Hosts Cool-season turfgrasses, such as annual bluegrass, rough bluegrass, and perennial ryegrass.
Cause Labyrinthula terrestris, a microscopic organism in the kingdom Chromista, which thrives when soils or water have elevated salt levels (salinity levels typically above 2.0dS/m are associated with symptom development) as may occur when poor-quality irrigation water is used or under reduced irrigation conditions. Cells of this organism can be observed microscopically within infected plant tissue. First confirmed as a disease in California in 1995, the disease has been found in numerous states including Washington. Symptoms are more severe when poor water quality or amount of irrigation to the turf results in an increase in salinity in the soil. The pathogen can thrive in the range between 50°F and 93°F maximum daytime air temperature. This organism colonizes the foliage, crowns, and roots in affected areas.
Symptoms Symptoms develop as irregularly shaped patches of darkened or sometimes chlorotic turf. Infected turf may look water-soaked and darker in color than surrounding healthy plants. Small patches a few inches in diameter, enlarge and may rapidly coalesce killing large portions of turf. Symptoms may resemble damage from salinity or may look similar to foliar damage from Pythium blight. Although infection occurred during warm periods, symptoms did not show up until it cooled.
Cultural control
- Reduce salinity of irrigation water and soil by leaching with less saline water.
- Application of calcium sources to reduce sodium in the soil may lessen disease severity but is unlikely to provide complete control.
- Avoid overseeding with highly-susceptible turf species.
- Aerate to improve surface drainage as needed
Chemical control Control is difficult as multiple fungicide applications are often needed to eradicate the pathogen from the crowns, upper roots, and thatch for complete control. Tolerant strains of many fungal pathogens can become troublesome when some fungicides are used exclusively in a spray schedule. To prevent developing resistant fungi, alternate or tank-mix materials from different groups with different modes of action. Under high disease pressure, fungicide effectiveness may be reduced. Limit the use of any one group during crop production.
- Compass 50 WDG at 0.1 to 0.25 oz/1,000 sq ft. Do not use organosilicate additives. Group 11 fungicide. 12-hr reentry.
- Fore 80WP at 8 oz/1,000 sq ft. Apply at 14-day intervals in conditions that are favorable for disease.
- Insignia SC at 0.4 to 0.7 fl oz/1,000 sq ft. Use preventatively. Do not use with organosilicate adjuvants. Group 11 fungicide. 12-hr reentry.
- Interface at 3 to 5 fl oz/1,000 sq ft. Maintain agitation while spraying. Do not mix with adjuvants. Group 2 + 11 fungicide. 24-hr reentry.
- Lexicon at 0.34 to 0.47 fl oz/1,000 sq ft. Group 7 + 11 fungicide. 12-hr reentry.
- Navicon at 0.7 to 0.85 fl oz/1,000 sq ft. Group 3 + 11 fungicide. 12-hr reentry.
- Velista at 0.5 oz/1,000 sq ft. Group 7 fungicide. 12-hr reentry.
Note: Azoxystrobin (Heritage) has not provided effective control.
Reference Kerrigan, J.L., Olsen, M.W., and Martin, S.B. 2012. Rapid Blight of Turfgrass. The Plant Health Instructor. DOI: 10.1094/PHI-I-2012-0621-01