Research Update from UMD

Efficacy of registered fungicides for the management of gray leaf spot, 2022

By Fereshteh Shahoveisi, Ph.D. Department of Plant Sciences and Landscape Architecture, University of Maryland

A field study was conducted at the University of Maryland Turfgrass Research Facility in College Park to evaluate five registered fungicides for the management of gray leaf spot on perennial ryegrass cv. Majesty planted on native silt loam soil. Mowing was performed two times weekly at the height of 3 in. Treatments were applied approximately every 14 or 28 days from 30 Jun to 23 Sep. Plots were 3 ft x 6 ft and arranged in a randomized complete block design with four replications. A CO2 backpack sprayer at 40 psi equipped with a single Teejet AI9508E nozzle in water equivalent to 2 gal /1000 ft2 was used. Gray leaf spot did not occur naturally and plots were inoculated with 4000 spore/ml spore suspension on 11 Sep. Nitrogen was applied as urea (46-0-0) three times at the following rates: 0.9 lb per 1000 ft2 on 13 Jun and 11 Jul, and 0.5 lb per 1000 ft2 on 25 Jul. Emerald and Prostar fungicides were applied to control dollar spot and brown patch. Both fungicides were applied at the highest label rates on 13 Jul, 25 Jul, 8 Aug, and 24 Aug. Herbicides including 3way Ester (1.2 fl oz/1000 ft2 on 8 Jul) and T Zone (1.5 oz/1000 ft2 on 27 Jul) were used for the management of weeds in the plots. Gray leaf spot severity was evaluated as a visual estimate of the percentage plot area affected starting 15 Sep until 13 Oct. The study was concluded on 13 Oct as disease symptoms started to recover due to weather factors. SAUDPC was calculated using the following formula: Σ [[(yi+ yi+1)/2] [ti+1 - ti]]/n, where yi and yi+1 represent disease rating at two consecutive dates, ti and ti+1 represent the corresponding rating dates to yi and yi+1, respectively, and n represents the number of days between the first and last rating dates. Data were subjected to analysis of variance and means separation using Fisher’s least significant difference (LSD) procedure at α = 0.05.

The first gray leaf spot symptoms were observed four days after inoculation on 15 Sep. The severity of the disease increased throughout the study; however, the maximum disease severity reached 9.0% in the non-treated plots. Non-treated plots showed significantly higher disease severity on 15 Sep compared to plots treated with fungicides. Plots treated with Rayora, Tekken, or Daconil Action alone had significantly lower disease severity than the non-treated control, except on 22 Sep. Differences among plots treated with Rayora, Tekken, or Daconil Action were not significant which could be due to the low disease pressure. Pillar G-treated plots had significantly lower disease severity than the non-treated plots on 15 Sep, 6 Oct, and 13 Oct. Plots treated with Banner Max II did not have significant differences with the non-treated plots on most of the rating dates except 15 Sep. •

Gray leaf spot symptoms on perennial ryegrass

z Letters indicate the date of application: A=30 Jun, C=14 Jul, E=28 Jul, G=11 Aug, I=24 Aug, K=8 Sep, M=23 Sep.

y Gray leaf spot severity was visually assessed on a 0 to 100% scale where 0 =no disease and 100 = entire plot area affected by the pathogen.

x Means in a column followed by the same letter are not significantly different according to Fisher’s least significant difference (α = 0.05).

w Standardized area under the disease progress curve is a quantitative measure of dollar spot intensity with time over all rating dates.