JACKSON, Tenn. — Tests conducted last week at the University of Illinois showed that a gene present in PPO-resistant waterhemp is now present in Palmer amaranth samples taken from Tennessee soybean fields.

Weed scientists expect additional testing will confirm Palmer amaranth resistant to PPO (protoporphyrinogen oxidase) inhibitors – or Group 14 herbicides – has moved another state north. Larry Steckel, University of Tennessee weed scientist, said “stout rates” of post-emergent PPO inhibitor herbicides have not consistently controlled Palmer amaranth at heights of 2 inches and less in some isolated West Tennessee locations.

With the existing evidence, “One can surmise it is likely that we have PPO-resistant Palmer in Tennessee,” he said.

That means Tennessee growers should plan for PPO-resistant Palmer in 2016. Weed scientists recommend growers adopt a program approach for resistant weeds: start with a weed-free field and apply two pre-emergent residual herbicides, followed by a post-emergent herbicide with residual action. “In other words, overlay residuals and do not allow the Palmer to emerge, is Plan A,” said Steckel.

If Palmer amaranth does emerge, university tests indicate some herbicides will control PPO-resistant weeds less than 1 inch. Commonly used post-emergent PPO inhibitor herbicides include Flexstar, Reflex, Ultra Blazer and Cobra, as well as combination herbicides like Prefix, according to Steckel.

A future strategy for controlling PPO-resistant weeds may be dicamba-resistant soybeans – like Roundup Ready 2 Xtend and Enlist. But since availability of that seed technology is uncertain, he and other weed scientists are advising growers plan their herbicide program to emphasize pre-emergent controls.

Palmer amaranth – a cousin of waterhemp – is in the pigweed (Amaranthus) genus. It can be confused with red-root and smooth pigweed. A native of the desert Southwest, Palmer amaranth grows more than six feet tall. Palmer spreads more rapidly than pigweeds native to the Midwest.

Weed scientists advise Corn Belt growers to eradicate Palmer amaranth before it can build resistance. “Likely areas of infestation would be near cattle or swine operations that import feed or bedding from southern states, especially those that use cottonseed products or gin trash,” according to Iowa State University weed scientist Meaghan Anderson.

A fact sheet detailing how to distinguish Palmer amaranth is available from ISU at www.weeds.iastate.edu/mgmt/2013/ palmerpostersmall.pdf