Bermudagrass Decline

Getting to the Root of the Problem

By Matthew Aaron Tucker

Bermudagrass decline (BD), is caused by the pathogen Gauemannomyces graminis var. graminis. It was first reported by Dr. Monica Elliot, University of Florida, in 1991. BD is a destructive disease of ultradwarf bermudagrass (UDB) putting greens. Symptoms begin as yellowing areas of turf, typically along the margins of putting greens. As the disease progresses, yellowing areas will become thin and in extreme cases, lead to turf loss (Figs. 1 and 2). Turf symptoms of BD become apparent in late summer and may progress into early fall (August – October). If symptoms persist into dormancy, scarring (lack of turf density) may appear during spring green-up. Affected roots of bermudagrass plants appear black and brittle (Fig. 3).

Figure 1: Symptoms of bermudagrass decline (BD ) on an ultradwarf bermudagrass putting green. Symptoms of BD occur along the margins of putting surfaces and, in early stages, show a general thinning of the turf. It is common to observe algae growing in thinned areas where canopy density has decreased and can serve as an indicator of BD .

Figure 2: Turf loss caused by bermudagrass decline (BD ). In this ultradwarf bermudagrass putting green, BD has caused large areas of turf loss. What was once a general thinning has progressed to total loss of turf and is extremely problematic to the superintendent. The current best option to correct this issue is to replace areas of turf loss.

Figure 3: Ectotrophic root-infecting fungi damage ultradwarf bermudagrass roots. Symptoms consist of black, necrotic roots affected at the crown of the plant. These roots are dysfunctional and restrict the movement of nutrients and water resulting in loss of turf density.

G. graminis var. graminis belongs to a group of ectotrophic root-infecting (ERI) fungi made up of soil-borne fungi that colonize the outer surface of roots. These fungi produce dark runner hyphae, growth cessation structures (a structure that is formed when fungal growth has slowed or ceased), and simple or lobed hyphopodia (infection structure of the fungus). Twenty-four years after the discovery of G. graminis var. graminis, Phillip Vines, working under the direction of Dr. Maria Tomaso-Peterson in Turgrass Pathology at Mississippi State University, identified and characterized several novel species of ERI fungi that were virulent to UBD. Species included an undescribed Gauemannomyces sp., Magnaporthiopsis cynodontis, and a novel genera/ species, Candidacolonium cynodontis. Vines found that in terms of virulence, C. cynodontis was the most virulent to UDB, followed by G. graminis var. graminis, and finally G. sp. and M. cynodontis were weakly virulent to UDB. Vine’s work showed these novel ERI fungi form a complex with G. graminis var. graminis and together can potentially cause significant root decline in UDB putting greens. Moving forward with these results, we conducted site specific sampling on two putting greens to determine if this ERI complex existed, and if so, to show the spatial distribution of these ERI fungi.

Our Research

Mr. Pat Sneed, Golf Course Superintendent at Mississippi State University Golf Institute (MSUGI), was gracious enough to provide us with two greens to conduct our research. One green was symptomatic for BD while the other green was asymptomatic (Fig. 4). The first step was to establish a grid in mapping software. Each point location within the grid represented an area of interest (AOI) which covered 30 ft 2 (Fig. 5). Soil cores, generated from a summer core aerification, were collected from each area (Fig. 6). Root material from each area was processed for total genomic DNA extraction. To identify the ERI fungi associated with BD and root rot, a method called quantitiatve PCR was used to identify four fungal pathogens and associated root material. As a result, we were able to determine if these ERI fungi were present in each root sample, and if so, at what proportion of fungal DNA to plant DNA occurred on average per green. This data was then used to create a spatial distribution map of each fungus within both greens.

Figure 4: Greens 12 and 2 located at Mississippi State University Golf Institute and used in this study. Green 12 is symptomatic for bermudagrass decline and shows areas of turf loss along the margin of the green.

Figure 4: Greens 12 and 2 located at Mississippi State University Golf Institute and used in this study. Green 2 is asymptomatic for bermudagrass decline and shows no areas of visible turf loss on the putting surface.

Figure 5: Shapefiles created using ArcMap for both greens. This image shows areas of interest (AOI s) for both greens using a fishnet grid. Green 12 (above) includes 66 AOI s.

Figure 5: Shapefiles created using ArcMap for both greens. This image shows areas of interest (AOI s) for both greens using a fishnet grid. Green 2 (above) includes 72 AOI s.

Figure 6: Summer core aerification sampling method. Displayed in this image is a summer core aerification organized into areas of interest (AOI s) in green 12. In order to organize each AOI , the green was aerified and point locations were marked with surveyor’s flags. A 25 ft 2 PV C grid was used to differentiate each AOI without overlap into neighboring areas. Cores from each AOI were then collected and taken back to the lab for further processing.

Results

The ERI fungal complex was identified in many AOIs and tended to be aggregated within each green. All root samples on the symptomatic green had at least one or more ERI fungi identified; whereas the asymptomatic green had AOIs where no ERI fungi were present. M. cynodontis had the greatest frequency of occurrence in both greens; however, it had the lowest ratio of fungal to plant DNA. In the symptomatic green, G. graminis var. graminis occurred in 75% more AOIs than G. sp. with 120 times more fungal DNA than G. sp. In contrast, G. sp. occurred in 50% more AOIs than G. graminis var. graminis in the asymptomatic green; however, the quantity of fungal DNA for both was less than 1:1 fungal to plant DNA ratio. C. cynodontis was more predominant in the asymptomatic green but its fungal to plant DNA ratio was numerically less as compared to the symptomatic green where C. cynodontis occurred in less than 25% of AOIs.

These results provided a spatial distribution of the ERI fungi within greens that were both symptomatic and asymptomatic for BD (Figs. 7–9 on pages 20–21). In addition, turf health data was collected for each AOI which included visual turf quality ratings, normalized difference vegetation index ratings, and root architecture.

Future Research and Application

Future research will anlayze results to determine if a significant relationship exists between agronomic factors and each ERI fungus or if there is a pathogen-pathogen interaction that may affect the distribution of these ERI fungi. All in all, a complex of these ERI fungi was identified in both symptomatic and asymptomatic greens. A distribution of fungal presence was determined for each fungus within both greens. An application of this research would be for superintendents to use these methods to sample greens of interest and send the samples to a diagnostic lab to determine ERI fungal distributions. In understanding the distribution of ERI fungi within a green, superintendents can focus on areas where fungal populations are present. This would introduce the option of a spot spray fungicide program for greens of interest which will reduce fungicide inputs and promote sustainability and environmental stewardship.

Figure 7: Green 12 ectotrophic root-infecting (ERI ) fungal distribution maps for each fungus. Each map represents an ERI fungus showing the distribution and frequency within the green symptomatic for bermudagrass decline. Gauemannomyces sp. (G. sp.), Gaeumannomycesgraminis var. graminis(Ggg), Magnaporthiopsis cynodontis (Mc), and Candidacolonium cynodontis (Cc).

Figure 8: Green 2 ectotrophic root-infecting (ERI ) fungal distribution maps for each fungus. Each map represents an ERI fungus showing the distribution and frequency within the asymptomatic green. Gauemannomyces sp. (G. sp.), Gaeumannomyces graminis var. graminis(Ggg), Magnaporthiopsis cynodontis (Mc), and Candidacolonium cynodontis (Cc).

Figure 9: Total distribution maps representative of all four fungi in both the green symptomatic for bermudagrass decline (left) and theasymptomatic green (right). Gauemannomyces graminis var. graminis (Ggg)in yellow, Gauemannomyces sp. (G. sp.) in blue, Magnaporthiopsiscynodontis (Mc) in orange, and Candidacolonium cynodontis (Cc) inpurple. Three important conclusions from this information are: 1. Acomplex of the four ectotrophic root-infecting (ERI ) fungi of interestconfirms Vines’ research in 2015. 2. The ERI complexes are present inboth symptomatic and asymptomatic greens. 3. The green symptomatic forbermudagrass decline (left) showed a greater occurrence of G. graminisvar. graminis when compared with the asymptomatic green (right).

Matthew Aaron Tucker is a graduate research assistant in the department of Biochemistry, Entomology, and Plant Pathology at Mississippi State University. He studies under the direction of Dr. Maria Tomaso-Peterson. He received his undergraduate degree in Agronomy and Soils while studying Golf Course and Sports Turf Management. He is an avid sports fan, loves his wife and dog, and is a member of Mississippi State University’s disc golf team.

References

Elliott, M.L. 1991. Determination of an etiological agent of bermudagrass decline. Phytopathology. 81:1380 –1384.

Vines, P.L. 2015. Evaluation of ultradwarf bermudagrass cultural management practices and identification, characterization, and pathogenicity of ectotrophic root-infecting fungi associated with summer decline of ultradwarf bermudagrass putting greens. MS Thesis. Mississippi State University.