AgronomySoil Fumigation in Potatoes

Soil Fumigation in Potatoes



While it is practiced extensively in the United States, soil fumigation is anything but common in potato production in Canada. But there’s a growing voice within the industry that views soil fumigants like chloropicrin as an important tool that should not be overlooked for ensuring healthier, better yielding potato crops.

For this edition of Roundtable, Spud Smart asked the following six experts to weigh in on the use of soil fumigation in potatoes:

  • Tom Dixon, a McCain Foods agronomist based in New Brunswick. Dixon is helping oversee a three-year trial of the soil fumigant chloropicrin that’s part of the New Brunswick Potato Industry Transformation Initiative.
  • Eugene Antworth, who grows potatoes in Upper Knoxford, N.B. He’s been taking part in the New Brunswick fumigation trials since the start of the project in 2014.
  • Roy Culberson, a potato producer in Jacksonville, N.B., who’s also participated in the trials since 2014.
  • Homer Vander Zaag, an Ontario potato grower who’s practiced soil fumigation for many years at his farm near Alliston, Ont.
  • Chad Hutchinson, director of research at TriEst Ag Group, the North Carolina-based distributor of chloropicrin.
  • Jeff Douglas, TriEst’s point person for chloropicrin in Canada and the fumigant applicator in the New Brunswick chloropicrin trials.

Soil Health

Dixon maintains any growing area that’s prone to verticillium wilt, nematodes and potato early die is a good candidate for soil fumigation. “Growers [in these areas] who have been cropping their land for many, many years … and may have shorter rotations, may have more soil disease presence as a result.”

Many farms in New Brunswick’s picturesque St. John River Valley, with its long history of potato production, fall into that category. It’s why the province’s potato industry decided to embark on the trials testing the efficacy of chloropicrin, a broad spectrum product that fights numerous soil-borne diseases that are important to potato.

Dixon is among those who believe soil health as important as canopy health when it comes to producing good potato crops.

“The way I look at it is if we have early blight, or late blight or any type of foliar disease, we’re going to try to spray the crop so that it mitigates the risk of it being devastated. If we have an insect problem, we’re going to try to keep insects out so that they don’t cause problems for the crop as well. So I don’t think soil disease should be looked at any differently,” he says.

Soil fumigation can be used to control a host of problematic soil diseases in potatoes such as verticillium wilt, common scab, pink rot, fusarium and rhizoctonia, Dixon notes, and it also helps control nematodes, plant parasites that play a role in potato early die, by reducing populations.

“Typically, root lesion nematodes will infect the roots. The roots may then produce more root exudates and attract more microsclerotia (verticillium), thus leading to potato early die,” he says, adding anything that can help keeps plants healthy right from planting to harvest is worthy of consideration.

“As an agronomist, I just consider soil disease and nematodes similar to any other type of plant disease or any type of insect that may be deterring your crop. I don’t think we’ve paid as much attention to it as other ones because you don’t really see it until it’s late in the season when it’s already devastated your crop,” Dixon adds.

“If you see a Colorado potato beetle, you look at your economic threshold and then decide whether to spray, or if you know there’s a rain event coming or some type of problem conducive to early or late blight you’re going to protect your crop in advance with different decisions and support systems.

“We should think of soil disease or nematodes like we do other challenges in potato production,” he says. “At the end of the day the goal is to keep the crop canopy healthy and green as long as we can to maximize yields, and I believe soil fumigation is just another means to help achieve this.”

Dixon maintains soil fumigant products are similar to other types of pesticide, “be it insecticide or fungicide or herbicide,” that are managed under Health Canada’s Pest Management Regulatory Agency rules and regulations.

“You have to do a fumigant management plan, you have to have buffer zones, you have to post notifications about the fields and have emergency plans and whatnot. There’s a list of things from Health Canada that the applicator knows and follows so that it mitigates the risk of any type of potential health problem,” he says.

“I believe education is key,” Dixon adds. “A lot of people get tentative when they hear the word fumigation. But I think educating the public and the grower base and agronomists … [about] how to use it effectively and safely is the key to address those concerns.”

Vander Zaag, who has practiced soil fumigation for over a decade and has helped play a role in the development of safety guidelines, says the protocols outlined in the chloropicrin label have become much more stringent within just the past two years. “But it is quite manageable, and we’ve done a lot of fumigation with no issues,” he says.

Chloropicrin advocates say because the product can be selectively injected into just the seedbeds, rather than throughout the whole field, this reduces the environmental impact. Because fumigation can mean healthier crops, it can be argued that this also helps reduce the overall carbon footprint of potato farming. “Once your plant is healthy then it doesn’t need as many other inputs,” says Antworth.

Better Yields and Quality

TriEst’s Hutchinson, whose work for the past 17 years has focused on potato production and disease control issues in many countries around the world, maintains fumigation has a positive impact on both yield and quality in potatoes.

“These products are used to promote the healthy growth of the potato crop,” he says. “You end up with a much healthier root system that’s better able to scavenge and make use of applied water [and] applied nitrogen or fertilizer. All that translates into healthier growth and higher yields and higher quality.”

Dixon agrees that healthy root systems enable potato plants to utilize water and fertilizer more efficiently. “The parts of the plant that uptake water aren’t blocked off due to verticillium,” he says. “It makes for a more consistent, uniform run so you don’t have a lot of the great big large oversize potatoes or a lot of very small unmarketable potatoes.”

A healthier potato crop obviously means more profit for producers. Dixon says the cost of fumigating the New Brunswick test plots with chloropicrin ranged from $250 to $500 per acre but “all our trials are showing a positive return on investment. The increase can be significant if the field is heavily infected by verticillium and nematodes.”

The three growers who participated in the first year of the trials saw yield increases of up to 100 hundredweight per acre in their test plots (compared to non-fumigated control areas) in 2015, says Dixon.

He adds five growers are participating in the program this year and their test plots will be tracked throughout the season to see what the yield response will be. Antworth and Culberson are both involved in the trials again this year.

“I’m impressed with it because we’re involved in the initiatives that McCain is doing, and this one is the most promising,” says Antworth. “If we get the same results this year, then we’ll know better that it’s working.”

Culberson is similarly impressed with the first-year results of the chloropicrin test on his farm, but he’d like to see another year or two of trials before forming a firm opinion.

“Fumigation made it clear we definitely have an issue with soil-borne diseases,” he says. “The preliminary results looked very promising and more testing and trials are definitely needed before arriving at a conclusion. … For 2016, more acres and trials will give us a much better idea where we as an industry go from here.”

Vander Zaag applies chloropicrin selectively to his potato fields with the most disease-susceptible varieties, and he’s currently seeing about a 50-hundredweight yield increase with the product. Fumigation has also contributed to more size uniformity, he says, and as an added benefit, there’s been a significant decrease in common scab issues in his crop.

Vander Zaag considers himself one of pioneers of soil fumigation in Ontario, initially experimenting with the practice in the 1990s. Vander Zaag renewed his fumigation efforts 10 years or so ago when chloropicrin entered the marketplace and proved to be effective in controlling common scab in addition to other soil-borne diseases.

“There was an increase of scab as a major potato problem, especially here in the Alliston area with our tight rotations,” he says. “That was another reason that drove us to figuring out how to fumigate.”

According to Douglas, chloropicrin is an attractive option for Ontario growers dealing with common scab because there aren’t really any other chemical options for controlling it. “We have not been successful with any of the other products with any kind of scab control, where we do see that with chloropicrin,” he says.

How Does Chloropicrin Work?

According to Hutchinson, the process of fumigation with chloropicrin starts with the product being delivered to growers in registered containers. Under an application system developed by Hutchinson’s company, TriEst, the containers are attached to the tractor and fumigation lines are hooked up to an implement with injection shanks that insert the fumigants into the soil within the seedbeds.

Dixon says the fumigation implement used by Douglas in the New Brunswick chloropicrin tests resembles a one-pass hiller. Douglas says it’s the same implement that the company rents out to customers in Ontario and Quebec, but most potato farmers there who’ve been fumigating for a while will adapt their own equipment to apply chloropicrin to their own fields.

Regardless of what type of applicator is used, “it’s a closed system, moving right from the container that the product is delivered in, to the soil,” says Hutchinson. “The fumigant moves from that container through those fumigant lines into the soil without ever coming in contact with the grower or the air. … Once that container’s empty, the grower disconnects those lines and we come and pick up that container from the grower.”

The fumigant is in a liquid form until it’s injected underground, when it then changes from a liquid into a gas that moves throughout that soil profile.

“Chloropicrin is degraded in the soil by microbes, so over a very short period of time, less than three or four days, it is chewed up and degraded into some very safe products. One is carbon dioxide, others are nitrogen and a little bit of chlorine,” says Hutchinson. “Basically no residue is left behind.

“A lot of people have a perception that soil fumigants sterilize the soil, and that’s simply not the case,” says Hutchinson. “In fact, they’re fairly specific on the types of organisms that they do suppress, and there are several ‘beneficial’ organisms that really rebound well, say in the case of chloropicrin.

“If you apply chloropicrin, they come back in and re-inhabit the soil [and] you can see massive growth in these populations. We think that that provides some of the growth benefit that you see in the crop, the fact that after the application, you get rebound in populations of beneficials.”

Fall Application

In places with shorter growing seasons like Canada, fumigants like chloropicrin are normally applied to potato fields in the fall.

“Soil temperatures are still relatively warm then and the diseases are still respiring; they’re active and they’re easier to kill,” says Hutchinson. “You put the product in the ground, kill the diseases, and then the soil freezes, and remains in that frozen state until spring … [when] you’re ready to plant.”

Dixon says August-September is the ideal time to fumigate in New Brunswick since it’s typically fairly dry then, which helps the gas flow properly through the soil and avoids compaction and other moisture-related problems.

Douglas points out that soil pathogens are usually at their highest levels in the fall, and adds that both soil temperature and moisture are key to getting chloropicrin sealed in.

“We need at least 10 C soil temperatures for a number of days to make sure the organisms are actively respiring. If they’re not actively respiring, the fumigants just won’t control them,” he says. “We believe the fall is our best window of application, when we have to have soil moisture of at least 50 per cent tilth capacity or a little higher.”

Too much moisture can also be an issue. According to Hutchinson, applying fumigants in really wet soils can be trouble since “the fumigant won’t move well through the soil pores if those soil pores are full of water.”

Antworth agrees it can be a balancing act deciding when to fumigate. “It can’t be terribly dry and it can’t be terribly wet.”

According to Vander Zaag, having to change agronomic practices to accommodate fumigation in the fall represents a significant adjustment for growers.

“We’ve had to figure out how to make seedbed conditions in the fall to fumigate with, and then come back in the spring and not disturb those beds yet still figure out how to fertilize and have loose ground to plant potatoes into. It was quite an overall shift in concept,” he says. “We really feel we’ve figured it out, but it’s quite different from what most people do around here.”

Antworth agrees that preparing fields in the fall rather than the spring represents the biggest change for growers considering fumigation.

“It has to be done in September, and that’s when people are starting to harvest. You basically have to prepare your ground as if you’re going to plant the following spring. That’s a minor hurdle,” he says.

“I guess the way I look at it is it’s going to cost you one time or the other to do it. It comes down to more manpower … so as long you have the men around to do it, what difference does it make when you do it?”

Best Practices

The experts agree that chloropicrin fumigation in potatoes works much better coming off a grain crop rather than corn or soy.

“It pretty well has to be grain … [because} corn or soybeans come off too late to really fumigate, in our part of the world, anyways,” says Dixon.

Douglas says his company will perform soil fumigation after a corn or soy crop, “but it’s not our preferred way certainly. A small grain crop is off in good time and we don’t have that residue to deal with when we’re trying to fumigate.”

Vander Zaag agrees that’s one of the challenges of fumigation – “to make sure that all the straw and the trash from the previous crop is incorporated well enough that it does not negatively affect the process.”

Hutchinson shares the view that fumigation works best when there isn’t a lot of leftover residual organic matter like corn stover in the fields. “Those types of things affect the way the gas moves in the soil, so you want any type of large organic matter fairly well chopped up and incorporated throughout the soil profile,” he says.

Hutchinson stresses that land preparation is a key consideration, particularly with heavier soils. “It comes down to tilth, so even in heavier soils, you can often work that soil or prep that ground to be acceptable for soil fumigation,” he says.

“Land preparation is important, without a doubt,” says Douglas. “The fumigant moves through the air spaces in the soil, so if there are clumps and clods and stover and whatnot in there, first of all, those act as chimneys and the fumigant will find its way out. The other part is that inside of those larger clods and clumps, the fumigant can’t penetrate to get the organisms that are in there.”

Vander Zaag says in his experience, potato early die isn’t as much of an issue on heavier soils as it is on lighter ones, which is why he’s moved to variable-rate fumigation on his farm in the last three years.

“We have tried fumigating heavier soil, but it doesn’t seem to have the same effect,” he says. “We’ve found low areas with deeper topsoil or higher organic matter or more moisture-holding capacity don’t really suffer from early die, so we’ve created zones in our fields. Our units are all GPS controlled, so we’ve actually gone to quite an extensive range of variable-rate application.

“We’re probably using a zero rate on 30 to 40 per cent of our fields on the lower topography and going up to 150 per cent on our worst spots and getting fantastic results with that,” Vander Zaag says.

Antworth says if he decides to opt for fumigation at the conclusion of the New Brunswick trials, he’d likely start with problem fields that are lower yielding and go from there.

Dixon agrees that targeted fumigation probably makes sense for many potato operations. “Its just another tool in the toolbox. I don’t think every field on every farm needs it, and I don’t it needs to be a broadcast application for 100 per cent of your farm acreage,” he says. “I think it’s just another tool to help in suppressing disease.”

Fumigation More Common in U.S.

The number of Canadian producers utilizing soil fumigation is small potatoes compared to their counterparts to the south.

“In the U.S., the practice is used widely in a lot of the potato production areas and has been for quite some time,” says Tom Dixon. “There’s very little in potato production in Atlantic Canada, or in Manitoba or Alberta for that matter. … Soil fumigation in potatoes is primarily practiced in Ontario and Quebec.”

Chad Hutchinson estimates that as much as 50 to 60 per cent of the potato crop south of the border uses some form of soil fumigant. Potato growing areas in the U.S. where it’s most common, he says, is “where the pest pressure is greatest, so probably where historically there has been less rotation or the growth of varieties that have less resistance to some of these different pests. That’s where you’re going to find soil fumigation.”

Jeff Douglas agrees. “I think it’s just the land has been farmed a lot more intensely in the U.S. and the pathogen levels have built up to a point and the land supply is short,” he says. “They work that land pretty hard and they need to clean it up. Now we’re starting to experience that here in Canada.”



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