Midwest: PPO Resistant Pigweeds on the Rise

PPO resistant pigweeds are increasingly more common in the cornbelt as we rely more on these herbicides for weed management in soybean. Using integrated weed management tactics that include effective cultural and mechanical control measures are more important than ever.

Farmers are increasingly relying on the Group 14 or PPO-inhibiting herbicides to control problem weeds in soybean. The common POST Group 14 herbicides include Reflex/Flexstar (fomesafen), Cobra (lactofen), Ultra Blazer (acifluorfen), and Cadet (fluthiacet) which often follow a preemergence Group 14 application that contains a Valor (flumioxazin) or Authority (sulfentrazone) product.

The repeated use of PPO’s in soybeans that is increasingly includes rescue POST applications (Group 14) on big weeds has likely led to the selection for PPO-resistant pigweed populations.

Travis Legleiter and Bill Johnson of Purdue University reported in the most recent issue of “Pest and Crop Newsletter” that populations of PPO-resistant waterhemp and Palmer amaranth are increasing in Indiana.

The Purdue Weed Science Group has been screening pigweed samples this summer sent in by farmers and industry reps for resistance to Group 14, glyphosate (Group 9), and ALS inhibitor (Group 2) herbicides. So far in 2016, 22 fields from 10 counties within Indiana have been tested for resistance with more samples waiting to be analyzed.

The results thus far show that 31 of 92 counties have glyphosate-resistant waterhemp. PPO-resistant waterhemp has been confirmed in 14 counties with most occurring in the southwest portion of the state.

In this year’s samples from 20 different fields, 10 have confirmed PPO resistance in waterhemp in one or more plants. Palmer amaranth, a more recent invader than waterhemp to Indiana is confirmed to be PPO-resistant in a single population from the samples submitted.

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Nationally, PPO-resistant waterhemp was first identified in 2001 in the state of Kansas and now has a presence in at least seven states from Kansas to Indiana and North to Minnesota.

PPO-resistant Palmer amaranth is less common likely only because it is a more recent problem, but is currently reported in Arkansas, Tennessee, and now Indiana. Interestingly, PPO-resistant smooth pigweed has been identified in Bolivia with resistant redroot pigweed in Brazil.

This is just another tale confirming that over-reliance on certain herbicide families quickly leads to herbicide resistant weeds. The utility of the Group 14 herbicides are in jeopardy, particularly for these problematic invasive pigweeds, but also for the ragweeds and likely marestail/horseweed.

The take home message is to diversify the weed management program not only rotating herbicide sites of action and using effective herbicides mixtures, but also relying on integrated weed management tactics that include effective cultural and mechanical control measures.