Development of improved management strategies for controlling thatch on tuifgrasses is limited by an incomplete understanding of thatch biology. Mechanisms whereby fungicides cause thatch to accumulate in turfgrass were investigated, by evaluating the influences of fungicides on rates of tissue production and thatch decomposition. Fourteen fungicides, one nematicide, and five mixed‐fungicide programs were applied repeatedly (up to nine times annually), over a 4‐yr period, to a field‐grown Kentucky bluegrass (Poa pratensisL.) turf on a soil of the fine, illitic mesic family of Glossoboric Hapludalfs. Measurements were made of thatch depths, root and leaf clipping weights, shear strength of sod, and decomposition rates for thatch implanted into the turfs. Compounds that caused thatch to become deeper (p= 0.05) than in the nontreated control included benomyl, cadmium succinate, fenamiphos, iprodione, and mancozeb. Treatments in which these pesticides were used were characterized by sod shear strengths greater (p= 0.05) than in the control. Thatch accumulations were related mostly to the amounts of roots in the surface 4 cm. None of the studied fungicides significantly (p= 0.10) reduced the apparent rate of thatch decomposition. Fungicides in this study therefore appeared to induce thatchiness in Kentucky bluegrass by increasing the rates of root and rhizome production and not by reducing the rate of litter decomposition.

As new herbicides are investigated for use on turf, it is important to study the effects continued use of these materials have on desirable grass species. A field investigation was conducted to determine the influence of prodiamine [2,4‐dinitro‐N3,N3‐dipropyl‐6‐(trifluoromethyl)‐1.3‐benzenediamine] application rate and frequency on Kentucky bluegrass (Poa pratensis L.) turf over a 4‐yr period. Root mass and turfgrass density decreased following prodiamine treatment. Turfgrass quality ratings decreased as prodiamine rate increased, but remained acceptable at rates below 1.1 kg ha−1 a.i. The projected use rate for prodiamine in the northeastern USA is 0.56 kg ha−1 a.i. Quality was reduced and necrotic ring spot (Leptosphaeria korrae Walker and Smith) disease more severe when prodiamine was applied annually vs. biennially. Percent turf area damaged by necrotic ring spot also increased as prodiamine rate increased, but was most severe at rates greater than 2.2 kg ha−1 a.i. The quality, density, and disease data indicated that annual applications of prodiamine can be detrimental to Kentucky bluegrass turf. These results suggest that it may be advantageous to avoid prodiamine applications in successive years or to alternate it with less phytotoxic preemergence herbicides.

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Excess thatch in turfgrasses is often associated with decreased plant vigor and increased disease susceptibility. Thatch is the primary rooting medium for many grasses, and is a substrate that possesses many prerequisites for anaerobiosis. The oxidation status and the gas composition in unsaturated thatch of mature Kentucky bluegrass (Poa pratensis L.) were measured to determine if anaerobiosis occurs under field and greenhouse conditions. Soils under the sods included a Hudson silty clay loam (fine, illitic, mesic Glossaquic Hapludolf), a Riverhead sandy loam (coarse−loamy, mixed, mesic Typic Dystrochrept), and an Arkport fine sandy loam (coarse−loamy, mixed, mesic Psammentic Hapludalf). Measurements of redox potentials (Eh) and concentrations of oxygen (O2), carbon dioxide (CO2) and ethylene (C2H4) were made. Conditions of poor oxidation, including low Eh and O2 and CO2 concentrations, and accumulations of C2H4, were measured in thatch on poorly drained soils in the field and in the greenhouse. Applications of lime and calcium arsenate amplified the extent of poor oxidation in thatch, whereas calcium nitrate improved the oxidation status. Thatch depth and a coring procedure did not influence thatch oxidation. The Eh varied diurnally and was lowest in wet thatches during warm, sunny days when thatch became warmer (by up to 7°C) than the air. Poorly oxidized conditions for periods over 7 h were measured in moist but unsaturated thatch in the field. The temperature of thatch appeared to be important in governing the oxidation status. Measurements of Eh were considered to provide rapid and useful insights into the gaseous composition of wet thatches. Our results imply that phytotoxic products of poorly oxidized environments may accumulate in wet thatch on warm sunny days. Such conditions may also be common in field−grown cores of mature turfgrass which are moved to the greenhouse for study. The relevance of these results to the occurrence of diseases such as Fusarium blight are discussed.

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