Fungicides in Kentucky Bluegrass Turf: Effects on Thatch and pH¹

Abstract

Management of thatch and pH are important aspects of turfgrass culture. The use of disease‐preventive fungicides is a standard practice on highly‐maintained turfgrasses, and fungicides may be toxic to many thatch‐decomposing microorganisms. However, little information exists concerning the effects of long‐term fungicide programs on thatch accumulation. The influence of 14 fungicides, one nematicide, and of five mixed fungicide programs, applied over a 3‐year period, on thatch decomposition and the resultant pH of underlying soil was investigated on a blended Kentucky bluegrass (Poa pratensis L.) sod. The physical depth of noncompressed thatch was measured after samples were removed from plots with a Noer Soil Profile Sampler. Cores of 2.54 cm diam were used to determine pH characteristics of the soil profile. Thatch depths among fungicide treatments ranged from 2.8 to 22.0 mm, and the pH (saturated paste in 10 mM CaCl2) of the surface 3 cm ranged from 5.6 to 6.5. Thatch depth and pH of the surface 3 cm of thatch plus soil were inversely related (r = −0.797; significant at p = 0.001), although differences in pH among treatments were associated with the mineral soil fraction and not with the thatch. Fungicide‐induced reductions in pH occurred to depths exceeding 20 cm. Acidification of soil was apparently not the result of organic acids leaching from decomposing thatch, but was correlated (r = −0.564; significant at p = 0.01) with the contributions of sulfur (up to 491 g S/are/year) from the fungicides. Oxidation of sulfur from decomposing fungicides reduced the soil pH which, in turn, reduced the activity of microorganisms responsible for degradation of thatch. Thatch accumulation was not considered to be due to inhibitory effects of fungicides toward earthworms.

Fusarium Blight and Physical, Chemical, and Microbial Properties of Kentucky Bluegrass Sod

“…Thatch is almost always present on highly maintained, mature bluegrass stands and on harvested sods, but the depth of thatch is not always related to the severity of Fusarium blight (14,20). The rate of thatch decomposition has been shown here to be important to disease incidence.…”

“…Fungicides were applied repeatedly from 1975 to 1977. Details of the plot's management, soil, the fungicides, and the application schedules and equipment have been described (14).…”

“…The 22 treatments, applied in three replicates of 1 X 5 m plot areas, included 15 individual fungicides, five combined or alternated "programs," one nematicide, and an untreated control. The installed sod had a thatch depth of 2 cm, and in November 1977 the depths among treatments ranged from 3 to 22 mm (14), which represented decomposition rates of 6 to -I mm/yr. The composite pH of the upper 3 cm of thatch and soil varied from 5.6 to 6.4 in the treated areas (14).…”

“…Occurrence of Fusarium blight on mature turfgrasses is very dependent upon the environmental conditions (3,20), but these conditions remain poorly defined due to the unpredictable nature of the disease in well-controlled experiments. We investigated the longterm (3 yr) effects of fungicide applications on nontarget microbial activities and soil processes in closely monitored Kentucky bluegrass sod (14)(15)(16). Fusarium blight occurred on the plot in 1977 (18), and the disease was unexpectedly controlled by several fungicides that have been ineffective in all 1-yr studies.…”

Recent Mechanical Cultivation of Lawns Enhances Lime Application Efficacy

“…Furthermore, saprophytic processes are often substantially reduced at suboptimal soil pH levels (Smith, 1979; Potter et al, 1985). Several long‐term studies report acidic soil pH levels (4–6) promote accumulation of thatch (a loose intermingled organic layer of dead and living shoots, stems, and roots that develops between turfgrass verdure and the soil surface) in managed Kentucky bluegrass ( Poa pratensis L.) swards (Edmond and Coles, 1958; Smiley and Craven, 1978; Murray and Juska, 1977). Likewise, turfgrass‐availability of soil P, K, Mg, S, and Mo is decreased at suboptimal soil pH levels (Carrow et al, 2001).…”