Greenhouse studies were conducted to determine the effect of soil dust on glyphosate efficacy. Eastern black nightshade and hairy nightshade were affected similarly by glyphosate, regardless of whether plants were or were not treated with dust, but glyphosate phytotoxicity decreased when dust accumulated on plants. Silty clay dust at 2, 4, 6, 8, and 10 kg ha⁻¹ on nightshade plants decreased the efficacy of glyphosate at 80 g ae ha⁻¹ by 1, 5, 12, 21, and 32 percentage points, respectively. The adverse effect of dust on glyphosate efficacy occurred regardless of whether dust settled before or immediately after herbicide spraying and varied with dust type, which was silty clay ≥ silty clay loam > loamy sand. Glyphosate efficacy was slightly influenced by dust pH, with a trend toward increased efficacy as pH increased. Glyphosate applied at 280 L ha⁻¹ spray volume controlled dust-treated plants better than when applied at 94 or 190 L ha⁻¹. Three adjuvants (ammonium sulfate, nonionic surfactant, and organosilicone surfactant) only partially overcame the adverse effect of dust on glyphosate efficacy in eastern black and hairy nightshade control. Methylated seed oil or petroleum oil adjuvants did not improve glyphosate control of dust-treated or untreated plants.

Nomenclature: Glyphosate; eastern black nightshade, Solanum ptycanthum Dun. SOLPT; hairy nightshade, Solanum sarrachoides Sendtn. SOLSA.