Speaker 1:The worldwide threat of glyphosate-resistant weeds to production agriculture has made headlines. In the Midwest, mare’s tail, waterhemp and giant ragweed has caused the greatest concern to growers.  
Giant ragweed has been the most problematic broadleaf weed in the Eastern corn belt for over two decades. Now, numerous fields are infested with glyphosate-resistant populations. The purpose of this video is to show a unique response of a glyphosate-resistant biotype compared to the response of a susceptible population following a standard glyphosate application.
First, the two susceptible clients on the left slowly wilt and new growth turns carotic and eventually, necrotic. Overall plant death normally occurs around 7 to 14 days.
Focus on the two resistant plants on the right. Within 48 hours, mature leaves become brown or necrotic. In between veins and leaf margins typically curl up and inwards. New growth yellows and turns carotic but continues to grow. The speed and severity at which symptoms appear is enhanced under conditions of high light and temperature.
The important issue to highlight is the unique resistance response. The rapid necrosis observed is similar to that of a contact herbicide and is not expected of a systemic herbicide like glyphosate.
In fields infested with weeds resistant to other modes of action, plants are generally either dead or alive by 21 days. However, in fields with this particular biotype, plants exhibit some degree of injury then recover and complete their life cycle making glyphosate resistance difficult to identify.
For more information on the biology and control of glyphosate-resistant weeds, please check out these publications.