Hard to control common chickweed may be a little tougher than before, as it becomes more likely this pesky pest has developed multiple resistance to several commonly used herbicides.

Herbicide resistant chickweed has popped up in Europe and the Pacific Rim in recent years and it appears the problem may have reached the Upper Southeast, which could be a major threat to cereal grain production in the area.

Virginia Tech Weed Scientist Scott Hagood has tested wild common chickweed versus chickweed suspected of having resistance to Harmony GT (thifensulfuron). “In terms of plant weight, using .5 up to 32 times the label rate of Harmony GT we saw 98 percent or better control of the wild chickweed. On the two samples from grower fields, the best we could get was 50 percent control and on some of these plants we got virtually no control,” Hagood says.

In chickweed collected from one particular field in New Kent County, Hagood says 32 times the label rate had no visible effect. Of more concern, he says, samples from this field were sprayed with a number of other sulfonylurea herbicides, with a similar lack of control.

“We also tested these resistant plants to other ALS herbicides other than the sulfonylurea herbicides. The New Kent site was resistant to both Pursuit and Arsenal. Arsenal in particular is a very persistent and efficient herbicide and resistance to this material should give growers some insights as to how difficult multiple herbicide resistant chickweed will be to manage,” Hagood says.

Considering how widespread chickweed is in the Upper Southeast, Hagood contends multiple resistance to commonly used herbicides could be a bigger problem than herbicide resistant Italian ryegrass in wheat.

There are some management tools left to handle multiple resistant chickweed, if it becomes a problem in the upper Southeast. Hagood says the same multiple resistant chickweed from New Kent County, sprayed with Valor at 2 and 4 ounces per acre, applied pre-plant, gave 98 percent to 100 percent control of the resistant weeds.

If Valent (manufacturers of Valor) can get their label restrictions down to 14 days pre-plant from 30 days, Valor will be a strong tool to use in controlling herbicide resistant chickweed, he adds.

Other effective herbicides in controlling herbicide resistant chickweed in wheat from Hagood’s test include:

• Prowl H20, which gave 98 percent control.

• Spike on one-leaf wheat gave 98 percent control.

• Axiom 94 percent control with no plant injury.

• Sencor at 1 and 2 ounces, safe on heavier soil, gave 94 percent control.

Hagood says there are fewer alternatives once resistant chickweed is found in wheat. Post applications of Harmony did not work in his tests. In fact, he says there was no visible difference between Harmony and check plots. If a grower gets into a situation where wheat is planted and resistant chickweed is found, 2,4-D plus Banvel provided about 63 percent control.

The most encouraging post-application tool is Starane, commonly used in pastures, but registered for wheat.

“I commend DuPont for their support of our work on Harmony resistant chickweed. They didn’t stick their head in the sand and try to ignore it — they have been in a leadership role in trying to help us find some solutions,” Hagood says.

Common chickweed (Stellaria media), a dicot weed in the Caryophyllaceae family, is a common problem for cereal grain producers throughout the World.

In Europe this weed first evolved resistance to Group B/2 herbicides in 1996, and has been a persistent management problem for wheat, barley and oat growers for over a decade. Group B/2 herbicides are known as ALS inhibitors (Inhibition of acetolactate synthase ALS (acetohydroxyacid synthase AHAS)). Research has shown that these particular biotypes are resistant to metsulfuron-methyl and they may be cross-resistant to other Group B/2 herbicides.

In New Zealand, chlorsulfuron-resistant chickweed was discovered in 1995. Experiments were conducted in the field and greenhouse to determine whether these resistant plants were also cross and multiple-resistant to other herbicides normally effective on chickweed.

In one test in New Zealand a population of chlorsulfuron-resistant chickweed growing in an oat crop was treated with 13 different herbicides. This population was not controlled by mecoprop, methabenzthiazuron and pendimethalin, and only partially controlled by bromoxynil + ioxynil, diflufenican +isoproturon and diflufenican + bromoxynil in the field trial.

This early research in New Zealand demonstrated chickweed’s ability to combine multiple resistance to different families of herbicides in the same plant. It appears multiple resistance may have developed in chickweed in the Southeast and other areas of the U.S.

Currently, common chickweed is one of eight weed species with documented resistance to herbicides in Virginia. These include:

• Redroot pigweed.

• Smooth pigweed.

• Lambsquarters.

• Johnsongrass.

• Annual bluegrass.

• Italian ryegrass.

• Shattercane.

• Common chickweed.

In North Carolina, these weeds are joined by goosegrass, Palmer amaranth, common cocklebur and horseweed as being resistant to one or more herbicides.

Hagood urges growers to be aware of the threat of resistant chickweed. It is common in wheat and barley, even in pastures. If it becomes widespread it can be a big problem for wheat and barley production in Virginia, he cautions.

e-mail: rroberson@farmpress.com