The possibility of sicklepod weed becoming resistant to herbicides is a potential concern for all Georgia peanut farmers, said Eric Protsko, a weed scientist with the University of Georgia College of Agricultural and Environmental Sciences.
Sicklepod weeds look similar to peanut plants, though the leaves are a little wider and a lighter green than those of the peanut plant. The weed is a concern for peanut farmers every year because the seed remains viable in the soil for at least five years and can germinate from a 5-inch soil depth. This makes the weed almost impossible to control with residual herbicides, and there are no peanut herbicides that provide adequate residual control.
Sicklepod is especially threatening, considering it is self-pollinating, meaning it doesn't require additional plants or insects to spread throughout a field. Approximately 14,000 seeds are produced per plant, far fewer seeds than Palmer amaranth.
Cotton farmers have struggled to contain Roundup-resistant Palmer amaranth in recent years, and now some peanut farmers are having difficulty managing sicklepod.
Through greenhouse research on the UGA Tifton Campus, Prostko is studying whether this is a production problem or a resistance issue.
"Now that our senses are heightened because of our problems with Palmer amaranth, we are looking at whether every failure we've had has been a true herbicide resistance problem. That's the issue," Prostko said. "Enough people start talking about it, a couple of good growers tell you they're experiencing management concerns, then maybe we do need to take a closer look at it."
To complete this research, Prostko, UGA graduate student Wen Carter and fellow UGA weed science researcher Bill Vencill are studying the effects of the herbicide Cadre, which may be farmers' best treatment option against sicklepod. The research project has more than a year left before it's complete, as 30 populations need to be screened.
In some preliminary screenings, Vencill identified a few suspect sicklepod populations, according to Prostko. "However, we don't know how widespread it is or how much of a problem it is. I think there are other issues that might be going on, such as delayed applications or reduced spray coverage at faster tractor speeds. The only way to know for sure is for us to do what we're doing and come up with a better picture of what might be out there," Prostko said.
Timing is crucial for the researchers as they don't want to allow the test plants to produce seeds. "If we let them go to seed, you've just made a big deposit in the soil seed bank, which could be a potential problem," Prostko said.
Farmers' best mode of action is to treat with Cadre; however, those actions may be futile if resistance is truly an issue.
"I wouldn't be surprised if we had a lot of resistance because it (the herbicide) has been around a while," Prostko said.
"I try to tell growers when we're talking to them, 'Just because you've had a failure, this doesn't mean it's always a resistance issue.' But if you have a good grower who's trying to do things right, and you know that grower is a good grower, if he observes something wrong, then it does make you wonder. That's how it starts, somebody says, 'I'm not really seeing what I used to see. What's going on?'"
Prostko said the best time to manage sicklepod is when it's 2 to 3 inches tall. This can be hard to see later in the year, when the peanuts are covering the ground. As the weed increases in size, the success rate of treating it decreases.
"By the time you do see it, it's up to the top of the canopy, and it's really too big to treat," Prostko said.