Growth Regulator Effects of Propiconazole on Redroot Pigweed (Amaranthus retroflexus)1

Nonfungicidal effects of agricultural fungicides on crop plants have been reported previously; however, there are few reports of nontarget effects of fungicides on weedy species. Field research trials in Oregon demonstrated that the growth of several broadleaf weeds was reduced after multiple applications of the fungicide propiconazole. Greenhouse experiments confirmed that preemergence applications of propiconazole reduced the biomass accumulation of several common broadleaf and grass weeds 15 to 63%. Laboratory experiments were performed on redroot pigweed, the most sensitive species, to examine the effects of propiconazole on germination and early seedling growth. Redroot pigweed germination and total seedling length (root plus shoot) were reduced at propiconazole concentrations above 37 and 0.36 mg/L, respectively. Growth-regulating effects of fungicides such as propiconazole on the germination and early growth of weeds may contribute to integrated weed management, especially when adequate moisture ensures the presence of germinating seeds and small seedlings throughout the growing season. Nomenclature: Propiconazole; redroot pigweed, Amaranthus retroflexus L. #3 AMARE. Additional index words: Fungicide, Lolium multiflorum, LOLMU, POAAN, Poa annua, nontarget effects, Senecio vulgaris, SENVU, sterol biosynthesis inhibitor. Abbreviations: DMI, demethylation inhibitor; POST, postemergence; PRE, preemergence; SBI, sterol biosynthesis inhibiting.