Lawn & Turf Tips: Necrotic Ring Spot

Necrotic Ring Spot

Necrotic Ring Spot is a soil born disease that infects the roots of the plant. Also known as, Leptosphaeria Korrae, this fungus affects Kentucky bluegrass and annual bluegrass.

Symptoms

Typically, the initial outbreak of the disease occurs during cool wet weather in either the spring or the fall. At times, it can be observed through out the growing season. This disease can be difficult to diagnose by it's symptoms alone especially during the early stages.

• Disease starts out as scattered light green patches approximately 5-10 cm in diameter.
• These patches can grow as large as 1 meter in diameter. They turn into dull-tan to reddish-brown patches.
• A common characteristic of this disease is that it looks like a "frog-eye" with the dark green, apparently healthy grass in the center surrounded buy a ring of dead grass.
• It tends to be worse during the cooler parts of the year.
• The fungus survives as mycelium in dead and decaying root tissues.
• It is spread thru the transport of soil with infected roots.

Disease Cycle

• It is able to survives during unfavorable conditions as dormant mycelium in infected plants.
• This fungus infects plants and produces the symptoms during early spring.
• As the fungus becomes dormant, the symptoms fade and then reappear when those conditions become favorable again. For example, you will see it in spring then it goes away during the heat of the summer only to reappear during the fall.
• The fungus can be spread through infected turf and lawn equipment.

Disease Control Strategies

It is important to first have a professional come and diagnose the fungus before any treatment is applied. The most common stresses that influence this disease are excessive thatch, fertilizer, and turf canopy temperature. This also includes incorrect timing of fertilizer applications, low mowing height and low soil pH. Fortunately, each of these stresses can be minimized by using correct lawn care maintenance.

Cultural Control

• Correct excess soil acidity by liming your lawn annually. Your soil pH can be assessed with a soil sample sent in to be tested.
• Deep watering is essential to promote root growth. You want to water the areas where it is disease prone to a depth of 15 to 20 cm every 7 - 10 days during the dry periods in summer.
• Watering should only be done in the morning.
• If you have turf that is prone to this disease, you should cut your lawns at 2 1/2 - 3 inches high. Decreasing the mowing height only increase the stress on the turf when it is already diseased. You only want to cut off 1/3 of the blad during each mowing.
• Your thatch (the layer between the soil and the green grass) should be no more than 1/2 inch thick. You can remove the thatch during the spring or fall by having it thatched with a vertical slicing machine.
  Aeration is essential in getting the air circulation down by the roots and promoting healthy root growth. This can only be done during the spring or fall (preferably during the fall).

Chemical Control

• If cultural treatments are not working, you can use a chemical treatment. It is only effective if the cultural controls have been tried.
• Applications must be timed correctly. It needs to be applied in the spring and in the fall.
• Depending on the severity of the disease, you may need 2 applications (14 - 21 days apart) in the spring and in the fall.
• This option should be considered only if the other options have failed.

Managing necrotic ring spot can be difficult. By following the steps above, you will help strengthen your turf and lessen the severity of the outbreak.