Necrotic Ring Spot in Lawns

OMAF Turfgrass Series (2000)

Table of Contents

1. Introduction
2. Symptoms
3. Life Cycle
4. Control

Introduction

Necrotic ring spot is probably the most damaging disease that occurs on Kentucky bluegrass home lawns in Ontario. Necrotic ring spot is commonly found on sodded lawns in the first 5 years after establishment. Necrotic ring spot is caused by the fungus Leptosphaeria korrae.

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Symptoms

Symptoms of the disease start as yellow patches which can coalesce. As the disease progresses, brown rings with healthy grass on the inside and outside develop (Figure 1). The turf in the approximately 10cm to 1m diameter rings can be pulled out easily because the roots and crowns are dead. On an individual plant infected by the fungus, outer leaves turn yellow and bleach out and younger leaves turn red- purple before turning brown. Blackened masses of hyphae (fibular filaments of mycelia) may appear on root surfaces(Figure 2). Growth of the pathogen is promoted by cool, wet weather in the spring and autumn. Summer heat stress may intensify the symptoms of the disease.

 

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Life Cycle

The fungus overwinters as mycelia on roots and lower crowns(Figure 3). In the spring the roots are infected by spores or mycelium. Symptoms from this infection may appear 12 to 18 months later. Heat and drought stress during the summer decreases the activity of the fungus yet enhances the symptoms of the disease on the weakened root system. Necrotic ring spot usually occurs where there are unfavourable rooting conditions for the grass.

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Control

The best method of control is to establish a good turf root system by:

• providing the necessary fertility requirements (such as a light application of a nitrogen potassium phosphorus fertilizer) will promote root growth

• reducing the clay in the soil

• irrigating deeply and infrequently but syringing turf at mid-day on hot dry days

• mowing at a higher height

• overseeding with a resistant Kentucky bluegrass cultivar (Table 1)

• (in southwestern Ontario) seeding damaged lawns with perennial ryegrass (which is also resistant to necrotic ring spot
  

Note: There is no chemical registered in Ontario for the control of necrotic ring spot.

Cultivar - Cultivar equivalences were determined from listings in 15 trials

Rank - To determine the overall ranking, data within each trial were ranked for susceptibility from 0 (lowest susceptibility) to 100 (highest susceptibility), and all data were subjected to analysis of variance.  

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