Thomas C Mueller

Potential Speaker for plant biology conference 2017-Thomas C Mueller

Title: Herbicide Resistant Amaranthus palmeri poses major evolutionary and ecological challenges to broad acre farming operations in the United States.

Thomas C Mueller

University of Tennessee, United States


Dr. Mueller grew up on a small grain farm in rural Illinois, where his family farm produced corn, soybeans, and wheat. Through several experiences in the private sector, he came to understand the pragmatic aspects of agronomic productions systems. He joined the University of Tennessee in April, 1991 as an assistant professor, was promoted to associate professor in 1996 and to professor in 2003.  The primary goal of Dr. Mueller's research is to develop and validate weed control systems that are effective, economical, and environmentally sustainable. Specific research areas include weed control in soybeans and corn, especially under no-tillage production systems, the environmental fate of herbicides, and a focused research effort examining glyphosate-resistant weeds.


A major purpose of plant biology is the interaction of plants that produce food for humans (crops) and those that compete and challenge those crops (weeds). The evolutionary struggle between crops and weeds is as old as agriculture itself. In recent decades, new technologies have simplified weed control and broad acre production agriculture, with herbicides commonly used to reduce weed competition with crops. However, the intense selection pressure placed upon vast areas of farmland has resulted in weeds developing immunity, or resistance, to these same herbicides. This report details the weedy plant Amaranthus palmeri, a plant that is infamous in the United States for rapid growth (up to 8 cm per day), large seed production potential (> 100,000 seeds per female plant), and wide genetic variability that has allowed it to develop resistance to multiple herbicide modes of action (MOA).

            A common response of many farmers in highly developed countries to herbicide resistance is to simply change to a different MOA. One reason why glyphosate resistant crops were so widely and rapidly accepted by producers in the Americas was that glyphosate applied to foliar plants controlled many weedy species, including A. palmeri, that have developed resistance to other MOA. However, once glyphosate resistance was apparent farmers change to use other MOAs, including PPOs. This report details PPO-resistant A. palmeri in the United States.

            Three putative PPO-resistant populations were collected from the Mississippi River Valley from the states of Tennessee and Arkansas, United States. These populations or biotypes were compared to a population that was known to be susceptible to the PPO herbicide fomesafen.  A. palmeri response on a 0 to 100 scale (no effect to complete death) 14 days after treatment was 98% for the susceptible population and less than 25% for all three of the resistant populations.  A separate study showed that the weed size at the time of planning affected control with less control of plants that were 12 cm compared to 4 cm in height. Studies examining alternate modes of action showed that the PPO-Resistant were also insensitive to glyphosate and ALS inhibiting herbicides. Additional studies growing the A. Palmeri plants in the absence of herbicide applications showed no phenotypic differences between Resistant and Susceptible biotypes, indicating no apparent fitness penalty for the observed herbicide resistance. Plants with herbicide resistance to multiple modes of action is a very real challenge to producers and all available choices should be considered to maintain viable production systems that are effective, economical and environmentally sustainable.