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Results on Herbicide Resistance in Illinois

May 10, 2002
Herbicide-resistant weed biotypes have plagued Illinois growers for more than 20 years. During this time we have confirmed nine different herbicide-resistant weed biotypes in Illinois. All of these biotypes were suspected herbicide-resistant weeds that were sampled and sent from various locations around the state to the University of Illinois to be tested for resistance. From these samples and others, we have been able to keep fairly good records on where, when, and how widespread the herbicide-resistance problem is. An extensive list of confirmed herbicide-resistant weed biotypes in Illinois is presented in Table 4. However, like most other problems in agriculture, herbicide resistance has continued to spread throughout the state.

In the last issue of the Bulletin, we reported results from the "Illinois Invasive Weeds Survey" that was conducted this past winter. A series of questions from this survey was focused on herbicide resistance to give us a broader perspective on how widespread the problem is, what new species are suspected to be resistant to what herbicides, and when the resistance problem was first encountered. From this survey, 43% of the participants answered yes, indicating that they have a herbicide-resistant weed problem in their area. There were 26 different weed species identified as resistant to nine different herbicide classes, showing up as early as 1986.

The results from this survey identified Amaranthus spp., ragweed spp., common lambsquarters, and common cocklebur as comprising 80% of the herbicide-resistant weed species in the state (Table 5). These four weed species have been confirmed resistant for a number of years to the ALS inhibitors, triazine herbicides, or both classes, depending on species (Table 4). However, the survey participants identified some of these weeds as also being resistant to other herbicides, including glyphosate and the PPO inhibitors (i.e., Flexstar, Ultra Blazer, and Cobra). Even though none of these species have actually been confirmed resistant to these herbicides in Illinois, there have been reports of waterhemp not being effectively controlled by glyphosate and the PPO inhibitors (diphenyl ethers). Some of these escaped waterhemp plants may be due to environmental conditions; however, there are a number of populations that don't seem to fit this explanation.

In Illinois, Iowa, and Missouri, weed scientists are currently working with waterhemp populations that have not been effectively controlled with glyphosate and have determined that some of these populations have increased tolerances. While there are no confirmed glyphosate-resistant waterhemp populations, there is still some concern that this could be a potential problem in the future. In addition, during this past growing season, there were some reports of waterhemp populations resistant to the diphenyl ether herbicides. These populations have not yet been confirmed in Illinois; however, this past year, Kansas State University identified a waterhemp population resistant to this family of herbicides.

The survey also identified other weed species that have not been confirmed resistant in Illinois. Some of these species include horseweed, velvetleaf, morningglory, and woolly cupgrass (Table 5). Most of these species were identified by only a few participants and may not truly be resistant to these herbicides. However, in the case of horseweed, there have been confirmed cases of resistance to ALS inhibitors in Ohio and glyphosate in Delaware and Tennessee. While none of these species are currently confirmed resistant in Illinois, there is still some concern that this could be a potential problem in the future.

To learn more about herbicide resistance and management strategies that will help delay the development of herbicide-resistant weed biotypes, refer to the article "Herbicide Resistance: Where Are We?" in issue no. 3, April 13, 2001, issue of the Bulletin.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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