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© 2008 Plant Management Network. Influence of Trinexapac Ethyl on the Efficacy of Chlorothalonil and Propiconazole for Control of Dollar Spot on Creeping Bentgrass J. M. Stewart, and R. Latin, Department of Botany and Plant Pathology, Z. Reicher, Department of Agronomy, and S. G. Hallett, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907-2054 Corresponding author: Richard Latin. rlatin@purdue.edu Stewart, J. M., Latin, R., Reicher, Z., and Hallett, S. G. 2007. Influence of trinexapac ethyl on the efficacy of chlorothalonil and propiconazole for control of dollar spot on creeping bentgrass. Online. Applied Turfgrass Science doi:10.1094/ATS-2008-0319-01-RS. Abstract Dollar spot, caused by Sclerotinia homoeocarpa, is a chronic disease of golf turf in the midwestern and northeastern United States, where it is routinely managed with repeated applications of fungicides. Trinexapac ethyl is a plant-growth regulator commonly applied to putting greens and fairways to limit grass clippings and improve turf quality. Field experiments were designed to assess the effects of trinexapac ethyl on dollar spot severity on creeping bentgrass putting greens and fairways, and to determine its influence on the efficacy of two fungicides, propiconazole and chlorothalonil. Research was conducted over three growing seasons where treatments were initiated before (pre-outbreak) and after (post-outbreak) dollar spot symptoms were evident in the experimental plots. In pre-outbreak trials on putting greens and fairways, trinexapac ethyl rarely affected disease progress, and did not consistently improve or diminish fungicide efficacy. In post-outbreak experiments, turf recovery from dollar spot damage following fungicide applications was significantly delayed in some cases in plots treated with trinexapac ethyl. Introduction Dollar spot, caused by Sclerotinia homoeocarpa F. T. Bennett, is a common disease of creeping bentgrass and annual bluegrass on golf courses in the midwestern and northeastern United States. Leaves of infected plants become blighted and die resulting in characteristic straw-colored spots that mar the appearance and adversely affect playability of fine turf (19). Dollar spot is a serious concern for golf course superintendents because the pathogen is active over a broad range of environmental conditions (16). Creeping bentgrass turf damaged by severe outbreaks of dollar spot is then vulnerable to colonization by certain algae and invasion by weed species, especially Poa annua (17). Because of low tolerance for damage on golf turf and the season-long threat of disease development, dollar spot is one of the most pervasive management issues for golf course superintendents, and significant resources are utilized to limit disease severity (2). Although numerous cultural options contribute to a reduction in the dollar spot, golf course superintendents must rely on repeated application of fungicides to avoid unacceptable levels of turf damage. Commonly used fungicides for dollar spot control include chlorothalonil and propiconazole. Chlorothalonil is a preferred contact fungicide because it is effective against a broad spectrum of turf pathogens and is considered to be at low risk for the development of fungicide-resistant pathogen populations (6). Federal restrictions on the use of chlorothalonil limit the amount that can be applied to golf course fairways and putting greens. Propiconazole is a penetrant fungicide that is able to quell existing infections as well as protect against new attacks. Because of its protective and chemotherapeutic properties, propiconazole is generally applied at 21- to 28-day intervals (13). There are numerous reports of resistance to fungicides such as propiconazole because the inhibitor is site-specific (9,12). Both fungicides are routinely included in comprehensive disease management programs for golf courses in the Midwest. Since plant growth regulators (PGRs) were introduced into the turf market in the early 1990s, their use has become a standard turf management practice for golf courses. Although they were initially developed to reduce costs associated with mowing, PGRs also are used to reduce the growth of annual bluegrass where it occurs in mixed stands with creeping bentgrass, to improve color and density of creeping bentgrass, and to suppress annual bluegrass seed-head development (14,18). Trinexapac ethyl is frequently used by golf course superintendents to reduce turf growth on fairways and to improve vigor of creeping bentgrass greens (18). It is classified as a Type A PGR because it reduces cell elongation by interfering with gibberellin biosynthesis (1). Since PGR use has become a conventional turf management practice, it is reasonable to consider whether or not they influence disease development and/or fungicide performance against common diseases of golf course turf. Most of the investigations on the effects of PGRs on disease development involved dollar spot. An early report described improved fungicide efficacy with use of trinexapac ethyl (Primo Maxx) against dollar spot on creeping bentgrass maintained at fairway height in Ohio (10). Several research briefs addressed combinations of trinexapac ethyl and various fungicides for dollar spot control. Most described a neutral effect of trinexpac ethyl on fungicide performance (7,8). A more thorough investigation addressed the interaction of PGRs with fungicides for dollar spot control (3). In this case, the effects of PGRs on fungicide efficacy against dollar spot varied among PGR active ingredients and fungicides for each year of the research. In general it showed that trinexapac ethyl, unlike two other PGRs (flurprimidol and paclobutrazol), is not fungistatic to S. homoeocarpa and was less likely to affect fungicide performance. Zhang and Schmidt (20) reported increased antioxidant (superoxide dismutase) levels in response to trinexapac ethyl applications and suggested that the growth regulator increased host plant resistance to the dollar spot fungus. Research on the influence of PGRs is not limited to their effects on dollar spot development. Results are mixed, but the majority of cases showed that trinexapac ethyl did not influence disease development or fungicide efficacy against Rhizoctonia blight of tall fescue (4). A more recent investigation of the effects of trinexapac ethyl on anthracnose of annual bluegrass demonstrated reduced disease severity where the PGR was applied (11). Some of the previous research was conducted on fairway-height turf, whereas other research focused on turf maintained at putting-green height. Most reports involve research conducted in only a single year. None of the investigations addressed the effects of PGRs, and specifically trinexapac ethyl, on disease progress and fungicide efficacy after outbreaks had already occurred. Since the growth reduction effects of PGRs may delay recovery from disease symptoms, the effects of trinexapac ethyl on post-outbreak disease suppression and restoration of fine turf should be examined. The objective of this research was to conduct a comprehensive study of the effects of trinexapac ethyl on the performance of chlorothalonil and propiconazole against dollar spot on creeping bentgrass greens and fairways, including the effects on fairway-height turf where disease was well established. Assessing Effects of Trinexapac Ethyl on Dollar Spot Severity and Fungicide Efficacy Research was conducted in 2004, 2005, and 2006 on swards of Penncross creeping bentgrass located at the Daniel Turfgrass Research and Diagnostic Center in West Lafayette, IN. Turf was established from seed in 1999 on Stark silt loam with a pH of 6.9 and 5.7% organic matter. Experiments were conducted on sites that were maintained at heights of cut of 12.7 mm and 3.3 mm, typical of golf course fairways and putting greens, respectively. General plot maintenance was similar for each year of the research. Fairway sites were mowed three times weekly and clippings were not collected during these routine mowings. Putting greens were mowed six days weekly throughout the season and clippings were collected and removed from the plots. Fertilizer was broadcast over the fairway and putting green swards at a rate of 146.4 and 170.8 kg of N per ha, respectively, with approximately 70% of the nitrogen applied September through November. Remaining nitrogen was applied equally between the months of April and June. These applications were designed to simulate a standard fertilizer program on golf courses in Indiana where at least 50% of the total nitrogen is water insoluble. Irrigation was applied as needed throughout the course of the experiments to prevent drought stress and to promote conditions favoring the dollar spot pathogen. Azoxystrobin (Heritage 50WG, Syngenta Crop Protection Inc., Greensboro, NC), a fungicide with no efficacy against dollar spot, was applied at a rate of 0.3 kg ai/ha to suppress the development of competitive fungi, specifically the pathogen of brown patch, Rhizoctonia solani Kuhn. Dollar spot was allowed to develop through natural infection. Fungicide and plant growth regulator treatments were applied to field plots using a customized boom sprayer with three Tee-Jet 8004 flat fan nozzles (Spraying Systems Company, Wheaton, IL) that delivered a spray volume of 813.9 liters/ha at 276 kPa. Trinexapac ethyl (Primo Maxx, Syngenta) was applied at a rate of 0.05 kg ai/ha beginning seven days prior to the start of each experimental run. A second application was made 14 days after the first, except in the season long experiments in which the PGR was applied six times at 14-day intervals through the entire season. Chlorothalonil (Daconil Ultrex, Syngenta) and propiconazole (Banner Maxx, Syngenta) were applied at rates of 8.1 and 0.5 kg ai/ha, respectively. Both were applied seven days after the initial trinexapac ethyl treatment as a single application, except in the season long experiments in which they were repeated at 21-day intervals through the course of the experiment. All experiments involved six treatments as described in Table 1. Field plots measured 1 × 2 m and were arranged in a randomized complete block with four replications, except in the post-outbreak experiments which had eight replications. An alleyway (2 m wide) separated the blocks; adjacent plots were separated by 0.5 m of unsprayed turf. The treatments were tested under three regimes defined by turf height (putting green or fairway) and disease status at the time treatments were initiated (pre-outbreak or post-outbreak). For experiments in the pre-outbreak regimes, dollar spot symptoms were absent from plots when trinexapac ethyl treatments were initiated. For the post-outbreak regime, trinexapac ethyl was first applied after dollar spot symptoms were observed in all creeping bentgrass fairway plots. Because of the near-zero tolerance for disease on putting greens at almost all golf courses, a post-outbreak putting green regime was deemed impractical and not included in this research. Table 1. Treatment descriptions and preplanned contrasts for all experiments.
Pre-outbreak putting green experiments. Two experimental trials were conducted per year on putting green sites in 2004 and 2005. Plots for each trial were located on adjacent sites within the same sward of turf. Each year these trials were initiated 14 days apart. In 2004, initial trinexapac ethyl treatments were applied to the putting green sites on May 12 and May 26. In 2005, initial treatments were applied on May 18 and June 1. Fungicide treatments were applied once, approximately seven days after the initial trinexapac ethyl treatment. Disease severity was evaluated three times per week by counting the number of dollar spot infection centers per plot. Counts were recorded on the day of the initial trinexapac ethyl treatment, the day of the fungicide application, and then thereafter at 2- to 3-day intervals for the next three weeks. Counts were used to calculate the area under the disease progress curve (AUDPC) to represent treatment effects over the experimental period (5). Pre-outbreak fairway experiments. Five experimental trials were conducted on fairway sites; two in 2004 and 2005, and one in 2006. Plots for each trial were located on adjacent sites within the same sward of turf. Each year these trials were initiated 14 days apart. Initial trinexapac ethyl treatments were applied on May 12 and May 26 in 2004, on May 18 and June 1 in 2005, and on June 1 in 2006. Fungicide treatments were applied once in each trial, approximately seven days after the initial trinexapac ethyl treatment. The number of dollar spot infection centers per plot were recorded on the day of the initial trinexapac ethyl treatment, the day of the fungicide application, and then thereafter at 2- to 3-day intervals for the next three weeks. Counts were used to calculate area under the disease progress curve (AUDPC) to represent treatment effects over the experimental period (5). Post-outbreak experiments. Initial treatments for the post-outbreak experiments were delayed until dollar spot symptoms were apparent in all plots. Four trials of this experiment were performed; one in 2004 and 2006, and two in 2005. Treatments, experimental design, and plot dimensions were identical to those described above for the pre-outbreak experiments. Initial trinexapac ethyl treatments were applied on August 9 in 2004, on July 26 and August 9 in 2005, and on June 28 in 2006. The second trinexapac ethyl treatment was applied 14 days after the first. As in the pre-outbreak experiments, fungicides were applied once, approximately seven days following the first trinexapac ethyl treatment. Disease severity was assessed by visually estimating the percentage of symptomatic turf in each plot. Severity assessments were recorded on the day of the initial trinexapac ethyl treatment, the day of the fungicide application, and then thereafter at 2- to 3-day intervals over the next three weeks. Disease percentages were used to calculate AUDPC values. Season-long experiments. The influence of trinexapac ethyl on disease severity and fungicide performance was evaluated over a period of three months in 2005 and 2006. In 2005 and 2006, initial trinexapac ethyl treatments were applied on June 1 and May 30, respectively, prior to symptom expression in the plots. Trinexapac ethyl applications were then continued at 14-day intervals. Fungicide treatments were initiated on June 8 and June 6 in 2005 and 2006, respectively, and were repeated at 3-week intervals throughout the remainder of the study. Disease severity was assessed by visually estimating the percentage of symptomatic turf in each plot. Severity assessments were recorded on the day of the initial trinexapac ethyl treatment, the day of the fungicide application, and then at 7-day intervals over the course of the experiment. Disease percentages were used to calculate AUDPC values. Statistical analysis. The same treatment pairs were compared in all experiments as described in Table 1. AUDPC values were transformed to square roots as suggested by Box-Cox analysis (SAS version 9.1, Cary, NC). Results of a Bartlett’s Test (15) for homogeneity of variance determined that experimental trials within years could not be combined and therefore were analyzed separately. Data were subjected to analysis of variance using the General Linear Model procedure in SAS (Proc GLM, version 9.1, SAS Institute Inc., Cary, NC). Orthogonal contrasts were used to examine differences in AUDPC means for the preplanned treatment comparisons. Trinexapac Ethyl and the Efficacy of Chlorothalonil and Propiconazole for Dollar Spot Control Based on AUDPC values, the application of trinexapac ethyl did not significantly affect dollar spot development on creeping bentgrass putting greens, nor did it affect the efficacy of propiconazole (Table 2). The only significant difference in AUDPC associated with trinexapac ethyl occurred in trial 2 in 2004, where chlorothalonil efficacy was increased in plots also treated with trinexapac ethyl (Table 2). Disease progress curves for the planned comparisons in putting green trial 2 in 2004 further describe the treatment effects (Fig. 1). Where untreated plots were compared with plots that received trinexapac ethyl alone, differences occurred on some individual evaluation dates, but over the course of the experiment statistically different effects did not occur. The disease progress curves clearly show the reduction in disease severity associated with chlorothalonil + trinexapac ethyl vs chlorothalonil alone (Fig. 1). Table 2. Dollar spot severity on creeping bentgrass putting greens as influenced by pre-outbreak applications of two fungicides and trinexapac ethyl.
x AUDPC values were calculated from infection center counts and represent the means of four replications. y Preplanned comparisons were tested with orthogonal contrasts where NS = differences were not significant, * = differences were significant at P = 0.05.
Similarly, trinexapac ethyl had little effect on the severity of dollar spot in pre-outbreak experiments on creeping bentgrass fairways based on AUDPC values (Table 3). Disease progress curves for planned comparisons show that trinexapac ethyl increased dollar spot severity in one trial in 2004 (Fig. 2) and decreased the efficacy of control of propiconazole in one trial in 2005 (Fig. 3). However, in 2006 plots treated with trinexapac ethyl alone had significantly less disease than the untreated plots (Fig. 4). In all other relevant comparisons, no significant effects could be attributed to trinexapac ethyl. Table 3. Dollar spot severity on creeping bentgrass fairways as influenced by pre-outbreak applications of two fungicides and trinexapac ethyl.
x AUDPC values were calculated from infection center counts and represent the means of four replications. y Preplanned comparisons were tested with orthogonal contrasts where NS = differences were not significant, * = differences were significant at P = 0.05, and ** = differences were significant at P = 0.01.
In the post-outbreak experiments, trinexapac ethyl reduced the efficacy of propiconazole in 2004 and 2005 and reduced the efficacy of chlorothalonil in one trial in 2005 (Table 4). Disease progress curves for the post-outbreak fairway trial in 2004 illustrate how the propiconazole + trinexapac ethyl treatment delayed recovery from turf damage during the 2-week period following the fungicide application compared to the propiconazole only treatment (Fig. 5). Disease progress curves for the post-outbreak fairway trial in 2005 show that greater AUDPC values in plots treated with fungicide + trinexapac ethyl resulted from an increase in dollar spot severity during the 7 days after the initial trinexapac ethyl application (Fig. 6). Table 4. Influence of post-outbreak applications of fungicides and trinexapac ethyl on the severity on dollar spot epidemics on creeping bentgrass fairways.
x AUDPC values were calculated from infection center counts and represent the means of four replications. y Preplanned comparisons were tested with orthogonal contrasts where NS = differences were not significant, * = differences were significant at P = 0.05, and ** = differences were significant at P = 0.01.
In the season-long experiments, the only significant effect of trinexapac ethyl was found in 2005 when it increased the efficacy of chlorothalonil (Table 5), but this difference was not observed in 2006. From inspection of the disease progress curves (Fig. 7), it appears that the difference has marginal practical importance. Table 5. Influence of trinexapac ethyl on disease severity and fungicide
x AUDPC values were calculated from infection center counts and
y Preplanned comparisons were tested with orthogonal contrasts
Results from the pre-outbreak experiments on putting greens and fairways suggest a net neutral effect of trinexapac ethyl on dollar spot development and fungicide performance. In most cases there were no differences in disease severity among relevant comparisons. In the few cases where differences were observed, treatment with trinexapac ethyl was not consistently associated with an increase or a decrease in disease severity. These results support other published research demonstrating that dollar spot was not suppressed and fungicide efficacy was not affected by the use of trinexapac ethyl (3,7,8). Our conclusions differ from reports suggesting that trinexapac ethyl contributes to a reduction in dollar spot severity (10,20). However, in one report (20) conclusions were based on results from a single experiment and where differences were not constant throughout the experimental period (20). While their observations are certainly valid, and we did observe this effect on a few occasions, differences appear to be the exception rather than the rule. The influence of trinexapac ethyl and fungicides on dollar spot over several years on both putting greens and fairways showed few significant differences, but strong evidence of anything other than a neutral effect was not apparent. In the post-outbreak experiments, disease progress curves reveal the basis for concern over using PGRs when disease is already present In 2004, there was a delay in turf recovery after the propiconazole application in plots also treated with trinexapac ethyl (Fig. 5). This result suggests that diminished turf growth reduced fungicide uptake and limited the suppressive effect of the active ingredient, thereby delaying recovery from the outbreak. Such a delay was not apparent for the chlorothalonil comparisons (Fig. 5), most likely because it is a contact fungicide and therefore not vulnerable to the consequences of reduced uptake into the plant. The significant differences between treatments for both fungicides in 2005 suggest a different interpretation. The disease progress curves show a marked increase in dollar spot severity after the initial trinexapac ethyl application (Fig. 6). For both fungicides greater AUDPC values in plots treated with trinexapac ethyl was more likely due to the high initial disease severity at the time of the fungicide spray than a result of delayed turf recovery. In all other post-outbreak trials, although there were individual assessment dates where differences occurred, trinexapac ethyl had only a negligible effect on overall disease progress. Conclusions were drawn from AUDPC values which tend to be more conservative because treatment effects are described in the context of the entire epidemic rather than appraisals on individual dates. Disease progress curves are presented for all trials where significant differences occurred between treatments based on AUDPC. The expression of results in terms of AUDPC is not common among turf disease research reports, but there is precedent for examining treatment effects on turf disease epidemics in this way (3). Also, AUDPC provides a more comprehensive representation of disease progress than severity estimates at a few evaluation dates (5). Epidemics caused by S. homoeocarpa on creeping bentgrass are somewhat unique because disease severity declines after each fungicide application due to turf recovery. Where fungicide applications are repeated over time, the application intervals essentially represent individual epidemics characterized by a given level of initial inoculum and a weather-dependent infection rate. Our investigation included experiments that involved single and multiple applications of fungicides. Where multiple applications were made, the season-long disease progress curves contain a certain amount of bias associated with different amounts of initial inoculum in plots of treatments that we intended to compare. That bias could be compounded over time, although it did not appear to occur in our experiments (Fig. 7). The advantage of an experiment that follows disease progress through the effective duration of a single fungicide application is that the bias associated with repeated applications is eliminated. Therefore, effects of the treatments (such as trinexapac ethyl) on fungicide performance can be more accurately determined. By conducting the experiments 13 times over three years, the planned comparisons were made under different environmental circumstances and with limited bias from initial inoculum. This research represents a comprehensive investigation of the effects of trinexapac ethyl on the development and control of dollar spot on creeping bentgrass greens and fairways. Significant effects were observed in only a few experimental situations indicating that in most cases trinexapac ethyl did not contribute to an increase or decrease in fungicide efficacy. The significant differences between treatments in the post-outbreak experiments, however, represent a legitimate concern, because superintendents may encounter occasional severe dollar spot outbreaks on fairways and question the continued use of trinexapac ethyl while disease levels remain high. Since we lack consistent evidence that trinexapac ethyl contributes to improved disease control, it may be prudent to suspend such treatments to fairways until turf has fully recovered from the disease-related damage. Given that fungicide performance is influenced by numerous interacting factors including disease pressure, chemical deposition and depletion phenomena, and fungicide sensitivity of the pathogen population, neutral results associated with a single factor should not be unexpected. Journal Series Article Number Purdue University Agricultural Research Programs Journal Series Article No. 2007-18088. Literature Cited 1. Adams, R., Kerber, E., Pfister,K., and Weiler, E. W. 1992. Studies on the action of the new growth retardant CGA 163’935 (Primo). Pages 818-827 in: Progress in Plant Growth Regulation. C. M. Karssen, L. C. van Loon, D. Vreugdenhil, eds. Kluwer, Dordrecht, the Netherlands. 2. Beard, J. B. 1973. Turfgrass: Science and Culture. Prentice-Hall, Englewood Cliffs, NJ. 3. Burpee, L. L., and Stephens, S. L. 1996. Interactive effects of plant growth regulators and fungicides on epidemics of dollar spot in creeping bentgrass. Plant Dis. 1245-1250. 4. Burpee, L. L. 1998. Effects of plant growth regulators and fungicides on Rhizoctonia blight of tall fescue. Crop Prot. 17:503-507. 5. Campbell, C. L., and Madden, L. V. 1994. An Introduction to Plant Disease Epidemiology. John Wiley and Sons Inc., New York, NY. 6. Couch, H. B. 2000. The Turfgrass Disease Handbook. Kreiger, Malabar, FL. 7. Fidanza, M. A., Wetzel, H. C., III, Agnew, M. L., and Kaminski, J. E. 2006. Evaluation of fungicide and plant growth regulator tankmix programs on dollar spot severity of creeping bentgrass. Crop Prot. 25:1032-1038. 9. Golembiewski, R. C., Vargas, J. M., Jones, A. L., and Detweiler, A. R. 1995. Detection of demethylation inhibitor (DMI) resistance in Sclerotinia homoeocarpa populations. Plant Dis. 79:491-493. 10. Golembiewski, R. C., and Danneberger, T. K. 1998. Dollar spot severity as influenced by trinexapac ethyl, creeping bentgrass cultivar, and nitrogen fertility. Agron. J. 90:466-470. 11. Inguagiato, J. C., Murphy, J. A., Park, B. S., Lawson, T. J., and Clarke, B. B. 2005. Anthracnose on annual bluegrass affected by nitrogen, plant growth regulators, and verticutting. Pagers 26-27 in: Proc. of the 14th Ann. Rutgers Turfgrass Symp., 2005. Center for Turfgrass Science at the Rutgers New Jersey Agricultural Experiment Station 12. Jo, Y-K, Niver, A. L., Rimelsbach, J. W., and Boehm, M. J. 2006. Fungicide sensitivity of Sclerotinia homoeocarpa from golf courses in Ohio. Plant Dis. 90:807-813. 13. Latin, R. 2006. Residual activity of fungicides for control of dollar spot on creeping bentgrass. Plant Dis. 90:571-575. 14. Lickfeldt, D. W., Gardner, D. S., Branham, B. E., and Voigt, T. B. 2001. Implications of repeated trinexapac ethyl applications on Kentucky bluegrass. Agron. J. 93:1164-1168. 15. Neter, J., Wasserman, W., and Kutner, M. H. 1985. Applied Linear Statistical Models, 2nd Edn. Irwin Press, Homewood, IL. 16. Smiley, R. W., Dernoeden, P. H., and Clarke, B. B. 2005. Compendium of Turfgrass Diseases, 3rd Edn. American Phytopathological Society, St. Paul, MN. 17. Smith, J. D., Jackson, N., and Woolhouse, A. R. 1989. Fungal Diseases of Amenity Turf Grasses, 3rd Edn. E. and F. N. Spon. New York, NY. 18. Turgeon, A. J. 2005. Turfgrass Management. Pearson Prentice Hall, Upper Saddle River, NJ. 19. Walsh, B., Ikeda, S. S., and Boland, G. J. 1999. Biology and management of dollar spot (Sclerotinia homoeocarpa); an important disease of turfgrass. HortScience 34:13-21. 20. Zhang, X., and Schmidt, R. E. 2000. Application of trinexapac ethyl and propiconazole enhances superoxide dismutase and photochemical activity in creeping bentgrass. (Agrostis stolonifera var. palustris). J. Am. Soc. Hortic. Sci. 125:47-51. |
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