Golfdom • September 2023 • Fungicide efficacy on spring dead spot-infected bermudagrass

Spring dead spot is a fungal disease that affects bermudagrass and zoysiagrass. As the name suggests, it typically appears as circular patches of dead or dormant grass in the spring. Ophiosphaerella spp. are the fungal pathogens that cause the disease by infecting the roots and crowns of the grass during the fall. These infections predispose the turf to winter injury.

Spring dead spot thrives in poorly drained or compacted soil with heavy thatch. Cultural practices such as proper watering, fertilization and aerating can help prevent and manage the disease. Fungicides may be necessary for severe cases.

We conducted three fungicide efficacy studies at the North Carolina State University Turfgrass Field Lab in Raleigh, N.C., on ‘Champion’ bermudagrass maintained as a golf course putting green with USGA spec mix. Two of the experiments focused on adepidyn (Syngenta) fungicide mixtures.

Adepidyn provides effective control of a wide range of plant diseases, including those caused by ascomycetes and basidiomycetes fungi. It exhibits preventative, curative and translaminar properties, which means it can protect treated plant parts and penetrate the plant tissue to combat existing infections.

We mowed plots six times per week at 0.15 inches with clippings collected. For each experiment, individual plots were 3-by-6 feet and arranged in a randomized complete block design with four replications.

We applied treatments in water equivalent to 2 gallons per 1000 sq. ft. for each experiment with a CO2-powered sprayer with a single TeeJet AI9508E nozzle at 50 psi.

We applied treatments on Oct. 18 and Nov. 8, 2018, and immediately watered in with 0.2 inches post-application irrigation. We assessed spring dead spot severity on April 16, May 7 and May 21, 2019, using a 281 intersect grid to determine the area within each plot exhibiting symptoms. The area under the curve (AUC) was calculated using the trapezoidal method with the formula Σ [(yi + yi+1)/2][ti+1 + ti], where i = 1,2,3,…,n-1, where yi is the rating, and ti is the time of the ith rating.

Spring dead spot severity peaked at 26.8 percent on April 16 in the nontreated control plots and above 17 percent in the nontreated control through May 21 (Table 2). All treatments suppressed spring dead spot compared to the nontreated control at each rating date. Plots treated with Lexicon Intrinsic (BASF) and Velista (Syngenta) had AUDPC values that were not statistically different from nontreated control plots.

In this experiment, we applied treatments on Oct. 17 and Nov. 22, 2019 and immediately watered in with 0.2 inches post-application irrigation. We evaluated spring dead spot severity on March 27, April 28 and May 26, 2020 to visually estimate the percent symptomatic area within the plot. Data were subjected to analysis of variance and mean separation by Fisher’s Protected LSD test (P = 0.05).

Spring dead spot severity peaked at 37.4 percent on March 27 in the nontreated control plots and declined to 26.1 percent in the nontreated control through May 26 (Table 3). All treatments suppressed spring dead spot when compared to the nontreated control at each rating date. On April 28 and May 26, plots treated with Posterity (Syngenta) at 0.24 fl. oz and 0.32 fl oz, A19649H at 0.32 fl oz, and Velista provided better suppression of spring dead spot than the other two treatments.

We initiated treatments for this experiment on Oct. 17 and reapplied either on Nov. 9 or Nov. 21, 2019. Fungicides were immediately watered in with 0.2 inches of post-application irrigation. We assessed spring dead spot severity on March 27, April 28, and May 26, 2020, to visually estimate the percent symptomatic area within the plot. Data were subjected to analysis of variance and mean separation by Fisher’s Protected LSD test (P = 0.05).

Spring dead spot severity peaked at 25.9 percent on March 27 in the nontreated control plots, only dropping to 21 percent in the nontreated control by May 26 (Table 4). All treatments suppressed spring dead spot when compared to the nontreated control on each rating date.

⦁ An October and November application of Kabuto (PBI-Gordon) 3.3SC at 1.6 fl. oz. per 1000 ft. sq. significantly controlled spring dead spot in bermudagrass.

⦁ Lower Kabuto application rates of 0.75 and 1.2 fl. oz. per 1000 sq. ft. in combination with Tekken (PBI-Gordon) 1.8SC at 3.0 fl. oz. provided adequate control.

⦁ Lexicon Intrinsic 4.17 SC at o.47 fl. oz. and Velista 0.5 WDG at 0.7 fl. oz. per 1000 ft. sq. provided some control; however, both fungicides did not have significantly different AUDPC estimates compared to the nontreated control in 2018.

⦁ Two fall applications of Posterity 1.6SC at the higher rates of 0.32 and 0.24 fl. oz. performed significantly better than the nontreated control.

⦁ In 2019 Velista 0.5 WDG at 0.5 oz per 1000 ft. sq. performed better than the previous fungicide trial in 2018.

⦁ All application rates of both Posterity Forte 2.5SC and Posterity 1.67SC alone and combined with Headway (Syngenta) 1.39ME applied at 1.5 fl. oz. per 1000 ft. sq. provided significant control compared to the nontreated control.