necrotic ring spot. There is no product registered for use in Ontario or North America for that matter which will kill the fungus or stop it from infecting grass. You CAN make your lawn resistant with good cultural practices.

 If you already have it: 

  •  Clean your lawn mower:  Mycelia tend to attach themselves to the underside of dirty lawn mowers and are carried from lawn to lawn.  We most often see necrotic ring spot on lawns that have a mowing service.
  • Sharpen your mower blade: Torn grass blades are more easily infected because they have larger surface areas to become infected by most lawn diseases
  • Fertilize with Nitrogen: LawnSavers professional fertilizer contains a high amount of slow release nitrogen which helps the lawn grow through this disease quicker. 
  • Syringing: (only recommended to help reduce an identified Necrotic ring spot problem)  Water very lightly during the high heat of the day, essentially to activate the fungus and expose it to the sun, therefore decreasing its life expectancy.
  • Aerate: Aerating the lawn reduces compaction thereby reducing the amount of infection.
  • Over-seed with resistant varieties of Kentucky bluegrass and resistant perennial rye-grass. (LawnSavers special overseed blend is very resistant)
  • Let it run it’s course!: any efforts to seed or dig it out will only be met with disappointment as disturbing it helps the fungus spread and new seedlings become infected quickly.


Strong roots and resistant strains of grass will both help shield your lawn from this attack. 

  • Water deeply, once or twice a week. Grass will grow roots to where the water is. Creating a deep reservoir and allowing the surface to stay drier will encourage the roots to grow long.
  • Mow frequently and high, to prevent thatch build up which can snarl root growth.
  • Core aerate every year to break up excess thatch and reduce soil compaction, so the roots have a better base to grow into.
  • Over seed, not only to replace any grass damaged by fungus, but to introduce new, stronger varietals which resist necrotic ring spot better.

We can over seed starting at $59.95 + tax and core aerate starting at $60.00 + tax, and both services can be bundled together for bigger savings.

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