Interaction of Paraquat‐Based Weed Control with Chlorothalonil‐Based Disease Control in Peanut

Understanding how weed and disease management strategies may be implemented in combination is important for improving pest management strategies in peanut (Arachis hypogaea L.) production. In greenhouse and field studies, tank mixtures of paraquat and other herbicides, with the fungicide chlorothalonil (tetrachloroisophthalonitrile), were evaluated for their effect on weed control and peanut injury, defoliation due to leafspot diseases (Cercospora arachidicolu and Cercosporidium personatum), and yield of peanut. In the greenhouse studies, the bipyridylium herbicide, paraquat, was tank‐mixed with each of two formulations of the substituted aromatic fungicidal compound, chlorothalonil. Tank mixtures consisted of 0, 0.06, and 0.12 lb a.i./acre paraquat and each of two chlorothalonil formulations (a liquid and a dry‐flowable) at 0, 038, 0.75, and 1.12 lb a.i./acre. Control of smallflower morningglory [Jacquemontia tamnifolin (L.) Griseb.] increased with increasing rate of paraquat application but was not affected with chlorothalonil in the tank mix. Control of Florida beggarweed [Desmodium tortuosum (Sw.) DC.] and injury on peanut increased with increasing rates of both pesticides, especially with paraquat at 0.06 lb/acre. In 2 yr of field studies, peanut injury due to paraquat, paraquat plus bentazon, paraquat plus bentazon plus 2,4‐DB, 2,4‐DB, or bentazon was not affected by the addition of chlorothalonil to tank mixtures. Three years of field studies on peanut show that end‐of‐season defoliation and yield, after a season‐long schedule of chlorothalonil applications, were not affected when the initial fungicide application was tank‐mixed with paraquat‐based herbicide systems or 2,4‐DB alone, regardless of chlorothalonil formulation. These data suggest that tank mixtures of chlorothalonil and early‐season herbicides are compatible for use on peanut. Research Question Paraquat‐based herbicide systems are widely used for weed control in peanut production in the southeastern USA. Paraquat can be applied to peanut from emergence to 4 wk after emergence. This application window is listed on the paraquat label and overlaps with recommended timing of the initial application of chlorothalonil fungicide for control of leafspot diseases on peanut. Because paraquat application causes foliar necrotic injury, growers often delay the first fungicide application until recovery from paraquat injury. However, any delay may partially negate the effectiveness of fungicidal disease management. An alternative to separate paraquat and chlorothalonil applications would be to apply a tank mixture of both pesticides at a time consistent with both pesticide labels. Would crop injury or season‐long leafspot disease control be altered when the early‐season application of paraquat‐based herbicide systems are tank mixed with chlorothalonil? Literature Summary When weather conditions are favorable for disease development, an early application of chlorothalonil fungicide is critical for disease control. However, when fungicides are applied with herbicides, the effectiveness of the pesticides may be altered. The few herbicide‐fungicide mixtures that have been studied were reported to reduce either weed or disease control. Reduction in effectiveness of herbicide‐fungicide mixtures was attributed to reduction in absorption or incompatibilities of the pesticides. No comparable studies have been published on paraquat or paraquat combinations and chlorothalonil mixtures. Study Description Greenhouse tests were conducted to compare weed control effectiveness of mixtures of paraquat and chlorothalonil. Two formulations (dry flowable and liquid) of chlorothalonil, each at four rates (0, 0.38, 0.75, and 1.03 lb a.i./acre) were mixed with each of three rates (0, 0.06, and 0.12 lb a.i./acre) paraquat and applied to sicklepod, smallflower morningglory, Florida beggarweed, and peanut plants. Plants were 1 mo old at the time of application and injury was visually rated 10 d after treatment. Field tests were conducted over 3 yr to evaluate the effects of tank mixtures of paraquat‐based herbicide systems and chlorothalonil on peanut injury and season‐long leafspot disease control. All combinations of 6 herbicide systems (none, paraquat, paraquat plus bentazon, paraquat plus bentazon plus 2,4‐DB, bentazon, and 2,4‐DB) tank mixed with three fungicide options (none, chlorothalonil liquid and dry flowable formulations) were applied to hand‐weeded peanut plots in mid‐May. Crop injury was rated 1 wk after tank mixture applications. Chlorothalonil was applied for the remainder of the season. Defoliation was rated 1 wk prior to harvest. Yield data were also recorded. Applied Question Is it feasible to tank mix paraquat or paraquat in combination with other herbicides with chlorothalonil fungicide for early‐season application to peanut without increasing peanut injury or decreasing disease control? In greenhouse studies, when paraquat was mixed with chlorothalonil and applied to greenhouse grown plants, control of Florida beggarweed and peanut injury increased, compared with paraquat applied alone. Improvement in weed control and increased injury may have been due to increased absorption of the chlorothalonil due to paraquat‐induced injury. At the recommended rates of paraquat (0.12 lb/acre) and chlorothalonil (l.12 lb/acre), mixtures of these pesticides did not increase peanut injury or decrease weed control compared with paraquat applications alone. In field tests, peanut injury was less than observed in the greenhouse. When paraquat and chlorothalonil were tank‐mixed, peanut injury was not greater than when paraquat was applied alone. In addition, peanut injury due to paraquat plus bentazon plus 2,4‐DB or paraquat plus bentazon tank‐mixed with chlorothalonil was not greater than when these herbicide systems were applied alone. In 1993 and 1995, levels of defoliation, assessed 1 wk prior to harvest, did not differ among treatments of herbicide systems tank‐mixed with chlorothalonil. In 1994, defoliation was greatest when chlorothalonil was not included in an early season tank‐mix with an herbicide, or when chlorothalonil was tank‐mixed with paraquat. Lowest defoliation was observed when chlorothalonil was applied early season in a tank‐mix with bentazon, 2,4‐DB, or alone. Yield was not affected by any treatments. Recommendation When weather conditions are favorable for development of leaf spot diseases (i.e., regular rainfall), and fungicide applications are recommended by an advisory, growers should not delay application of chlorothalonil fungicide for leafspot control until peanut recovers from any early herbicide injury. When used at labelled rates, chlorothalonil can be applied early‐season in a tank‐mix with an herbicide application.