Rapid blight is a disease that affects cool-season grasses and typically occurs in areas where drought and accumulation of sodium salts coincide. It was first identified as a disease of turfgrasses in 2002, although symptoms were reported as early as 1995. Rapid blight damages golf greens, tees, fairways, and roughs cultivated with Poa annua, P. trivialis, Lolium perenne, and sometimes Agrostis stolonifera. It has also been observed in L. perenne overseedings on athletic fields. The disease has been reported in several states in the United States and in countries such as Argentina, Ireland, Spain, and the United Kingdom.

Credit: PACE Turf

Causal agents

Labyrinthula terrestris

Primary Hosts

Poa annua, P. trivialis, Lolium perenne, and sometimes Agrostis stolonifera. 

Disease Cycle

The disease cycle of rapid blight begins with the dissemination of spindle-shaped vegetative cells, the main propagules of Labyrinthula terrestris. These cells can be spread through activities like mowing and traffic. Infection occurs through natural openings or wounds on turfgrass. Wounding can accelerate the appearance of infected leaves.

Labyrinthula terrestris does not produce mycelium but may create slime pathways for movement within the turf canopy. While cyst formation has not been confirmed in turfgrass isolates, some Labyrinthula species form thick-walled cysts for survival. Symptomless hosts like bermudagrass and seashore paspalum can harbor L. terrestris, potentially serving as inoculum sources for susceptible cool-season grasses during overseeding or other interactions.

Epidemiology

Rapid blight is influenced by variable environmental conditions. Epidemics can occur during any season when there are dry periods and the soil and water salinity levels range from 1 to 3.5 dS/m (640 to 2,240 ppm or 1,000 to 3,500 mmhos/cm). The disease is more likely to occur during mild to warm conditions when cool-season turfgrasses are susceptible to drought and salinity stress. Increased precipitation that leaches sodium cations from the soil can decrease the severity of rapid blight. However, if dry conditions persist for at least 2 weeks and irrigation water with a salinity level of 1.5 dS/m or higher is applied, the disease can develop in highly susceptible hosts such as Poa trivialis or Poa annua. The pathogen can move locally through slime pathways and on a larger scale through the spread of infected tissues via mowing, equipment transfer, or foot traffic.

Symptoms

Rapid blight manifests as chlorotic or darkened turf patches with irregular shapes, typically measuring 2 to 3 meters in diameter. In some cases, the patches may appear circular. On low-cut greens with overseeded grasses, the patches may exhibit dark borders of water-soaked turf. Upon closer inspection, the chlorotic leaves display mottling and have a water-soaked appearance. The affected leaves can experience tip dieback and eventually become shriveled and necrotic.

Rapid blight can cause significant damage to turf, including the complete elimination of a stand within a week. Even mature stands of turf, such as putting greens with Poa annua, can be severely affected. The chloroplasts in infected leaf tissues deteriorate as the disease progresses.

Credit: PACE Turf and John Kaminski/Penn State University

Signs

Microscopic examination reveals hyaline, spindle-shaped vegetative cells within leaf epidermal and mesophyll cells.

Credit: PACE Turf and John Kaminski/Penn State University

Management

Rapid blight is a significant disease that affects cool-season grasses, causing considerable damage and leading to turf decline if left unmanaged. Effective management of rapid blight requires a comprehensive approach that combines cultural practices and chemical treatments. Cultural strategies focus on manipulating host, pathogen, and environmental factors to create conditions less favorable for disease development. These practices include proper host selection, optimizing soil and water conditions, and implementing good turf management practices. In addition to cultural practices, fungicides can be employed to suppress the disease and provide control.

Cultural

Strategies for managing rapid blight involve manipulating host, pathogen, and environmental conditions. Host manipulation strategies include using more-tolerant cool-season grasses or reducing or eliminating overseeding. Salt-tolerant cultivars of perennial ryegrass or mixing alkali grasses with more-susceptible species can also be effective.

Environmental manipulation includes reducing salinity in irrigation water or soil through leaching with less-saline water and the use of gypsum to displace sodium. Increasing potassium fertilization may also help reduce the disease. Adequate drainage is important to prevent waterlogged conditions. Sodium can be removed from irrigation water by reverse osmosis systems, however this treatment can be costly.

Chemical

Fungicides such as pyraclostrobin, trifoxystrobin, and mancozeb have shown effectiveness against rapid blight, with the best control achieved using pyraclostrobin or a mixture of pyraclostrobin and mancozeb. However, under high disease pressure, fungicides alone may not provide complete control.