Mesotrione carryover into soybean examined
Mesotrione, an active ingredient found in Syngenta's Callisto, Lumax, Lexar and Halex provides excellent preemergence control of black nightshade, lambsquarter, pigweeds, annual smartweeds, velvetleaf and waterhemp. It also provides excellent postemergence control of these weeds and several others. It is labeled for preemergence and in come cases postemergence in corn. Mesotrione is an HPPD inhibitor or a group 27 herbicide that causes bleaching in susceptible weeds. Purdue University's own weed guide gives a good tolerance rating for mesotrione for corn. In rare cases it has been reported to carry over into soybean.
click image to zoomGlenn NiceFigure 1. Bleaching and mottling effect in soybean leaves. In the 2004 Weed Control update we reported on a few cases of mesotrione carryover injury to soybean in North East Indiana . In these cases injury was most prevalent on sandy soils with low organic matter and low CEC. The soil pH was often reported to be low, below 6.0. The conditions leading up to carryover are not completely understood. Mike Owen of Iowa state university suggested that they occurred in areas where overlap could have taken place added to this that the summer of application was dry and they experienced a cold wet spring .
Mesotrione is broken down in the soil by microbial activity into an end product of CO2. Any soil quality that would inhibit microbial activity, such as dry conditions or changes in pH, could potentially lead to a decrease in the breakdown of the herbicides. Of course, doubling up on rates due to overlapping or improper calibration of spray equipment would also lead to a higher potential for carryover. In 2008, Darren Robinson of the University of Guelph reported that the addition of atrazine increased injury in horticulture crops planted one year after a mesotrione application.
In cases where mesotrione carryover is suspected, symptoms observed on soybean are bleaching of the tips and margins of new growth (figure 1). This bleaching often bleaches out the veins or causes a mottling effect (figure 1). In cases where drift is the suspected delivery method, bleaching of the veins and mottling is not as evident. In a carryover situation the introduced herbicide is taken up from the soil and if the herbicide is soluble in water it moved into the leaf to accumulate in the tips and margins. Oddly enough, in some cases the leaves may have a slight strapping effect (figure2). Strapping is typically connected to growth regulator herbicide injury, but in these cases of possible mesotrione carryover new growth may show a strapping effect. In some of the cases it has not always been clear if the injured soybean may have also come in contact with a growth regulator; however, in all cases a HPPD inhibitor was used the previous year and there are no reports of growth regulator herbicides causing a bleaching effect.
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