A deeper dive into Pythium root rot control on bentgrass and ryegrass

Researchers at North Carolina State University, Clemson University and Rutgers University evaluated fungicides to control Pythium root rot on creeping bentgrass and perennial ryegrass. The research teams conducted two studies on creeping bentgrass putting greens, and the other on perennial ryegrass rough.

Fungicides used in the studies included Segway 3.33 SC (PBI-Gordon), Union 0.79 SC (PBI-Gordon), Serata 20 WDG (FMC), Banol 6 SL (Envu), Subdue MAXX 2 ME (Syngenta) and Fame 3.98 SC (FMC) (see Table 1).

At all three locations, Pythium root rot peaked during the summer months in 2022 at Clemson’s Pee Dee (S.C.) Research and Education Center, and in 2023 at NC State, Turfgrass Field Lab in Raleigh, N.C., and at Rutgers, Horticulture Farm No. 2 in New Brunswick, N.J.

Overall, at North Carolina State, the disease exhibited moderate severity in 2023, peaking in mid-July, with the combination of Segway and Serata treatments being notably effective in suppressing disease development.

In 2022 at Clemson, Pythium root rot symptoms throughout the summer demonstrated the effectiveness of Serata alone or in combination with Fame SC in controlling Pythium root rot and were comparable or superior to Segway. On perennial ryegrass at Rutgers, the Banol treatment was among the best in reducing disease severity compared to untreated plots.

The following is a more in-depth review of experimental methods and results for each fungicide trial to control Pythium root rot.

North Carolina State University

Researchers at the North Carolina State University Turfgrass Field Lab in Raleigh, N.C., conducted this study on Dominant Plus creeping bentgrass maintained as a golf course putting green. The research teams mowed plots six times per week at 0.150 inches. Researchers fertilized the plots with urea at 0.125 lbs. N per 1000 sq. ft. every other week.

The NC State researchers irrigated plots at 5 p.m., 12 a.m., and 5 a.m. daily for 3 minutes starting in May to induce root rot symptoms. Individual plots were 3-by-6 ft. and arranged in a randomized complete block design with four replications.

Researchers applied treatments in a water carrier at 2 gallons per 1000 sq. ft. with a CO2-powered sprayer equipped with a TeeJet AI9508E nozzle at 50 psi. They irrigated all treatments immediately after application with 0.125 inches of water.

The team initiated treatments on June 21 and applied them at 14-day intervals.

Pythium root rot developed naturally in June. The researchers assessed the percent turf area exhibiting Pythium root rot symptoms on July 18, July 31 and Aug. 15. Data were subjected to analysis of variance and means separation Fisher’s Protected LSD test (P ≤ 0.05).

Pythium root rot disease severity was moderate in 2023 due to high relative humidity and nighttime temperatures at or above 70 degrees F. Disease severity was highest on July 18 at 25.0 percent in the nontreated control. Disease severity in other treatments continued to increase throughout July and August. All treatments significantly suppressed Pythium root rot based on AUDPC values except for NB40950 and Subdue MAXX. The rotation of Segway with Serata almost completely suppressed the development of Pythium root rot.

Clemson University

A research team at Clemson University initiated a trial in the summer of 2022 — at the Pee Dee (S.C.) Research and Education Center — to evaluate the impact of product applications on Pythium root rot development. Researchers grew Penn A1 creeping bentgrass in a sand-based root zone maintained as a golf course putting green.

Researchers mowed the experimental site five times weekly at a cutting height of 0.125 inches but reduced the frequency to three times weekly in July and August during excessive heat. Fertilizer applications totaled 1.0 lbs. N (46-0-0) through foliar applications (0.25 lbs. N/1000 sq. ft.) made every 14 days.

Fleet (polyoxyalkylene polymers) was applied at 4.0 fl. oz./1000 sq. ft. every 28 days to maintain water infiltration and uniformity in the root zone. Primo Maxx (trinexapac-ethyl) was applied every 14 days at 0.138 fl. oz. per 1000 sq. ft. to limit turfgrass vertical growth following standard management practices in the region.

Researchers applied Prostar WG (i.e., flutolanil) at 3 oz. per 1000 sq. ft. on June 7, and Daconil Weatherstik (i.e., chlorothalonil) at 3.5 fl. oz. per 1000 sq. ft. on July 26 as curative applications for brown patch disease and algae.

Treatment plots measured 3-by-10 ft. and were arranged in a randomized complete block design with four replications. Treatments were applied as a foliar spray in water equivalent to 2.1 gallons per 1000 sq. ft. using a CO2-powered walk-behind sprayer equipped with dual TeeJet 8002 nozzles.
The team applied irrigation immediately after treatment through a hose calibrated to deliver 0.125 acre-inches. Treatments were initiated on May 17 and applied at 14-day schedules through July 26.

The researchers assessed Pythium root rot (i.e., percent diseased turf area per plot) every 7 to 14 days throughout the trial period. All data was transformed before analysis of variance, and means were separated using Fisher’s least significant difference (α < 0.05).

Pythium root rot developed naturally across the experimental site in late June and into early July. They observed some variation initially, but continued disease progression resulted in more uniform symptom development in late July and early August.

On July 7, Serata 0.6 fl. oz., Serata 0.4 fl. oz. + Fame 0.36 fl. oz., and Segway 0.67 fl. oz. all reduced Pythium root rot compared to the non-treated control.

All treatments reduced Pythium root rot compared to the nontreated control on July 29 and Aug. 5, when they observed peak disease pressure. The results show that Serata offers comparable control of Pythium root rot disease to Segway. Tank mixing a lower rate of Serata (0.4 fl. oz.) with Fame SC 0.36 fl. oz. can provide comparable control to higher rates of Serata or Segway alone.

Rutgers University

At Rutgers University Horticultural Farm No. 2 in North Brunswick, N.J., researchers conducted a field study on a newly established and highly susceptible perennial ryegrass turf grown on sandy loam, maintained as a golf course rough.

The researchers mowed plots three days per week at a height of 3 inches using a Turfmaster rotary mower with clippings collected. They arranged the treatments in a randomized block design with four replications having a plot size of 3-by-5 ft.

They applied fungicides in 2.0 gallons of water carrier per 1000 sq. ft. using a CO2 pressurized boom sprayer at 40 psi equipped with a single air induction even flat spray tip nozzle (AI9505EVS). Researchers initiated the treatments on June 20, with the last application on Aug. 15.

Pythium blight severity was assessed as a visual estimate of the percentage of symptomatic turf per plot. Disease severity was summarized as the area under the disease progress curve (AUDPC) to quantify disease severity using the trapezoidal method with the formula Σi=1n-1 [(yi + yi+1)/2](ti+1 – ti), where “ti” was time in days, “i” the order index for the ratings, “n” the number of ratings, and “y” the dependent variable (count) at each rating. Data were subjected to analysis of variance (ANOVA) and means separation using Fisher’s protected LSD test (P = 0.05).

Pythium disease severity was first observed on June 26 and continuously increased through July 17. At the peak of disease on July 17, Banol (1.33 fl. oz. per 1000 sq. ft. – 14 days) had a Pythium blight severity of 5.6 percent, which is not statistically different from Subdue Maxx (1.0 fl. oz. per 1000 sq. ft. – 14 days) with 17.1 percent disease severity.

AUDPC values for all treated plots were significantly lower than those for the untreated check. Plots treated with Banol (1.33 fl. oz. 1000 sq. ft. – 14 days) had significantly lower AUDPC values than Segway at both rates but were not statistically different from plots treated with Union (2.9 fl. oz. per 1000 sq. ft. – 14 days) and Subdue Maxx (1.0 fl. oz. per 1000 sq. ft. – 14 days).

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