It’s a catch-22 for the potato industry on the Northern Plains. The soil harbours verticillium inoculum, which is responsible for the yield-robbing early-dying complex. But one of the most susceptible varieties is Russet Burbank, a dominant variety in the region.
“There are resistant varieties, but none of them seem to have gotten much of a toehold in the industry,” North Dakota State University Professor Neil Gudmestad told growers at this winter’s Manitoba Potato Production Days. This makes soil fumigation with metam sodium a common management choice, but he says some of his recent research shows that getting applications done properly can be a big challenge.
Gudmestad said that’s because of the dispersion of the disease-causing infections in the soil. Most is found in the top few inches of the soil profile and typically the metam sodium is shanked in between six and 10 inches deep — but there’s no guarantee it will stay put.
“It’s really difficult to trap it at six inches or less,” Gudmestad said. “You can do a water trap or pack it and try to keep the gas in, but ultimately if that top four inches is drier, it’s going to escape and your kill is going to be less effective. Our work looks at getting that top four inches.”
So far what that work has found is few easy answers, but several management practices may play a role. For example, rotation is important because it takes 18 to 20 months after a potato crop before all the verticillium is released from the soil, and if it isn’t released metam sodium won’t get at it.
“Every other year is not enough time to break down that organic matter and release that verticillium,” he said. “Two years doesn’t work for us. There isn’t a single farm that I can think of in North Dakota or Minnesota on a two-year rotation. They’ve all gone to a three- and four-year rotation.”
The study also found metam sodium was most effective on sandy loam soils, likely because it had less organic matter to absorb the product.
“Organic matter is really problematic when it comes to getting metam sodium to work,” Gudmestad said.
Fewer control options
For U.S. growers, recent regulatory changes have also highlighted the challenges, he said. New EPA regulations have all but eliminated chemigation applications.
“The regulations call for buffer zones so huge it’s impractical,” he said. “It’s all shank from here on out.”
He said that’s causing “real issues” around whether farmers can even practically make these applications, given temperature recommendations for the product. It requires soil temperatures above 70 F (21 C) to work best according to the label, something that’s rare in North Dakota after Sept. 15 or so.
“We run out of time in North Dakota,” Gudmestad said.
One of the most surprising findings of the study is one Gudmestad says he’s still struggling to explain. Despite the fact that more verticillium was found in the top four inches, that didn’t appear to be the greatest source of infections. Those seemed to be coming from the mid-range of four to eight inches.
“There was a more robust and significant relationship with the amount of verticillium in the four- to eight-inches zone than in the zero- to four-inch zone, even though the amount of inoculum was significantly less,” he said.
While more research is needed to find out why, Gudmestadt says he suspects something is making the inoculum in the top part of the soil less efficient at causing infections — something he ruefully conceded he didn’t expect when he began working on verticillium 10 years ago.
“If I’d have thought a bit more deeply 10 years ago, I’d probably have the answers today, but I’m only getting a little closer,” he said.