It’s what allows farmers to spray fields with the class of herbicides known as HPPD-inhibitors, which kill weeds such as waterhemp and Palmer amaranth and leave corn unscathed. But in more and more fields, the method is failing; waterhemp isn’t dying. Scientists have studied wa-
terhemp’s response to two common HPPD-inhibiting herbicides,
mesotrione
(trade name Callisto) and tembotrione (Laudis), and found the weed uses the same cellular mechanism as corn to detoxify the chemi- cals. However, no one had studied waterhemp’s meta- bolic response to a third HPPD-inhibiting herbicide, topramezone (Impact or Armezon), which is in a dif- ferent chemical subclass than mesotrione and tem- botrione, but is just as
Study Explains Waterhemp’s Metabolic Resistance To Topramezone C
URBANA, ILL.
orn naturally tolerates certain herbicides, detoxifying the chemi- cals before they can cause harm.
widely used in corn. A new study from the University of
Illinois identifies the detoxification pathway in two Midwest waterhemp populations that plays a role in rapidly metabolizing topramezone. Unfortu- nately, the finding is not good news for corn growers. “Our initial theory was that water- hemp would mimic corn as it does for
the other two HPPD-inhibitors, but, no, it found a different way,” says Dean Riechers, weed scientist in the Depart- ment of Crop Sciences at U of I and co- author on the Frontiers in Plant Science study. “We don’t know how or why, but it has a different mechanism from what corn naturally has. Bottom line is that you can’t use any of the three HPPD-inhibitors to control this population.” The waterhemp population
Riechers refers to is from a field in McLean County, Illi- nois. During the past decade, the field of continu- ous seed corn has been treated with all three HPPD- inhibitors, and waterhemp was showing resistance to them all. Riechers and his co-authors planted seeds from that population in a greenhouse and sprayed the plants with all three herbi- cides to assess the degree of damage. Compared with two populations sensitive to the chemicals,
the McLean County waterhemp plants
looked great. The researchers also grew up water-
hemp plants from a Nebraska field that only had been treated with mesotrione and tembotrione. Despite never having been exposed to topramezone, the plants appeared to be resistant. They didn’t look as good as the McLean County population, but they looked much better than the sensitive popula- tions. Riechers says, “The greenhouse ex-
periment showed the Nebraska popula- tion did have resistance to an herbicide it had never been exposed to. Did the other two herbicides select for toprame- zone resistance? My colleagues at Syngenta and I believe so. Our long- term goal is to find out if each herbicide has its own resistance gene or if there are genes that one or the other could select for.” Using an excised leaf assay they de-
veloped to identify herbicide-detoxifying enzymes, the research team discovered the McLean County plants were using a different pathway than corn to detoxify topramezone. Riechers says the finding is scientifically interesting, but might
CONTINUED ON PAGE 9
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www.caseih.com
December 7, 2018 / MidAmerica Farmer Grower • 5 .....
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Hwy. D • Caruthersville, MO 63830
www.donmedlinco.com
573-333-0663
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