Southern blight is a disease that can affect various turfgrass species, including Agrostis, Cynodon, Festuca, Lolium, Poa, Zoysia, as well as broadleaf turf such as Dichondra. It is commonly observed in warm temperate climates across the globe.

Credit: PACE Turf

Causal agents

Athelia rolfsii fungi

Primary Hosts

• Agrostis
• Cynodon
• Festuca
• Lolium
• Poa
• Zoysia spp
• broadleaf turf 

Disease Cycle

The disease cycle of southern blight involves the pathogen, Athelia rolfsii, producing abundant sclerotia that serve as survival structures in soil and thatch. Germination of these sclerotia occurs in warm and moist conditions. A. rolfsii can act as a facultative parasite, capable of existing as a saprophyte or a pathogen, and it can attack a wide range of plant species. The pathogen spreads rapidly through thatch or soil and can infect the plant canopy. It is easily transmitted from infected turf to surrounding areas, causing secondary infections and the formation of satellite rings around older patches. Infected plants experience a loss of cellular integrity, resulting in chlorosis, gray discoloration, and eventually reddish-brown death. Sclerotia are formed on dead grass and in thatch, where they can persist for extended periods of time.

Epidemiology

The epidemiology of southern blight involves the growth of Athelia rolfsii being favored by hot and moisture-saturated conditions, as well as the presence of organic matter. The pathogen is most active when temperatures range from 30 to 35°C (86 to 95°F) and the relative humidity remains above 90% for a prolonged period. Cooler weather and poorly aerated or neutral to alkaline soils can limit the growth of A. rolfsii. Germination of sclerotia increases after dry periods, and subsequent high moisture levels following droughts create favorable conditions for extensive plant damage. Sclerotia formation is most abundant in well-aerated, acidic (pH less than 7), and moist soil under light exposure.

Symptoms

The symptoms of southern blight include the chlorosis and thinning of grass within the affected patches, eventually leading to its death. Recolonization of the affected patches typically occurs during autumn. Southern blight can affect various turfgrass species. In Agrostis and Poa spp., yellow rings, arcs, and patches develop, turning reddish brown as the grass dies. P. pratensis exhibits initial reddish brown or gray rings that fade to tan. During hot and humid weather, rapid enlargement of dead grass rings can be observed, growing up to 20 cm per week.

Credit: PACE Turf

Signs

White aerial mycelium may develop on dying grass leaves, and dark brown sclerotia, ranging from 1-3 mm in diameter, can be found on dead grass or thatch within the ring.

Credit: PACE Turf

Management

Management of southern blight involves thatch management and, in areas where the disease is chronic, fungicide applications.

Cultural

To manage southern blight, it is recommended to reduce thatch accumulation through practices like aerification or vertical mowing. Maintaining vigorously growing and well-fertilized grass can help suppress the disease.

Chemical

Certain fungicides are labeled for controlling southern blight in turfgrasses. These include:

• quinone outside inhibitors
• demethylation inhibitors
• succinate dehydrogenase inhibitors