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Key Takeaways

  • Autumn applications of fluazinam, or fluazinam and propiconazole, consistently delayed dollar spot onset the following spring, while propiconazole alone did not. 

  • High initial pathogen population (load) reduced the effectiveness of autumn-applied fungicides for delaying disease onset the following spring, especially when conditions were favorable for disease.

  • Low initial pathogen load in autumn delayed the onset of dollar spot symptoms the following spring by two weeks compared to the high initial inoculation, even on the nontreated control.

  • The fungicide chemistry used and the timing of autumn applications is more important for fairways managed with low inputs that have a higher initial pathogen load headed into autumn.

  • Other research to identify the best timing of autumn applications is underway; however, this research on three autumn applications of fluazinam delayed spring disease onset enough to potentially eliminate one or two early season fungicide sprays and suggests significant cost savings if autumn fungicide applications are already part of the management program.
     

Dollar spot (Clarireedia jacksonii) is a destructive disease that affects both cool- and warm-season turfgrasses throughout the world. Multiple fungicide applications are often required to maintain adequate control of this disease on golf courses during the growing season. Some research studies have suggested that fungicides such as chlorothalonil, fluazinam, propiconazole and triadimefon applied the previous year may influence dollar spot progress the following season, however, the effects were not always consistent and little is known about the impact of fungicide timing and chemistry on this phenomenon (Burpee et al., 1990; Niver, 2005; Koenig, 2009; Koch & Kerns, 2013; Groben, 2023; Zhang, 2024).

Observations from our previous research suggested that a cleanup application of fluazinam and propiconazole at the end of the year (autumn) delayed the onset of dollar spot the following spring on creeping bentgrass (Agrostis stolonifera L.). Therefore, from 2021 to 2023, the USGA’s Davis Program for Advancing Golf Course Management funded research at Rutgers University to evaluate the impact of autumn-applied fungicide chemistries on dollar spot onset and development on creeping bentgrass the following growing season. If using a particular fungicide or combination of fungicides as part of normal fall disease management results in significantly delayed onset of dollar spot the following spring, that could lead to cost savings for golf courses.

Investigation Methods

Two field trials were conducted on creeping bentgrass managed as fairway turf in North Brunswick, New Jersey. Both trials were initiated in September and concluded in July of the next calendar year (2021-2022 and 2022-2023). Each trial evaluated the factor of fungicide chemistry. The four levels of fungicide chemistry (and specific products) evaluated were:

  1. Fluazinam (Secure) applied at 0.71 pounds of active ingredient per acre, or 0.5 fluid ounces of product per 1,000 square feet.

  2. Propiconazole (Banner Maxx II) applied at 0.44 pounds of active ingredient per acre, or 1.0 fluid ounces of product per 1,000 square feet.

  3. A tank mixture of fluazinam and propiconazole applied at 0.71 and 0.44 pounds of active ingredient per acre, respectively.

  4. Nontreated control.
     

Fungicides were applied three times from late September to early November at a 21-day interval to the creeping bentgrass cultivar ‘007’ – which has been reported to have moderate resistance to dollar spot (Koch et al., 2009; Weibel et al., 2022).

The second factor – the initial pathogen load prior to the initiation of fungicide applications in autumn – was included in the second trial (2022-2023) to simulate differences in dollar spot pathogen load that may develop on fairways at low- and high-input (i.e., budget) golf courses, and how this might influence the effect of autumn-applied fungicide on dollar spot onset and development the following growing season. This was achieved by applying two different fungicide programs prior to the initiation of the autumn fungicide treatments. To develop the high initial pathogen load, chlorothalonil (Daconil Ultrex), a fungicide with short residual activity, was applied from July to mid-September on a curative basis to suppress severe dollar spot outbreaks. The low initial pathogen load was established by alternating applications of two highly efficacious fungicides, mefentrifluconazole (Maxtima) and fluazinam, during the same period on a preventive basis to maintain disease-free turf. It’s worth noting that fluazinam is the only Fungicide Resistance Action Committee (FRAC) Group 29 fungicide (C5: uncoupler of oxidative phosphorylation). The high initial pathogen load measured approximately two weeks prior to the initiation of autumn fungicide treatments using the qPCR method developed at Rutgers University, was found to be approximately 100 times higher than the low initial pathogen load (Groben et al., 2020).

All treatment combinations in both trials were evaluated for their impact on the onset and severity of dollar spot during the spring and early summer of the following growing season. A delay in disease onset was measured as the number of days between the first appearance of disease on any plot in the trial and the onset of disease in each of the other plots.

Results: First Trial (2021-2022)

Applying propiconazole in autumn did not delay dollar spot onset the following spring compared to the nontreated control. However, the fluazinam and the fluazinam and propiconazole tank mixture applications during autumn delayed the onset of dollar spot by six and nine days, respectively, compared to the nontreated control (Figure 1). Adding propiconazole to fluazinam did not significantly enhance the delay in onset compared to fluazinam applied alone. Although reduced sensitivity to demethylation inhibiting (DMI) fungicides such as propiconazole has not been reported at the study site, it is possible that the rate of propiconazole (1.0 fluid ounces per 1,000 square feet) in the two trials was not high enough to significantly delay disease onset the following spring, even though this fungicide has been shown to effectively control dollar spot at the site when applied during the growing season at a wide range of label rates (0.5-2.0 fluid ounces per 1,000 square feet). Therefore, while fluazinam was effective at delaying onset, propiconazole may have had a similar effect if used at a higher rate, but additional research would be needed to evaluate this possibility.

Results: Second Trial (2022-2023)

All factors evaluated in the second trial – fungicide chemistry and the initial pathogen load – affected the onset of dollar spot. Like the first trial, fungicides applied in autumn delayed disease onset; however, the specific fungicide treatment effects depended on the initial pathogen load in autumn.

Disease pressure was lower in the second trial during the onset period compared to the first trial, which resulted in a longer delay overall in dollar spot onset on fungicide-treated plots. Under the high initial pathogen load, applying propiconazole three times the previous autumn delayed the onset of dollar spot on ‘007’ creeping bentgrass by 17 days the following spring, whereas three applications of fluazinam delayed onset by 30 days and the fluazinam and propiconazole tank mixture by 31 days (data not shown). Thus, propiconazole alone was least effective at delaying onset and adding propiconazole to fluazinam did not significantly enhance the delay in onset compared to fluazinam applied alone.

Interestingly, when the pathogen load was low in autumn, dollar spot symptoms on the nontreated control developed 14 days later the next spring compared to the high pathogen load (data not shown). In addition, all fungicide treatments applied to turf with a low pathogen load delayed disease onset by an average of 17 (14-20) days and resulted in no significant difference in onset among chemistries. Thus, the selection of fungicide chemistry in the autumn would appear to be more important for golf courses with a low-input management strategy, which would likely lead to a higher initial pathogen load headed into autumn. Treatments that delayed disease onset also resulted in slower disease development into summer, as illustrated in Figure 2.

Implications for Golf Courses

When fluazinam or the fluazinam and propiconazole tank mixture was applied from late September to early November on a moderately resistant cultivar ‘007’, dollar spot onset the following spring was delayed up to four weeks when disease pressure was low to moderate. Thus, under these conditions, it is likely that a savings of one to two fungicide applications can be realized at the beginning of the next season when fungicides are applied the previous autumn to control dollar spot. However, it is not clear whether this effect requires three fungicide applications the previous autumn or whether the effect will vary on disease-susceptible or highly resistant varieties.

It is apparent from our research that golf courses applying limited fungicides (similar to our high initial pathogen load scenario) would benefit from applying an efficacious fungicide such as fluazinam in autumn to delay dollar spot onset and reduce disease severity the following spring. Research is currently being completed to better understand fungicide timing effects on dollar spot occurrence and to look at differences in disease susceptibility among creeping bentgrass cultivars. Since many fungicide chemistries are available to control dollar spot, further research is needed to determine whether other fungicide chemistries applied in autumn can also delay disease onset and progress the following spring, and what impact cultivar resistance has on this response.

References

Burpee, L.L., Mueller, A.E., & Hannusch, D.J. (1990). Control of Typhula blight and pink snow mold of creeping bentgrass and residual suppression of dollar spot by triadimefon and propiconazole. Plant Disease, 74(9), 687-689.

Groben, G., Clarke, B.B., Murphy, J., Koch, P., Crouch, J.A., Lee, S., & Zhang, N. (2020). Real-time PCR detection of Clarireedia spp., the causal agents of dollar spot in turfgrasses. Plant Disease, 104(12), 3118-3123.

Groben, G. (2023). Molecular identification of fungal organisms associated with cool-season turfgrass. [Doctoral Dissertation, Rutgers, The State University of New Jersey, School of Graduate Studies].

Koch, P.L., & Kerns, J.P. (2013). Impact of spring and fall fungicide timings on the development of Typhula blight and dollar spot on golf course turfgrass. The Grass Roots, 42(4), 6-12.

Koch, M.J., Weibel, E.N., Smith, D.A., Lawson, T.J., Dickson, W.K., Clark, J.B., Bonos, S.A., Murphy, J.A., Clarke, B.B., & Meyer, W.A. (2009). Performance of bentgrass cultivars and selections in New Jersey turf trials. In D. Spinella (Ed.), 2008 Rutgers Turfgrass Proceedings. Research Report, Vol. 40. (pp. 1-33).

Koenig, J.L. (2009). Timing of fungicide applications for the management of dollar spot. [Master’s Thesis, The Ohio State University, Columbus, Ohio].

Niver, A. (2005). Impact of fall and spring fungicide applications on dollar spot.
[Master’s Thesis, The Ohio State University, Columbus, Ohio].

Weibel, E.N., Lawson, T.J., Clark, J.B., Wright, O., Halterman, J., Murphy, J.A., Meyer, W.A., & Bonos, S.A. (2022). Performance of bentgrass cultivars and selections in New Jersey turf trials, 2021. In D. Spinella (Ed.), 2021 Rutgers Turfgrass Proceedings. Research Report, Vol. 53. (pp. 1-28).

Zhang, P. (2024). Bentgrass disease resistance affects fungicide schedule and dollar spot control. [Doctoral Dissertation, Rutgers, The State University of New Jersey, School of Graduate Studies].