A probable case of glyphosate (Roundup) resistant Palmer amaranth, Amaranthus Palmeri, has been discovered in an Arizona cotton field where glyphosate was the only herbicide used.

Seed collection and tests are underway to confirm the case.

Arizona cotton farmers should adopt a zero tolerance attitude towards Palmer amaranth to avoid dire consequences.

Palmer amaranth plants are male or female, and resistance genes can be spread by pollen and seed. Studies indicate Palmer amaranth pollen can move at least 164 feet in gentle breezes - longer distances with winds or ‘dust-devils.’

One female plant can produce over a million seeds.

Sanitation or the physical removal of suspected resistant plants will stop pollen release and seed production and may slow the spread of Palmer amaranth.

Arizona’s isolated agricultural valleys may also help delay the spread of resistance for a significant period of time if all growers follow the following practices.

In-field sanitation

Hand remove all Palmer amaranth escapes in the field or on the ends of fields. Do not allow pollen spread or seed production. Plants with large diameter stems will re-sprout when cut. Regrowth must be controlled.

Clean and remove all plant parts including seeds and soil from machinery before leaving infested fields. Do not transport seeds to other fields. Harvest machines have the potential to spread resistant seed.

Farmstead areas

Kill all Palmer amaranth plants on the farm with ‘burn down’ herbicides (e.g., Aim, ET, or paraquat), propane, or by mechanical means to stop pollen and seed production, especially along irrigation ditches, canals, fence lines, farm roads, and equipment yards.

Community action

Work with state and local governments plus neighbors to control Palmer amaranth infestations on public roads, other public rights-of-way, and in nearby residential enclaves.

Limiting the spread of glyphosate-resistant Palmer amaranth is a challenge. Using a diversity of herbicide mechanisms of action and weed control practices can help in cotton.

Cotton growers with resistant Palmer amaranth will face increased weed control costs by using herbicides and control techniques which require greater management input, hand labor to remove escapes, and more in-season tillage for weed control.

To view the entire University of Arizona web page on this issue, click on this link: http://ag.arizona.edu/crops/cotton/files/ResistantPalmerShort.pdf .