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Department of Crop Sciences
No Picture Available Josie A. Hugie
Department of Crop Sciences
Can Weed Resistance to HPPD Inhibitors Be Prevented?
Current weed management practices frequently exploit combining herbicides with different modes of action to broaden the weed control spectrum and control resistant biotypes. Broadleaf weeds, such as waterhemp and kochia, have evolved resistance to many different classes of herbicides, with some biotypes having "multiple resistance", meaning resistance to more than one class of herbicides in the same plant. HPPD-inhibiting herbicides such as Callisto and Balance Pro generally provide excellent control of these problem broadleaf weeds. Inhibition of the HPPD enzyme leads to the depletion of protective pigments in plant tissue, resulting in bleaching of young tissue which leaves the plant vulnerable to damage by light. HPPD-inhibiting herbicides are relatively new, and because their use has been common only in the past few years, weed resistance to HPPD inhibitors has not yet developed. Callisto is often tank-mixed with a low rate of atrazine, a photosynthetic inhibitor, and displays synergistic herbicidal activity on waterhemp and pigweeds, meaning herbicidal activity observed in the target plant is greater than the expected sum activity of the combined herbicides. In addition to improving weed control with the Callisto plus atrazine tank mix, this weed management strategy may also aid in preventing or delaying the development of weed resistance to Callisto. Resistance may be delayed through combining two herbicides with two distinct target sites and modes of action. This combination of herbicides having synergistic activity offers not only a valuable weed management tool in reducing the likelihood of herbicide resistant populations, but may also allow for the reduction of rates at which the herbicide is applied through gaining higher activity per unit of active ingredient. Furthermore, the synergism between Callisto and atrazine also has been documented in atrazine-resistant waterhemp and pigweed biotypes, suggesting that when combined with Callisto, atrazine has some level of activity in these atrazineresistant weeds. In other greenhouse studies with waterhemp and pigweed, it has been noted that the synergism is dosedependent and may also be dependent on the ratio of Callisto to atrazine. Greenhouse studies are currently underway with the goal of better understanding the dose dependency of the interaction, in addition to greenhouse and laboratory studies aimed at determining the underlying physiological basis for the interaction between Callisto and atrazine.
Velvetleaf treated with Callisto (left) displays bleaching of young tissue Applications of Callisto alone (far left) and atrazine alone (left center) display enhanced and synergistic activity when applied in combination (right center) to waterhemp as compared to an untreated control (far right).