Tar spot on seashore paspalum in Georgia

Tar spot has been identified for the first time on seashore paspalum turfgrass at the University of Georgia.

Alfredo Martinez-Espinoza, Ph.D.; Omar Martinez-Uribe; and Dae Kim

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Seashore paspalum (Paspalum vaginatum) is a warm-season grass native to tropical and subtropical regions of North and South America. In the United States, it is found in the coastal regions of Texas, Florida, Georgia, South Carolina and North Carolina. With the exception of one commercially available seeded cultivar, seashore paspalum propagates via rhizomes and stolons. Seashore paspalum has been growing in popularity for use on golf course tees, fairways and greens because of its high tolerance to saline soils and water (3,8).

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Figure 1. Tar spot lesions, which were black to dark brown with
yellow to light brown tissue surrounding the spots, were discovered
on seashore paspalum plants in a greenhouse at the
University of Georgia in Griffin, Ga.
Photos by Alfredo Martinez

Symptoms on plant tissue

Tar spot was first observed on seashore paspalum turfgrass plants in late 2010 at a research greenhouse on the campus of the University of Georgia in Griffin, Ga. Symptoms of the disease included what were later determined to be tar spot lesions, which were black to dark brown with yellow to light brown tissue surrounding the spot (Figure 1). The spots were small and compact, and most were circular to slightly oval. Some spots coalesced and formed a short delineated strip (Figure 2). Spots were located mainly on the tops of the leaves, with some discoloration on the back of the leaves. However, tar spots did not reach the back tissue of the leaves. These observations are consistent with tar spot disease observed in other turfgrasses, and in other crops, where lesions are dark, small, compacted and located on the upper part of the leaves (Figures 1,2) (1,2,4-7,9-11). Clypei. Light and electron microscope observations conclusively showed that the lesion on the paspalum plants was made up of a shield-like structure called the clypeus (plural clypei) (4,7,10,12). The clypei are oval in shape with a slightly raised center and seem to push through the epidermis of the plant tissue, dramatically changing the structure and anatomy of the plant epidermis. The size of clypei ranges from 170 micrometers × 152 micrometers to 203 micrometers × 495 micrometers.

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Figure 2. Some of the tar spots coalesced to form a continuous strip on the grass blade

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Figure 3. An electron microscope photo shows an ascocarp, which forms a nest-like or spherical supportive structure for asci, the bean-pod-like structures that contain the ascospores by which the tar spot fungus reproduces.

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Figure 4. A light compound microscope photo shows an ascus containing eight ascospores arranged in a single row.

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Figure 5. An electron microscope photo with a close-up of ascospores, the fungal spores from the tar spot fungus.

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Figure 6. The reproductive spores (ascospores) of the tar spot fungus are seen breaking through the tissue of an infected seashore paspalum plant to infect other plants.

Ascocarps and asci. An ascocarp is a nest-like or spherical structure that contains the asci (plural of ascus), bean-pod-shaped structures that are the sexual spore-bearing cells produced by ascomycete fungi (12) (Figure 3). When seen under an electron microscope, the asci are cylindrical and arranged in a palisade formation with vegetative tissue dispersed throughout the ascocarp. The asci range from 7 to 8 micrometers in width × 48 to 51 micrometers in length.

Ascospores. An individual ascus containing eight ascospores (fungal spores) is shown in a light microscope photo (Figure 4). The ascospores are uniseriate, that is, they are arranged one by one in a single row (12). The ascospores also have a smooth surface and are devoid of any ornamentation (Figure 5). Their shape is ellipsoidal, and many of them have a slightly conical or parabolic end. No division, separation or any other structure was observed. Ascospore size ranges from 5.7 to 7 micrometers in width × 10 to 11 micrometers in length. In an electron microscope photo, masses of ascospores are shown oozing through small ruptures or holes in the clypei; ascospore release appears to be limited to the inter-vein sections of the leaf (Figure 6).

The fungal organism can only grow on living tissue. After evaluating several methods of infecting the turfgrass and determining the environmental conditions conducive to infection, we collected grass clippings from the infected source (initial infection was observed in the cultivar SeaIsle 2000). Infected grass clippings were placed on healthy seashore paspalum turfgrass grown in plastic pots in the greenhouse. Greenhouse temperatures were kept at 77 F-80 F (25 C-27 C) and high relative humidity.

Conclusions

A careful visual evaluation of physical symptoms was conducted. Plants were inspected for size, shape, color, appearance, distribution, quantity and location of le- the lesions and observed under a compound microscope and under electron scanning microscopy. Visual observations, light microscopy and electron microscopy yielded conclusive evidence of the pathogen infection and symptoms.

The process was replicated at least four times. Healthy plants of the cultivars SeaIsle 2000, Aloha, SeaIsle Supreme and an experimental line (106L-1) were successfully infected with tar spot. All the inoculated pots were infected, but the incidence of the infection was rather low (mean of 5%).

Infection tests

Clypei, ascocarp, asci and ascospore morphology and measurements corresponded exactly to those described for Phyllachora paspalicola (also known as Phyllachora vaginata) (1,2,4-7,9-12).

Healthy grass was infected with P. paspalicola isolates, fulfilling Koch’s postulates and demonstrating the causal agent of the disease. Therefore, we propose that the causal agent of tar spot in seashore paspalum is P. paspalicola. To our knowledge this was the first report of tar spot on seashore paspalum in Georgia.

The research says

 
  • Visual observations, light microscopy and electronic microscope techniques yielded conclusive evidence of the pathogen infection and symptoms.
  • Fungal morphology and measurements corresponded exactly to those described for Phyllachora paspalicola.
  • Healthy grass was infected with P. paspalicola isolates, demonstrating that P. paspalicola is the causal agent of tar spot in seashore paspalum.

 

Acknowledgments

The material in this article was adapted from Martinez-Espinoza, A.D., Martinez- Uribe, O. A., and Kim, D. 2012. Identification and characterization of tar spot on seashore paspalum in Georgia. (Abstr.) Phytopathology 102(Suppl.):S4.76, and was also taken from the accompanying poster as presented at the 2012 Annual Meeting of The American Phytopathological Society, Providence, R.I., Aug. 4-8, 2012.

Literature cited

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Alfredo Martinez-Espinoza, Ph.D., is an associate professor in plant pathology at the University of Georgia-Griffin; Omar Martinez-Uribe is a student at the University of Georgia, Athens, Ga.; and Dae Kim is a student at Georgia State University, Atlanta.