Labyrinthula terrestris
Author: Leticia Kumar, University of Florida
Reviewed by: Jeffrey Rollins, University of Florida
Pathogen
Labyrinthula terrestris is a relatively new pathogen to emerge affecting turfgrass. In 2003, L. terrestris was identified as the causal agent of the rapid blight disease, although symptoms of the disease were reported on golf course greens as early as 1995 in California (1, 8, 11). Labyrinthula genus members are characterized by production of an ectoplasmic network, called “slimeways,” upon which the spindle-shaped vegetative cells travel (20). This trait is the basis for the common name, “net slime mold.” Their alternate moniker, “marine slime mold,” indicates the preference of this genus for saline environments (3). Increased usage of irrigation water with high salinity, in terms of sodium cations, is considered a factor in the transition from marine to terrestrial environments, giving rise to this turfgrass pathogen in the Labyrinthula genus (10).
Although the extent of host range has yet to be determined, this pathogen has been reported on several types of bentgrass. Affected types include velvet (Agrostis canina), creeping, (Agrostis stolonifera), and colonial (Agrostis tennis), as well as Chewing’s fescue (Festuca rubra var. commutata), perennial ryegrass (Lolium perenne), and bluegrasses such as annual (Poa annua), and rough (Poa trivialis). Of those listed, the turfgrass species considered most susceptible include colonial bentgrass, perennial ryegrass, and annual bluegrass (7, 14). Turfgrasses of the warm-season variety have also been reported to host L. terrestris, including bermudagrass (Cynodon spp.) and 'paspalum (Paspalum spp.), although these cultivars are generally non-symptomatic (13, 19).
Symptoms and Signs
An early symptoms of the disease is irregular chlorotic patches of turfgrass. Chlorosis may not be readily distinguishable upon rapid disease progression, where quick transition from chlorosis to necrosis will be observed as darkened, sunken patches on the field, course, or lawn. Water-soaked, chlorotic leaves with dark, rust-colored lesions will often be present upon closer inspection. Discoloration of roots may be evident at later stages of the disease. On overseeded close-cut greens, a clear delineation of water-soaked turf will be apparent. The diameter of blighted patches may range from 2 cm – 3 m (1 inch – 10 feet). Extensive loss can occur at the seedling stage if the pathogen is present during the first mowing. Such a scenario has potential to eliminate stands within one week. However, the disease may cause significant damage on mature stands as well. It is worth noting that this disease often resembles salt damage and Pythium blight disease (8, 11, 17).
Signs of the pathogen are not seen at the macroscopic level but are present within leaf tissue. Signs may be visualized using a compound microscope with a water mount of an infected leaf blade bisected longitudinally. Single vegetative cells are spindle-shaped (ovoid with tapered ends), hyaline, uninucleate, guttulate, vacuolate, and range in size from 5–10 µm (17). Diameter variability of the cells in culture has been reported as 13.0–17.5 µm × 4.5–7.0 µm (6).
Ecology and Spread
The L. terrestris life cycle is defined as asexual (i.e., mitotic division of the vegetative cells). Cell division can occur slantwise or longitudinally. Indications of sexual reproduction, such as structural features or cells, are absent. The disease cycle begins with infection through natural openings or wounding. Colonization occurs through colony expansion via formation of new slimeways as well as fusion of the existing networks. Labyrinthula terrestris cells have been observed to travel along slimeways produced by other cells (16). As the colonies senesce, aggregates begin to form and are observed as globules hyaline to yellowish in color. These aggregates are believed to be the overwintering structures as they can remain viable for over a year. Aggregates resume growth with favorable growth conditions or on a new host, thus completing the disease cycle (10).
An environment conducive to rapid blight disease development is characterized by warmer temperatures and a high degree of salinity, for which irrigation water and soil are significant factors. Increased precipitation that leads to leaching of sodium cations from the soil will decrease rapid blight disease severity. As such, dry conditions are favorable to the pathogen and disease can develop within two weeks. Irrigation water considered to be highly saline includes levels at or above 3.5 dS/m, but the disease can develop with water salinity at 1.5 dS/m (15, 19). Due to the positive association between salinity and disease development, onset and severity of symptoms become progressively more severe with prolonged favorable environmental conditions. Under extreme conditions, where the only source of moisture is high salinity irrigation water, the disease can completely kill stands within one week. The pathogen has a preference for warmer temperatures, although the optimal temperature range, 10-34°C (50-93°F), is extensive (6, 12).
Primary pathogen dispersal mechanisms include infected irrigation water, contaminated equipment such as mowers, or by foot traffic from an infected patch to an uninfected patch of turfgrass. Although the pathogen is known to infect through mow-injured tips, wounding is not a necessary factor for infection of the host. L. terrestris may also infect through natural openings such as stomata (12).
Geographic Distribution
Worldwide, rapid blight has been reported in Argentina, Ireland, the United Kingdom, and Spain (4, 6, 10). In the USA, rapid blight disease has been reported in Arizona, California, Colorado, Florida, Georgia, Louisiana, Nevada, North Carolina, South Carolina, Texas, and Utah (19).
Management
Several management strategies to mitigate rapid blight disease have been put forth, the most effective of which is based on management of irrigation water (14, 19). Controlling the salinity of irrigation water also influences soil salinity.
Since disease severity is heavily influenced by environmental factors, it is recommended to overseed with salinity-tolerant cultivars. Overall, turfgrasses considered to be cool-season are less tolerant to salinity than their warm-season turfgrass counterparts, but there is also a large range of salinity tolerance within both types of turfgrasses. In general, selection of cultivars with more salinity tolerance appears to contribute to lower rapid blight disease severity (2, 7, 14).
Trials aimed at identifying fungicides effective against L. terrestris have resulted in three primary chemicals, trifloxystrobin, pyraclostrobin, and mancozeb. Efficacy of these chemicals decreases under severe infection (7, 8, 9, 18). As such, fungicides are best used as a preventative measure or for containing epidemics, rather than as a curative treatment. Contact the county extension office for guidance in determining the appropriate chemical product. As with any chemical application, instructions for product usage should be carefully read and exactly followed.
Diagnostic Procedures
Successful culturing of L. terrestris from infected turfgrass samples, including diseased leaf and root tissues, has been conducted using modified forms Vishniac’s medium. Vishniac’s medium is a selective medium used for isolation marine phycomycetes. The recipe per 1L is as follows: 1g glucose, 1g gleatin hydrolysate, 0.01g liver extract (1:20), 1000ml sea water, 12g agar, 0.1g yeast extract, 0.5g streptomycin sulfate USP, and 0.5g penicillin “G” USP sodium (1). Instructions are as follows: 1) mix all ingredients except for the streptomycin sulfate and penicillin “G”; 2) autoclave; and 3) while still hot, add streptomycin sulfate and penicillin “G” to the medium.
As successful axenic culturing of L. terrestris remains notoriously challenging, further work has been conducted on optimization of the medium used. Research involved comparison of the modified form of horse serum and seawater agar ubiquitously used among research groups studying the pathogen (1, 10) with more optimized forms involving grass extracts and additives similar to Vishniac’s medium. Results indicate a trade-off between colony growth and viability over an extended period of time on the modified serum seawater agar versus the grass extract serum seawater agar. However, it was also demonstrated that media constituency would need to be coupled with amelioration of methods for long-term culturing (21).
Diagnostic features of L. terrestris in culture includes growth of the colonies in a labyrinthine or maze-like, meandering manner and a slimy appearance. These features may be observed either with a: 1) compound microscope using the dark field or phase contrast settings; or 2) dissecting microscope with a light source underneath the culture plate. Aggregation of colonies may appear as hyaline to yellowish globules which could be mistaken as bacterial contamination (1, 6, 11).
Resources and References
1. Bigelow, D.M., M.W. Olsen, and R.L. Gilbertson. 2005. Labyrinthula terrestris sp. nov., a new pathogen of turf grass. Mycologia 97: 185 – 190.
2. Camberato, J.J., P.D. Peterson, and S.B. Martin. 2006. Salinity and salinity tolerance alter rapid blight in Kentucky bluegrass, perennial ryegrass, and slender creeping red fescue. Applied Turfgrass Science Online, DOI 10.1094/ATS-2006-0213-01-RS.
3. Cienkowski, L. 1867. Ueber den Bau und die Entwicklung der Labyrinthuleen. Archiv für mikroscopische. Anatomie 3: 274 – 310.
4. Entwistle, C.A., M.W. Olsen, and D.M. Bigelow. 2006. First report of a Labyrinthula spp. causing rapid blight of Agrostis capillaris and Poa annua on amenity turfgrass in the UK. Plant Pathology 55: 306 – 306.
5. Fuller, M.S., B.E. Fowles, and D.J. McLaughlin. 1964. Isolation and Pure Culture Study of Marine Phycomycetes. Mycologia 56: 745 – 756.
6. Kerrigan, J.L., M.W. Olsen, and S.B. Martin. 2012. Rapid Blight of Turfgrass. The Plant Health Instructor Online, DOI: 10.1094/PHI-I-2012-0621-01.
7. Kopec, D., M.W. Olsen, J.J. Gilbert, D.M. Bigelow, and M.J. Kohout. 2004. Cool-season grass response to rapid blight disease. Golf Course Management 72: 78 – 81.
8. Martin, S.B., L.J. Stowell, W.D. Gelernter, and S.C. Alderman. 2002. Rapid blight: A new disease of cool season turfgrasses. (Abstr.) Phytopathology 92: S52.
9. Martin S.B., M.W. Olsen, P.D. Peterson, and J.J. Camberato. 2004. Evaluation of fungicides for control of rapid blight on cool season grasses. (Abstr.) Phytopathology 94: S66.
10. Olsen, M.W. 2007. Labyrinthula terrestris: a new pathogen of cool‐season turfgrasses. Molecular Plant Pathology 8: 817 – 820.
11. Olsen, M.W., D.M. Bigelow, R.L. Gilbertson, L.J. Stowell, and W.D. Gelernter. 2003. First report of a Labyrinthula sp. causing rapid blight disease of rough bluegrass and perennial ryegrass. Plant Disease 87: 1267.
12. Olsen, M.W., D.M. Bigelow, M.J. Kahout, J.J. Gilbert, and D. Kopec. 2004. Rapid blight: A new disease of cool-season turfgrass. Golf Course Management 72: 87 – 91.
13. Olsen, M.W. and M.J. Kohout. 2006. Isolation of the rapid blight pathogen, Labyrinthula terrestris, from Bermudagrass in Arizona. Super Journal of PACE Turfgrass Research Online, http://www.paceturf.org/PTRI/Documents/051121sj.pdf.
14. Peterson, P.D., S.B. Martin, and J.J. Camberato. 2005a. Tolerance of cool-season turfgrasses to rapid blight disease. Applied Turfgrass Science Online, DOI 10.1094/ATS-2005-0328-01-RS.
15. Peterson, P.D., S.B. Martin, J.J. Camberato, and D.E. Fraser. 2005b. The effect of temperature and salinity on growth of the turfgrass pathogen, Labyrinthula terrestris. (Abstr.) Phytopathology 95.
16. Porter, D. 1969. Ultrastructure of Labyrinthula. Protoplasma 67: 1 – 19.
17. Smiley, R.W., P.H. Dernoeden, and B.B. Clarke. 2005. Compendium of Turfgrass Diseases. 3rd Edition. St. Paul: APS press.
18. Stowell, L.J. and W.D. Gelernter. 2003. Prograss in understanding rapid blight of cool season turf. PACE Insights 9: 1 – 4.
19. Stowell, L.J., S.B. Martin, M.W. Olsen, D.M. Bigelow, M.J. Kohout, P.D. Peterson, J.J. Camberato, and W.D. Gelernter. 2005. Rapid blight: A new plant disease. APSnet Features 10: 2005 – 0705.
20. Watson, S.W. 1957. Cultural and cytological studies on species of Labyrinthula. Ph. D. Thesis, University of Wisconsin.
21. Yadagiri, K.K., J.L. Kerrigan, and S.B. Martin. 2012. Improved methods for axenic culture of Labyrinthula terrestris, causal agent of rapid blight of turfgrasses. Canadian Journal of Microbiology 58: 1230 – 1235.
