UGA researchers confirm PPO-resistant Palmer pigweed

georgia PPO herbicide trial
Figure 1

• By Stanley Culpepper and Taylor Randell •

Research over the past three seasons has confirmed a Georgia population of Palmer amaranth to be resistant to topical applications of PPO herbicides including Reflex, Cobra and Blazer (Figure 1).

For cotton and peanut farmers, the question on everyone’s mind is if the residual activity of Valor and Reflex will still control these resistant pigweeds? The initial study results are concerning. Figure 2 shares the comparison of a nontreated control in the left photo, the response of a “normal” pigweed population in the center and the response of the concerning Georgia population in the right photo (the population with confirmed resistance to topical PPO applications).

Similar results are being observed with Reflex, although symptomology is quite different. Obviously, this is a very serious threat to farm sustainability and must be taken seriously. If you are concerned that you have a field or two with pigweed escaping through the residual activity of Valor or Reflex, contact your Extension agent or the manufacture to develop a specific management program that limits development and spread.

palmer response to valor
Figures 2

For others, a sound program following the practices below is suggested:

1. No weeds emerged at planting (cover crops, tillage, burndown).

2. Two residual herbicides at planting.

3. Timely post herbicide tankmix applications.

4. Layby directed or hooded application in cotton.

5. Remove escapes

And finally, one must accept that relying on herbicides alone is futile when managing Palmer amaranth (or ryegrass)…..those farmers understanding the value of diversity in their management approach and implementing sound programs will be more sustainable in this battle.

Dr. Stanley Culpepper is a University of Georgia professor and Extension agronomist. He may be reached at Taylor Randell is a UGA graduate student.

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