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Annual Bluegrass

Timely Fungicide Applications, Salinity Reduction Help Control Rapid Blight

September 1, 2008 By: Mary W. Olsen TurfGrass Trends


Rapid blight is a relatively new turfgrass disease (Stowell et al., 2005). It was first described in 1995 when microscopic football-shaped structures were routinely observed within leaf cells in symptomatic cool-season turfgrasses, but their identity and relevance to disease remained a mystery for several years.

A common symptom of rapid blight in rough bluegrass (Poa trivialis) is the development of orange- to gold-colored sunken patches of turfgrass.
A common symptom of rapid blight in rough bluegrass (Poa trivialis) is the development of orange- to gold-colored sunken patches of turfgrass.

The causal organism, Labyrinthula terrestris, was identified in 2003 (Olsen et al., 2003). Rapid blight occurs on golf courses, commercial lawns and sports turf that use irrigation water with moderate to high salinity. Generally, it appears on cool-season turfgrasses irrigated with water at electrical conductivity greater than 2 deci-Siemens per meter (EC >2.0 dS/m) over 1,300 total dissolved salts (TDS). It is most severe on rough and annual bluegrasses, perennial ryegrasses, and colonial and velvet bentgrasses. Early symptoms of disease include patches of turf that appear water soaked and slightly sunken. Infected patches in Poa trivialis are first appear orange to golden (Photo 1), and large areas of straw-colored dead turf develop quickly in bentgrasses (Photo 2).

In susceptible bentgrasses, large areas of straw-colored dying turfgrass develop quickly, often in discrete areas with both heavy traffic and elevated salinity.
In susceptible bentgrasses, large areas of straw-colored dying turfgrass develop quickly, often in discrete areas with both heavy traffic and elevated salinity.

Characteristics of the pathogen

Understanding the nature of the pathogen that causes rapid blight is the first major step in developing control measures.

Early attempts to identify the pathogen led to the disease mistakenly being associated with a chytrid fungus. However, Labyrinthula terrestris belongs to a unique group that is not a fungus, oomycete or a bacterium. It is difficult to place it neatly among other turfgrass pathogens. Labyrinthula species are often referred to as marine net slime molds, but they are not related to true slime molds at all. Their most prominent features are the microscopic football-shaped vegetative cells and networks of tubes in which the cells "glide." Labyrinthula cells can move more than 0.5 inches per hour in lab cultures and are found on all plant parts.

Until identified as a pathogen of turfgrass, Labyrinthula species were thought to be associated only with marine systems, but L. terrestris grows best in culture at salt-adjusted salinity of 2.0 dS/m to 10.0 dS/m.

Long-term survival mechanisms of L. terrestris are not well known. It survives over the summer in the absence of its cool-season turfgrass hosts either on dry plant tissue or non-symptomatic warm season turfgrasses. In Arizona, L. terrestris has been isolated from non-symptomatic bermudagrasses and non-symptomatic Poa trivialis irrigated with recycled water (EC 0.8 dS/m) indicating that it probably has a wide distribution but causes disease only under certain conditions. Genetic studies indicate that Labyrinthula terrestris may have originated from a marine species (Craven et al., 2005), but the rapidity with which it apparently spread is still a mystery. 

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