Plant Growth‐Regulating Effects of Systemic Fungicides Applied to Kentucky Bluegrass1

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Abstract
Systemic fungicides applied to Kentucky bluegrass (Poa pratensis L.) turf often cause visible alterations in plant morphology. Compounds derived from 5‐pyrimidinemethanol and 1,2,4‐triazole may induce changes in color, growth rate, and density of the stand while also providing outstanding disease control. Because plant growth and stress responses contribute to the severity of many turfgrass diseases, nontarget plant growth regulating effects could influence the disease control activity of systemic fungicides. Studies were conducted to determine the effects of several systemic fungicides on Kentucky bluegrass morphology and physiology. Fungicides were compared, when possible, to growth‐regulating compounds with similar chemical and/or biological properties. Commercially available or experimental formulations were applied as soil drenches to greenhouse‐grown seedlings and as foliar sprays to field‐grown sod of ‘Merion’ and ‘Fylking’ Kentucky bluegrass. Application rates of the fungicides were as recommended for controlling Fusarium blight. Growth regulators were applied at rates comparable to the fungicides or at rates previously reported to have an effect on plant growth. Oven‐dry root weights, shoot densities (tillers plus nonemerged rhizomes), and leaf growth rates were measured. The chlorophyll content of young leaves was determined by extraction with dimethylsulfoxide. Chlorophyll retention of excised, mature leaves was measured following dark incubation. Total nonstructural carbohydrate (TNC) content in mature foliage was determined using the anthrone method. When applied to greenhouse‐grown seedlings, pyrimidinemethanol and triazole derivative fungicides reduced leaf growth rates, shoot densities, and root weights of one or both cultivars. Pyrimidinemethanols increased the chlorophyll content of Fylking and increased TNC contents of both cultivars. Triazoles also increased TNC levels while increasing chlorophyll retention of excised leaves. Chlorophyll retention was also enhanced by cytokinin analogs, including a benzimidazole fungicide. Results obtained from field experiments were inconsistent although similar growth‐regulating effects were observed following certain treatments. Our results show that the systemic fungicides studied have nontarget plant growth‐regulating activity on Kentucky bluegrass. These effects could influence the severity of stress related diseases such as Fusarium blight.