Off-type grasses in ultradwarf bermudagrass greens

A) Off-type grasses (lighter in color and noted by red arrow) present in an ultradwarf bermudagrass putting green. B) Close-up of an off-type grass (noted by red arrow) present in an ultradwarf bermudagrass putting green. (Photo: Eric Reasor)

Golf course superintendents have reported issues with off-type grasses in ultradwarf bermudagrass (Cynodon dactylon x C. transvaalensis) putting greens over the past several years. These “weedy” grasses have disrupted putting green aesthetics and surface uniformity (Figure 1).

Fundamentally, off-type grasses are defined by differences in morphology and performance when compared to the surrounding desirable turfgrass. Furthermore, a genetic distinction does not have to exist for a grass to be considered an off-type. Leaf texture, turfgrass color and density as well as response to management practices are reported differences among off-types and desirable cultivars.

Ultradwarf green history

It’s important to understand the history and development of ultradwarf bermudagrass putting greens to better understand the issue with off-type grasses.

Ultradwarf cultivars such as Champion, MiniVerde and TifEagle were selected from natural or induced mutations of older cultivars (i.e., Tifgreen 328 and Tifdwarf). These selections primarily were based on desirable differences in morphology and performance when compared to the surrounding turfgrass. Therefore, ultradwarf cultivars were once desirable off-type grasses.

The propensity of off-type development in bermudagrass putting greens is largely unknown. Tifgreen and Tifdwarf were reported to be genetically unstable compared to Tifway; however, the origin of this genetic instability is unknown. Several hypotheses concerning genetic instability have been summarized, but none have been confirmed by research. Regardless of the origin, off-types were a problem in older bermudagrass putting green cultivars and continue to be an issue in current ultradwarf cultivars.

Off-type morphology and genetics

We conducted research from 2013 to 2017 at the University of Tennessee to better understand off-type morphology and genetics. In the first step of this research, we built an off-type collection. Beginning in summer 2013, we harvested 52 samples from 21 golf courses in Alabama, Arkansas, Florida, Mississippi, South Carolina and Tennessee. The superintendent at each golf course determined the difference between an off-type and a desirable grass. It’s important to note that off-types were found in Champion, MiniVerde and TifEagle putting greens.

Photographs of samples representative of each morphological cluster. Grasses in cluster 1 had significantly longer internode lengths than those in clusters 2 and 3. Grasses in cluster 3 had significantly longer leaves than those in clusters 1 and 2. (Photo: Eric Reasor)

The next step was to characterize the morphology of sampled bermudagrasses. We measured internode length and stolon diameter between the third and fourth node and quantified leaf length and width using the outer leaf from the third node. All measurements were made using digital calipers.

We analyzed the morphological data using a cluster analysis, with the goal of grouping these grasses according to the variability among samples. This analysis grouped all the measured samples into three distinct morphological clusters (Figure 2).

Each morphological cluster included off-type and desirable samples. Internode length and leaf length were statistically different among the three clusters, whereas stolon diameter and leaf width were similar. Figure 2 contains a representative sample from each morphological cluster. Internode and leaf lengths varied greatly among desirable and off-types as well as grasses measured during other experiments. This is an indication of the amount of morphological variability that can occur in an individual putting green, both golf course to golf course and cultivar to cultivar. Moreover, differences in internode and leaf length within the same putting surface can lead to decreased turfgrass density and reductions in putting surface quality and playability.

Inconsistencies and diversity

Multidimensional scaling plot (MDS) of nucleotide variants from 47 off-type and desirable bermudagrasses sampled from golf course putting greens, six bermudagrass cultivars (Champion, MiniVerde, Tifdwarf, TifEagle, Tifgreen and Tifway) and two progenitor species (Cynodon dactylon and C. transvaalensis). The asterisk and box note the region with the majority of off-type and desirable samples from golf course putting greens and ultradwarf cultivars and samples in this region are considered genetically similar. (Photo: Eric Reasor)

The inconsistencies of morphological measurements among bermudagrasses used on putting greens suggest genetic techniques may be more accurate in evaluating the diversity of these grasses. Several methods (i.e., DAF, AFLP and SSR) have been used to explore the genotypic differences among bermudagrass cultivars and off-types. Most previous experiments have shared the same conclusion: Cultivars within the Tifgreen-derived family are not readily distinguished from one another.

One molecular marker method, genotyping-by-sequencing (GBS), had not been used. This method is capable of efficiently identifying large numbers of single-nucleotide (DNA base) variants for genetic comparison. Based on the robustness and its successful use in other grasses (switchgrass, wheat and barley), we hypothesized that GBS may be able to identify genetic variation among off-types and desirable ultradwarf cultivars.

We included samples of Champion, MiniVerde, Tifdwarf, TifEagle and Tifgreen as standards in the analysis, along with off-type samples from putting greens. The GBS results were surprising because most samples harvested from golf courses clustered with the standard cultivars (Figure 3).

This clustering suggests that these samples were similar genetically to those cultivars. Only five (~11 percent) of the 47 unknown off-type samples were genetically divergent from the standard cultivars. Furthermore, this method failed to identify Tifgreen and Tifdwarf from the ultradwarf cultivars. Our results using GBS were similar to previous molecular genetics research, which also failed to readily distinguish among ultradwarf cultivars and most off-type grasses.

So why did most of the grasses included in our experiment (and others) exhibit variable morphological characteristics while being genetically similar?

The majority of bermudagrass putting green cultivars were selected from other bermudagrass cultivars, and the off-types studied herein also were selected from existing plantings. The difference in morphology could be driven by differential gene expression influenced by environment or management practices. The intense management practices implemented on ultradwarf putting greens could result in the up-or-down regulation of genes that control important turfgrass characteristics (i.e., internode and leaf length). However, no research has been conducted to explore this hypothesis.

PGR research on off-types

Response of three ultradwarf bermudagrass cultivars and three off-type grasses 28 days after initial treatment (DAIT) following trinexapac-ethyl application (12 fl oz/acre Primo MAXX). An off-type of morphological cluster 1 (OTC1), 2 (OTC2) and 3 (OTC3) are included with Champion, MiniVerde and TifEagle cultivars. (Photo: Eric Reasor)

Anecdotal observations during off-type sampling suggests the off-types and ultradwarf cultivars respond differently to plant growth regulator (PGR) applications. Therefore, the next step in this research was to evaluate response of three off-types and ultradwarf cultivars (Champion, MiniVerde and TifEagle) to trinexapac-ethyl applications. One off-type was selected from each distinct morphological cluster previously mentioned. The grasses were established in greenhouse culture in a sand/peat mix meeting USGA root-zone specifications. Trinexapac-ethyl (Primo MAXX, Syngenta) was applied at rates of 0, 0.1875, 0.375, 0.75, 1.5, 3, 6 or 12 fl oz/acre. Daily clipping was suspended at the time of trinexapac-ethyl application and growth above one centimeter was harvested every 7, 14, 21 and 28 days.

Primo MAXX applied at 3, 6 or 12 fl oz/acre led to similar growth suppression. However, the six tested grasses responded differently. Off-types from cluster 1 were less sensitive to trinexapac-ethyl and produced 21 percent to 27 percent more clippings than Champion and MiniVerde but was similar to TifEagle (Figure 2). In addition, the three ultradwarf cultivars had a similar response across all trinexapac-ethyl rates. Off-types present in putting greens similar to off-types from cluster 1 have the potential to disrupt the functional and aesthetic characteristics because of differential susceptibility to trinexapac-ethyl.

Prohexadione-calcium is a PGR active ingredient with a similar mode of action as trinexapac-ethyl. Based on golf course observations, prohexadione-calcium has the potential to help manage off-types in putting greens. A second PGR experiment evaluated the response of the same six grasses in greenhouse culture to applications of trinexapac-ethyl (Primo MAXX) and prohexadione-calcium (Anuew, Nufarm). The six grasses were treated with Anuew at 6 oz./acre, Primo MAXX at 3 fl oz/acre or Anuew+Primo at 6 and 1 oz./acre. Clipping yield was measured using methods similar to the previous experiment.

Application of Anuew at 6 oz./acre resulted in less growth regulation than an application of Primo MAXX alone (3 fl oz/acre). However, an application of Anuew (6 oz./acre) combined with Primo MAXX (1 fl oz/acre) yielded similar growth regulation compared to Primo alone and minimized the rebound effect that occurred after growth regulation subsided. The lower rebound growth associated with Primo MAXX and Anuew combinations may aid in management of off-types. This response is likely to help balance the growth between off-types and ultradwarf cultivars. Moreover, this response may explain anecdotal improvements in putting green quality observed by golf course superintendents following applications of Anuew+Primo.

Off-type management using PGRs

The off-types sampled in this research had varying morphology compared to ultradwarf cultivars Champion, MiniVerde and TifEagle. Despite these differences, most off-types were genetically similar to ultradwarf cultivars. Certain off-types are less sensitive to trinexapac-ethyl applications compared to ultradwarf cultivars, and this can make managing growth rate difficult in putting greens. The off-type characteristics associated with lower trinexapac-ethyl sensitivity are long internode and leaf length as well as a lighter green when compared to ultradwarf cultivars. Therefore, it’s important to have an idea of the off-type present in the putting green.

Two information-gathering methods can better identify off-types present in ultradwarf putting greens. One method is to collect an off-type and desirable sample and grow them at your facility without clipping. You can observe and measure morphology as the samples grow. The second method is to send samples to the University of Tennessee Weed Diagnostic Center (www.weeddiagnostics.org). This service provides an off-type assessment to morphologically characterize off-type and desirable samples. Knowledge of the specific off-type at the facility can further refine PGR management of this issue.

The first PGR consideration for off-type management is to not apply more than 3 fl oz/acre Primo MAXX in a single application. Our research suggests there is no benefit when off-types are present in ultradwarf putting greens. Another consideration is to apply Anuew in combination with Primo MAXX. The addition of Anuew with Primo MAXX lessened the rebound growth of off-type grasses, which likely will help balance the overall growth rate among off-types and ultradwarf cultivars.

This PGR and off-type research was conducted in greenhouse culture on bermudagrass samples not maintained at golf course putting green mowing heights, and results of this research need to be confirmed in the field. However, this initial research provides insight on how PGRs have the potential to lessen the impact of off-types in ultradwarf bermudagrass putting greens.

Eric H. Reasor, Ph.D., is a turfgrass scientist and the Southeast research scientist for PBI-Gordon.

Acknowledgements

I would like to thank Jim Brosnan, Ph.D., John Sorochan, Ph.D., Robert Trigiano, Ph.D., Gerald Henry, Ph.D., Brian Schwartz, Ph.D., Greg Breeden, Javier Vargas, Sarah Boggess and Phil Wadl for their assistance with this research. I would also like to thank the University of Tennessee Institute of Agriculture, the United States Golf Association and the Memphis Area Chapter of Golf Course Superintendents Association for supporting this research.

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