Interaction of Cupric Hydroxide, Paraquat, and Biotype of American Black Nightshade (Solanum americanum)

Abstract

Experiments were conducted to determine the interaction between cupric hydroxide and paraquat on the control of American black nightshade. Three cupric hydroxide applications lowered paraquat toxicity with no further decrease in toxicity with six or nine cupric hydroxide applications when paraquat was applied 3 days after the last cupric hydroxide application. The interval between the last cupric hydroxide application and paraquat application was more important in decreasing paraquat toxicity than the number of cupric hydroxide applications. Three cupric hydroxide applications with a 10-day interval before paraquat application decreased paraquat toxicity less than three cupric hydroxide applications with a 3-day interval before paraquat application. Plants collected from three American black nightshade populations (Naples, Bradenton, and Gainesville) differed in their sensitivity to paraquat. I50values, based on dry weight, showed Naples plants to be 12 times more tolerant to paraquat than Gainesville plants without cupric hydroxide, and 14 times more tolerant when cupric hydroxide was applied. More plants of the Naples biotype regrew from apical buds after paraquat treatment following cupric hydroxide application, and from axillary buds with paraquat but not cupric hydroxide treatment, than did the Bradenton biotype. Plants of the Gainesville biotype did not regrow after paraquat application, regardless of cupric hydroxide treatment.