A case of resistant Palmer amaranth (pigweeds) to glyphosate (Roundup) has been confirmed in the Marana area, according to University of Arizona (UA) Area Extension Agent Ayman Mostafa.

Marana is located just north of Tucson in Pima County.

Based on the pattern, it is possible resistant seeds have been spread with water. 

Growers in the Cortaro Irrigation District and downstream users of the San Carlos should be particularly concerned. In this area and others watch for the efficacy of Roundup.

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If a glyphosate application has not significantly ‘melted’ these pigweeds within several days, it is safe to assume resistance. The plant should be mechanically destroyed or sprayed with an alternative herbicide.

“The good news is we caught this outbreak relatively early, and we have the opportunity to limit the impact on growers if they take actions right away,” Mostafa said.

“To wait until next season and just add ‘yellow’ residual herbicides is insufficient, and risks the amount of resistant pigweed seed increasing exponentially, along with the costs growers will incur to save their crops,” Mostafa said.

Click here for information from UA Weed Scientist Bill McCloskey on Palmer amaranth with an updated status on herbicide resistant and options available to deal with this issue in Arizona.

The first confirmed case of glyphosate-resistant pigweed found in Arizona in Buckeye in a cotton field several years ago in Maricopa County, just west of Phoenix..