Fertility and Nutrients
Brassicas in potato rotations
By Carolyn King
One of the more than 70 USDA studies in Maine looking at the effects of crops like mustard, rapeseed and barley in potato rotations. Photo by USDA-ARS
If you’re dealing with some tough soil-borne diseases, adding canola, mustard or rapeseed to your potato rotation could help.
That important finding emerged from recent potato rotation studies in Maine, led by Dr. Bob Larkin, a research plant pathologist with the United States Department of Agriculture (USDA).
Over the last 12 years or so, the USDA researchers have conducted more than 70 trials to investigate the effects of different rotations on soil-borne diseases in potatoes and on potato yields. Although the results varied from year to year and field to field, overall, Larkin and his research team found that crops in the Brassicaceae family, such as canola, rapeseed and mustard, consistently reduced potato diseases like black scurf, common scab and Verticillium wilt, and significantly improved potato yields.
Now researchers in Atlantic Canada will be examining the effects of Brassica crops as part of a major potato rotation project.
Larkin explains there are three general mechanisms by which rotation crops may reduce soil-borne diseases – and Brassica crops likely act in all three ways.
“The first mechanism is that the rotation crop serves as a break in the host-pathogen cycle,” he says. “This mechanism is in effect any time you have a rotation crop that does not have the same pathogens as your host crop. This is a general strategy of increasing rotation length by adding other types of crops. The longer the time between your host crop, the more its pathogen population declines.”
“The second mechanism is where the rotation crop causes physical, chemical or biological changes in the soil environment,” Larkin says. “It may stimulate microbial activity and diversity, it may increase beneficial organisms, and things like that, which then help compete with pathogens and reduce pathogen populations.” This mechanism varies with different rotation crops.
“The third mechanism is where the rotation crop has a direct inhibiting effect on either particular pathogens or general pathogens,” he says. The rotation crop may release suppressive or toxic compounds in its roots or residues, or it may release compounds that stimulate certain beneficial microbes that suppress pathogens. Only some crop species have this mechanism.
The first mechanism by itself may not be very effective for controlling some soil-borne pathogens that can survive for many years without a host plant.
Brassicas are well-known for the third mechanism. They contain compounds called glucosinolates, and when Brassica plant materials are incorporated into the soil, the glucosinolates break down to produce other compounds, called isothiocyanates. Isothiocyanates are biofumigants that are toxic to many soil fungi, especially fungal pathogens, weeds, nematodes and other pests.
Larkin’s research shows Brassica biofumigant activity is greatest when the crop is incorporated into the soil as a green manure. However, even when a Brassica crop is harvested and then the remaining crop residues are incorporated, there is still some biofumigant effect. The amount of the biofumigant effect also depends on the Brassica crop’s glucosinolate levels; canolas have relatively low levels, rapeseeds somewhat higher, and mustards have the highest.
“With any of those Brassicas, you will get some benefit from incorporating the crop residues. And it is a measurable and real effect on both potato yield and on reduction of potato diseases,” he notes.
As well, Larkin’s studies indicate Brassicas also provide the second mechanism.
“Brassicas seem to have an ability to alter soil microbial communities in different ways which is not necessarily related to their amount of glucosinolates or their ability to act as a biofumigant. I think the aspect of how they change the soil microbiology is equally important to their biofumigant effect,” he says.
For example, the USDA researchers found that canola and rapeseed sometimes do a better job at reducing black scurf (Rhizoctonia solani) than some of the higher glucosinolate mustards, and the effect on black scurf works even without incorporating the Brassica crop.
However, Larkin’s studies also show that managing some other diseases – like powdery scab (Spongospora subterranean) and Verticillium wilt – requires a full green manure.
Two-year rotation is too short
If disease suppression is a major goal of your potato rotation, then Larkin’s research results provide some key factors to consider.
First, a Brassica’s disease suppression effect won’t last forever, so the potato crop should immediately follow the Brassica in the rotation to get the greatest benefit.
Second, a two-year rotation will not effectively reduce disease in the long run. Larkin found that no matter what crop was in a two-year rotation with potatoes, certain pathogens tended to build up over time.
For example, in one 10-year study the researchers compared two-year rotations in fields where common scab and Verticillium wilt were not problems at the beginning of the period. But by the end of the study, both diseases had become substantial problems in all of the two-year rotations. The canola-potato and rapeseed-potato rotations had significantly lower disease levels than the other rotations, but they still had gradually increasing amounts of common scab and Verticillium wilt.
“So we recommend a three-year rotation as your first line of defence, and then including a disease-suppressive rotation crop in one of the years of that three-year rotation,” Larkin says.
A Brassica green manure could be a good choice for the disease-suppressive crop, or you could grow a Brassica as a full-season crop and follow it with a disease-suppressive cover crop. “The addition of a cover crop like winter rye or ryegrass, in combination with your Brassica, can provide a significant addition to the disease reduction,” he notes.
Even though the grower would not be earning any direct income from a green manure or a cover crop, these options can be valuable tools to get serious soil-borne disease problems under control.
“That’s really how we first got into this research,” Larkin explains. “Some potato growers [in Maine] had some soils with substantial disease problems, and they wanted to try whatever they could to get those soils back to where their potatoes would be producing better. So they were willing to give up a seasonal crop for a year or two, to try to get the pathogen populations down to controllable levels.” (Two seasons of a green manure might be necessary if the field has very high pathogen populations.)
A third factor to consider is whether the rotation crops share any pathogens with potatoes. In Maine, the only shared pathogen that increased in potato-Brassica rotation trials was sclerotinia.
In his own studies, Larkin hasn’t had any sclerotinia issues because sclerotinia is not common in Maine potato fields. However, a researcher at the University of Maine found two fields with sclerotinia and did some rotational trials there. Sclerotinia increased in those two fields when canola or rapeseed was in a rotation with potatoes.
“So if you have a field with a history of sclerotinia problems, then a Brassica may not be the best rotation crop for you,” says Larkin. Alternatively, adding a cereal crop to a potato-Brassica rotation may help because cereals are not susceptible to sclerotinia.
Maritimes rotation project
The major potato rotation project now underway in Atlantic Canada is examining various crop options including canola and some other Brassicas.
Dr. Aaron Mills, a research scientist with Agriculture and Agri-Food Canada (AAFC) in P.E.I., is leading the project. He is conducting trials at AAFC’s Harrington Research Farm in the province, involving nine different three-year rotations.
The project is funded under Growing Forward 2, with support from AAFC and the Eastern Canada Oilseeds Development Alliance Inc. McCain Fertilizer division is collaborating by conducting a similar study in New Brunswick.
Generally, the project’s three-year rotations involve a year of potatoes, a year of another high-value crop, and a year of a more diverse crop mix or a biofumigant crop. “We’re looking at canola, soybean and corn [as the high-value crops], and at other, more diversified phases in the rotation, including blends of a Brassica, a grass and a legume all planted at the same time,” Mills explains.
He notes, “The canola acreage is increasing slightly in Prince Edward Island, it is one of the higher-value oilseed crops, and it does very well under our climate. And it’s important to diversify the cropping system, so if you can add in a different crop and it’s a higher value crop, then that’s a win-win situation.
“Canola has also been touted to have some biofumigatory effects, and the Brassicas in general produce certain compounds shown to have effects on diseases and insect pests,” Mills says. “Buckwheat is another [crop that supresses pests], based on research by my colleague Dr. Christine Noronha, so we’re also including buckwheat in the trials.”
Mills and his research team will be scouting all the crops in the different rotations for disease and insect pests. Sclerotinia is one of the issues they’ll particularly watch for. Mills notes, “We are starting to see an increase in sclerotinia [in P.E.I.], and a lot of the higher value crops in these rotations are hosts for sclerotinia.”
Along with collecting data on crop yields, diseases and insect pest issues, the researchers will also be monitoring such factors as crop biomass and soil organisms including nematodes. And Dr. Judith Nyiraneza, an AAFC nutrient management specialist, will be tracking nutrient dynamics in the soil.
The researchers conducted preliminary work in 2013, and 2014 was the project’s first full year. The current funding will take the project to 2017-18, but Mills hopes to run it for nine to 12 years. “You can’t really look at the trends until you get at least a couple of phases of each rotation. So one of the big determinants for the project’s success is how long we can run the rotations,” he says.
Putting it all together
The effects of different rotation crops on potato diseases and yields may differ somewhat from region to region. Mills emphasizes the importance of evaluating rotations in different regions: “P.E.I. soils are different than those in Ontario or out west, and how the crops respond is not exactly the same.”
Mills’ overall advice for effective potato rotations is that more diversity is better. “From what we’ve seen so far with some of our other studies, it’s all about increasing the crop diversity. You can increase the length of the rotation by adding different crops. Or, if you have a shorter rotation, you can increase [the in-year diversity]. That seems to show some benefits to the soil and to the organic matter especially,” he says.
Similarly, Larkin advises using multiple rotation-related practices for enhanced disease suppression. Examples include: increasing the rotation’s length, adding crops that also have the second and third mechanisms of disease suppression, and including cover crops and green manures.
His research shows that, although these practices will not completely eliminate potato diseases, they will reduce soil-borne potato diseases and improve potato yields.
As well, these kinds of sustainable practices provide other long-term benefits for a farm’s production capacity and potential longevity. These benefits include improving overall soil health, enhancing soil microbial diversity and activity, increasing soil organic matter and building a healthier agro-ecosystem. “All these practices are components of making a better, more sustainable system,” Larkin says.
For potato growers in Maine, Larkin’s general rotation recommendation is “a three-year rotation, with one year of a grain such as barley, then a cover crop like ryegrass or winter rye, then a Brassica, which could be a mustard green manure or a harvestable oilseed Brassica crop, and then potato in the third year of the rotation.”
He notes, “That recommendation is based on a lot of different studies looking at what is the best system for reducing disease, what is the best system for improving soil quality. Now [in our current studies] we are trying to combine those into a rotation that incorporates aspects of all of those things and seeing if it really does everything we hoped it would.”