What is necrotic ring spot?

This lawn disease is a parasitic form of plant life called fungi. Fungi live in the soil, thatch and dead leaves all year round. These fungi feed off the grass by drawing nutrients from it. The disease spreads throughout the lawn in the form of spores. Wind, air, water and humans spread these spores. The disease may occur if the lawn is under stress by a variety of factors such as weather, temperature and improper watering or mowing. The disease is commonly found on lawns over one year old that were established from sod, but can occur in seeded lawns. The symptoms can be intensified in compacted or heavy clay soils.

How can I tell if I have necrotic ring spot?

The disease takes the form of perfectly shaped rings of yellowed out grass with healthy grass growing within the ring. The grass blades appear straw yellow and die from the tip down. As a result of its appearance, the disease is also referred to as “frog eye”. Initially the patches are only 5 to 10 cm in diameter but they can expand to form larger patches ranging from 30 to 50 cm in diameter.

Although the symptoms are most apparent when the fungus is most active during cool moist conditions, symptoms can show throughout the growing season. During the hot dry summer months, the symptoms may reappear due to heat and drought stress.

How can I treat necrotic ring spot?

Necrotic Ring is a very difficult disease to control. When the disease first occurs rings may be dug out with a shovel to rid the soil of the disease spores. Remove the grass and soil to a depth of 15 to 20 cm. Wash the shovel and any other gardening equipment including the lawn mower before using again to prevent the spores from spreading. Rake the area; fill the hole with topsoil and over seed with a perennial ryegrass seed mixture that is more resistant to the disease.

If the area is so infected that removing the rings is unreasonable, correct maintenance practices must be used to control the problem. These practices include:

  • First and foremost prevent your lawn from becoming droughted. A lawn requires 2.5 to 4 cm of water weekly. Under normal circumstances deep infrequent watering is important but recent theories suggest light frequent watering only if you have necrotic ring active in the lawn.

  • Do not water at night - Water laying on the grass plants overnight encourages disease to spread in the water droplets.

  • Mow your lawn frequently to a height of 6 to 8 cm with a razor sharp blade.

  • Ensure adequate amounts of phosphorus, potassium and slow release nitrogen. Regular applications of Weed Man's exclusive slow release fertilizer will provide adequate levels of all of these nutrients to the lawn.

  • Control thatch and soil compaction, which can increase the symptoms as the spores thrive in the thatch layer. Mechanical core aeration can help reduce both thatch and compaction.

If you have any further questions about Necrotic Ring Spot in your lawn or have any other questions or concerns, please feel free to contact your local Weed Man Professional.