| Diquat, a fast-acting contact herbicide, has been used for weed control in the US for over 45 years. It was widely recommended for control of hydrilla, often in combination with copper. Restriction of copper use in public waters in Florida in the 1980s and development of fluridone resistant hydrilla in the 1990s has resulted in renewed interest in using diquat for hydrilla control. Diquat is also being evaluated for aquatic weed control in irrigation canals due to its short half-life in water. Greenhouse studies indicated that rice seed germinated and maintained in diquat-treated DI water was very sensitive to diquat, with EC10 values of 0.016 and 0.004 mg L-1 for shoots and roots, respectively. Seeds of corn, cotton, squash, wheat and soybean germinated in sand and irrigated once with diquat-treated water at planting had EC10 values from 3 to 6 mg L-1. When treated at the seedling or more mature stages all crops exhibited EC 10 values from 1 to 5 mg L-1, suggesting that diquat-treated water (0.370 mg L-1) may be used for irrigation with limited potential for phytotoxicity, except for freshly planted rice. Biomass and chlorophyll studies indicated that surfactants (CT-301, Cygnet Plus, SilEnergy and Timberland 90) applied alone were phytotoxic to hydrilla at concentrations greater than those recommended for aquatic weed control. Regression models yielded EC50 values from 26 to 592 mg L-1 for hydrilla dry weight and from 433 to 4,814 mg L-1 for chlorophyll reduction. Addition of surfactants to diquat at aquatic and terrestrial labeled rates was additive on hydrilla and landoltia based on Colby method. When applied on petunia, the aquatic and terrestrial surfactant rates plus diquat were additive and synergistic, respectively. The combination of copper, flumioxazin and endothall salts at different concentrations with diquat gave an additive effect based on dry weight. Acibenzolar added to diquat was synergistic based on hydrilla dry weight, as was carfentrazone based on hydrilla total length. These results indicate that some herbicides can be combined with diquat for improved hydrilla control, but the benefit of adding surfactants at aquatic labeled rates provides no more than additive effects. |
