Herbicide Resistance in Waterhemps and Related Species

	Patrick Tranel Assistant Professor Department of Crop Sciences (217) 333-1531; tranel@illinois.edu
		William Patzoldt Graduate Research Assistant Department of Crop Sciences (217) 333-4723; patzoldt@illinois.edu

Common and tall waterhemp (Amaranthus rudis and A. tuberculatus) are dynamic weed species that have become a major concern to crop producers in Illinois. Efficient, long-term management of these weeds may be possible only by studying mechanisms of their biology and interactions within the Amaranthus family of plant species.

Counties (green) in which waterhemp and related Amaranthus species samples were collected in 1998 and 1999. The collection includes 131 samples, 63 of which are waterhemp.

Figure 1. Counties (green) in which waterhemp and related Amaranthus species samples were collected in 1998 and 1999. The collection includes 131 samples, 63 of which are waterhemp.

A major problem associated with waterhemp control has been its ability to develop resistance to herbicides, most notably the family of acetolactate synthase (ALS) inhibitors. ALS herbicides are typically not considered for use in controlling waterhemp populations because widespread levels of resistance are currently found throughout the state. However, ALS herbicides are routinely used for the control of other species within the Amaranthus family (e.g., smooth and redroot pigweed).

To address questions of herbicide resistance and management of waterhemp and related species, research has been conducted on collections of waterhemp and other Amaranthus species from 36 counties in Illinois (Figure 1).

Our research has confirmed that Illinois waterhemp populations are highly variable in their responses to ALS herbicides. Over 70 percent of the samples contained at least one plant that showed little or no injury following application of imazethapyr (Pursuit) at one-half the normal field use rate. Much less variability in response to glyphosate (Roundup Ultra) was observed among the waterhemp populations, ranging from 60-100 percent injury with a one-fourth normal field use rate.

Crossing experiments with waterhemp and other Amaranthus plants have been performed and resulted in the production of hybrid plants that inherited ALS resistance (Figure 2). The hybrids were produced by crossing ALS-resistant plants with sensitive plants in a greenhouse or growth chamber. Progeny were confirmed to be hybrids by herbicide screening and molecular genetic analysis. Our ongoing work will determine the significance of hybridization as a non-conventional mechanism for the evolution of herbicide resistance in Amaranthus species.

Leaves from hybrid Amaranthus plants. Common waterhemp (L), redroot pigweed (R), and waterhemp X redroot pigweed hybrids (center two).

Figure 2. Leaves from hybrid Amaranthus plants. Common waterhemp (L),
redroot pigweed (R), and waterhemp X redroot pigweed hybrids (center two).

Waterhemp is only able to produce seed by the dispersal of pollen. Pollen carrying herbicide resistance genes potentially could spread this trait into new areas. Field experiments to measure the pollen dispersal from common waterhemp were performed in 1998 and 1999. Male waterhemp plants that carried ALS resistance were placed in the center of a spoke arrangement of female waterhemp plants in varying directions and distance. Seed from each female plant was collected and is currently being screened for ALS resistance. Preliminary results suggest pollen may spread further than previously expected.

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Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension