As if Virginia farmers didn’t have enough problems with drought, high prices for fertilizer and a plethora of other production problems, now they have to contend with Palmer pigweed.
Virginia Tech Cotton Specialist Joel Faircloth says populations of the highly troublesome weed have been identified in at least five counties in southeast Virginia. “We have located it in soybean and cotton fields primarily, but it is also showing up in neighborhoods and common areas brought in by topsoil. These populations have been fairly isolated and not very dense in most cases, being specific to fields or in some cases whole farms,” he says.
Though no positive identification has been made of glyphosate resistant Palmer pigweed, Faircloth points out that resistant plants have been documented in North Carolina fields that are no more than 20 miles away from some of the Palmer pigweed found in Virginia.
Currently, the populations tested in Virginia have displayed resistance to several ALS-inhibiting herbicides (Staple and Envoke). While all have not been tested in Virginia, they are also likely to be resistant to other, if not all ALS-inhibiting herbicides (Accent, Ally/Cimarron, Beacon, Beyond, Cadre, Strongam, Classic, Exceed, Express, Finesse, Glean, Harmony Extra, Maverick Pro, Osprey, Peak, Permit, Pursuit, and Scepter).
Of the many weeds that are a challenge to cotton and soybean production in the Southeast, Palmer pigweed is widely regarded as the most dangerous in terms of yield loss. While all plants have the ability to develop resistance to any herbicide, Palmer pigweed has a huge advantage in ‘bad’.
Palmer amaranth, commonly called Palmer pigweed, can produce up to 500,000 seeds per plant. In mathematical terms, if developing resistance is a one in a million chance, it only takes two mature Palmer pigweeds to provide enough seed for resistance to occur. Up the ante to one in a billion and it only takes 20 pigweed plants to produce enough seed.
In addition to ALS and glyphosate resistance, Palmer amaranth has also demonstrated the ability to develop resistance to dinitroanaline herbicides (yellow herbicides like Prowl, Treflan, Sonalan, etc.) in North Carolina and South Carolina.
Far from panic time, Faircloth says there are various strategies for controlling this weed that were and will again be covered in winter cotton and peanut production meetings. “We are conducting on-farm research and depending upon the experience of other Southeastern weed scientists to develop recommendations,” he points out.
Faircloth adds that relative to other states in the Southeast and Mid-south, there is a negligible population of Palmer amaranth in Virginia. With few exceptions the areas where this weed has been observed thus far are mostly isolated and populations are manageable.
This is not the case in many areas of the Southeast. “Recently driving through the Mid-South and southeastern U.S., it was astounding to see the populations in cotton and soybean and roadsides and medians,” Faircloth says.
“While realistically we cannot eliminate the spread of this pest in Virginia, we have the opportunity to slow its spread. Inspecting fields now, when Palmer amaranth is most recognizable due to its distinct seed head is critical,” Faircloth stresses.
The Virginia scientist encourages dealers and producers to report any suspicious plants to county agents. To help in identifying Palmer amaranth, Virginia Tech weed scientists have posted pictures and description on the pesky weed on a Web site: http://www.ppws.vt.edu/scott/weed_id/amapa.htm.
Because of the multiple herbicide resistance capabilities of Palmer pigweed, it is critical that farmers know the mode of action of chemicals they use on their crops. Of the herbicide families used to control Palmer pigweed, only the protoporphyrinogen oxidase inhibitors, or PPOs, have shown no resistance problems. However, weed scientists urge growers to carefully use the PPOs, including such popular herbicides as Blazer and Reflex, to avoid resistance problems.
In addition to the PPOs, herbicide manufacturers, Syngenta in particular, has brought back some old herbicides that can provide some options for growers who have problems with Palmer pigweed or other difficult to control weeds.
Despite some limited success on mature Palmer pigweed with PPOs and other herbicides, the consensus of weed scientists around the South seems to be once these pests get 7-9 inches tall growers have few options for control.
The best option, glyphosate is a dangerous one because of Palmer pigweed’s known ability to develop resistance to this family of herbicides.
A number of herbicide resistance programs are available to help growers avoid Palmer pigweed resistance and to reduce, if not eliminate, the problem should it arise. In Virginia, Faircloth urges growers to be aware the weed is in the state and is likely to increase in occurrence, but that it is manageable.
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