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Issue No. 18, Article 3/July 27, 2007

Remain Vigilant for Herbicide-Resistant Weeds

In the July 7 issue (no. 15) of the Bulletin, we described the occurrence of an Illinois waterhemp population that is not controlled by glyphosate at rates exceeding field use rates. Prior to this, we described the development of a molecular/laboratory assay that can determine the presence of a specific trait known to confer resistance to PPO-inhibiting herbicides in waterhemp. Without doubt, waterhemp continues to present significant challenges to Illinois corn and soybean farmers.

Reports of poor control of waterhemp with glyphosate or PPO inhibitors have been common the past several weeks. We strongly suspect that glyphosate- or PPO-resistant waterhemp populations are not confined to any one particular area but are perhaps widespread across central and south-central Illinois. We are still interested in testing waterhemp samples for resistance to PPO inhibitors (issue no. 12 of the Bulletin, June 15, 2007), and usually we can provide results within 24 hours of receiving samples. Screening samples for glyphosate-resistant biotypes still requires a more time-consuming process of seed collection and greenhouse experiments. We are interested nonetheless in trying to determine the occurrence and extent of glyphosate-resistance in Illinois weed populations.

In addition to the waterhemp reports, we have received several reports of giant ragweed and common lambsquarters populations that have not been controlled following single or multiple applications of glyphosate. Similar instances have occurred in the past, but remember that selection for glyphosate resistance occurs with each application of glyphosate and the exclusion of other weed management practices. Glyphosate-resistant giant ragweed populations have been identified in Ohio and Indiana, and we have no evidence to suggest that a similar situation will not occur in Illinois. Unfortunately, limits of time and resources have kept us from pursuing some of these reports as thoroughly as we would like, but we very much appreciate learning about instances of poor weed control with glyphosate. Thank you for keeping us informed.

Our research efforts on herbicide-
resistant weeds will continue to focus on those weeds and herbicides of great importance to Illinois agronomic systems. As we learn more we'll share our results with you through a variety of media and meetings.--Aaron Hager

Aaron Hager

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