What is Necrotic Ring Spot?Necrotic ring spot is a disease caused by the fungus Ophiosphaerella korrae. This fungus spreads through the soil and colonizes the roots of plants. Over time, it eats away at those roots and deprives the plants of moisture and nutrients, causing them to weaken and die. The fungus thrives at temperatures between 65 and 80 degrees Fahrenheit and can occupy the same patches of your lawn for years at a time.
Necrotic ring spot is named after the large rings of dead or weakened grass that it produces, which can be anywhere from a few inches to several feet in diameter. These rings are yellow, tan, or brown in color, and often look like a donut or the eye of a frog. The longer they are allowed to fester, the more likely it is that the grass inside them will die. Ring spots tend to be most severe in shady parts of your yard, especially if there is a large amount of thatch in the soil.
Besides the rings, you can identify symptoms of this disease by cutting sod patches out of the ground and using a hand lens to examine them. If the fungus is present, it will leave dark threads running along the surface of the root. This method is useful if you’re having trouble telling the difference between ring spot disease and other issues that cause turfgrass to become discolored.
Because necrotic ring spot thrives under certain temperatures, it tends to die down after September, and your grass may recover in the final months of the year. But the disease will still be there in a dormant state and will attack your grass again once the warm weather returns. If you don’t get a handle on it soon, it will cause severe damage to the roots and crowns of your grass, making recovery harder and harder as time goes by. It is thus critical that you take action as soon as possible after you find symptoms of necrotic ring spot.
How Necrotic Ring Spot Disease is Treated
There are three main methods for treating necrotic ring spot outbreaks, each suited to different levels of severity:
- Physical Removal- For disease outbreaks that are limited to a particular part of your yard, you can get rid of the disease by physically removing the affected grass. Dig down to a depth of at least 6 inches and remove all the grass and soil that the fungus has spread to. You should then dispose of what you’ve taken out in an isolated area and wash the shovel immediately.
- Sulfur Amendments- You can mitigate necrotic ring spot by adding sulfur to the grass, administering it either as elemental sulfur or as part of the fertilizer. The more severe the disease is, the higher the concentration of sulfur you’ll need to administer. Once you’ve spread it over the lawn, you need to water it in so that it does not damage the leaves.
- Fungicides- For the most severe infections, you may need to kill the disease with a fungicide. It’s usually necessary to apply fungicides at least once every two weeks during a season. Typically, you’ll need a professional to administer them.
Besides treating the disease, you can lower the likelihood of an outbreak in the first place through proper lawn care. By dethatching and aerating your lawn on a regular basis, you make it harder for the fungus to spread. It’s also important to water, mow, and fertilize your lawn sufficiently, but not excessively; too much water or fertilizer containing nitrogen helps the disease grow. You should also avoid watering your lawn at night.