An infected potato psyllid insect carries the Lso (Candidatus Liberibacter solanacearum) pathogen that can cause zebra chip disease in potato crops.
Zebra chip has affected potato crops in the U.S., Mexico and New Zealand and caused millions of dollars in losses. Potatoes with zebra chip develop unsightly dark lines when fried, making affected potatoes unsellable.
The first detection of Lso came from sampling cards collected at one site south of Highway 3, near Lethbridge, Alta. For the full story, click here.
Researchers are hoping Canadian potato growers will soon be able to use an innovative approach to control wireworms. This method uses just a few grams of insecticide per hectare applied to cereal seeds that are planted along with untreated seed potatoes. It provides very good wireworm control for the whole growing season, with a lower environmental risk than currently available insecticide options.
Just because black cutworms don’t overwinter in Canada doesn’t mean they aren’t a threat to potato crops. The insects spend their winters in the southern United States but travel north on low-level jet streams and, once they cross the border into Canada, they look for a tasty food source. Black cutworm moths prefer some of the weeds that grow in and around fields and, while potatoes are not their favourite food, they will adapt and can wreak havoc on an unmonitored field.
A researcher at the University of Minnesota says the cutworms’ new interest in potatoes could be the result of a change in potential host plants. If the moth’s desired weed is being well controlled in a field, it will eat what is available where the wind sets it down.
“Black cutworm moths are active flyers,” explains Ian MacRae, the extension entomologist at the university’s Crookston research station. “These insects can travel hundreds of miles in a short period of time aided by an extremely efficient bug highway [a jet stream].”
MacRae says if the wind and temperature are conducive and Canadian potato producers are able to get their crop planted in good conditions, there is a chance the moths will arrive about the same time as the plants are emerging. The possibility exists that early arrival could spawn a second generation of the insects later in the season. He says, once landed, reproduction occurs when the moths lay eggs. The emerging larvae will feed on the foliage, but once at the fourth or fifth larval stage they will begin actively eating near the base of the host plant, cutting it off.
“The first you might notice a cutworm problem will be plants that are cut at the base or wilting,” MacRae explains. “At night, the worms burrow into the soil and if the tubers are close to the surface, they will burrow into the tuber. They can do more damage to tubers in dry conditions because cracks in the soil will give the cutworms access to what is underneath.”
Damage to potato crops early in the season can be a greater problem because the young plants will not recover from being chewed off. There is a possibility that the seed piece might send up another shoot, but the crop will be set back. MacRae suggests early scouting will help identify the problem and allow time for control. There are effective insecticides for control of black cutworm and there are sources of natural mortality, such as predators or parasitic wasps. Birds may be less effective because of the location of the worms and their habit of eating at night.
“If you find yourself at a threshold of about 30 per cent of your plants cut, you may want to apply an insecticide,” MacRae says. “If defoliation is this high, it may be that natural mortality sources are not functioning well.”
Ensure proper identification of the larvae as black cutworms so the correct product can be chosen for control. Combining regular field scouting with pheromone or light traps to catch the male moths is an effective way to identify the insects.
“When scouting, look for stalks at an odd angle or wilting,” MacRae suggests. “Look in the evening when the cutworms come out to feed, and look as much as a half metre away from the plant because they are good walkers. Black cutworms are aptly named because they are a dark caterpillar with a waxy appearance. They will often curl into a C if disturbed. They hang out during the day under clods of soil or in cracks.”
MacRae says climate change may be the reason black cutworms are being seen farther north. He doesn’t believe they will begin to overwinter in Ontario or the Prairies, but a warmer climate means they develop faster and may overwinter in more northern states, making the migration north earlier and causing greater problems.
“Black cutworms have certainly become a problem in Ontario in the last few years,” MacRae says. “They can be a significant pest issue.”
MacRae adds there are some cultural practices that may minimize the impact of black cutworms when they arrive. Planting late can put new, young plants directly on a collision course with the moths and their offspring, so plant early, if possible. He says controlling weeds will reduce the areas where the moths might lay eggs. Growers in the United States use pre-plant tillage to turn over the soil to destroy potential habitat.
To date, there is no accurate monitoring system in place for potato crops, according to MacRae, but the cutworms also like corn and the corn growers in some states, such as Iowa, have a black cutworm monitoring network. “The moths seem to appear in Ontario about three weeks after they are seen in the United States,” he says. Ontario growers could tap into the monitoring networks south of the border and use that information as an early warning system, he suggests.
Black cutworms could be considered a stealthy yield robber because by the time you begin to notice a problem, it could be a challenge to execute effective control. The best defence is early and frequent field scouting and adopting cultural practices that could minimize the attractiveness of the crop. MacRae believes Canadian potato growers will see black cutworm more often in the coming years, so preparation for and understanding of the pest is a wise approach.
Sept. 29, 2016 – Second growth is a physiological potato problem induced by soil temperatures of 24 C or above and water stress. These two factors interact to limit the tuber growth rate, causing second growth. Inadequate soil moisture alone does not result in the initiation of second growth.
Heat and drought prevailed during the 2016 Ontario growing season, which explains why second growth has been reported in some fields.
Potato varieties differ in their susceptibility to second growth. European varieties appear to be more susceptible because they were bred and evaluated in countries where the growing seasons are rarely hot.
There are three distinct types of second growth:
Tuber chaining: A series of small tubers are produced on a single stolon.
Heat sprouts: Sprouts develop from stolons or daughter tubers. The sprouts may emerge from the hills developing into leafy stems.
Secondary Tuber: Small tubers form on daughter tubers. The secondary tubers are formed on short sprouts or directly on the tuber surface. This disorder is usually associated with physiologically old potatoes. High temperatures and water stress during the growing season are major factors contributing to the physiological aging of potatoes.
Cultural practices that promote uniform growth of plants and tubers throughout the season help minimize second growth. Among them are:
● Do not plant physiologically old seed in cold, dry soil.
● Space seed pieces as uniformly as possible at planting.
● Apply an adequate amount of fertilizers.
● Maintain uniform soil moisture sufficient to meet crop needs (this was easier said than done this past season!).
August 26, 2016 - According to Dr. Vikram Bisht, of Manitoba Agriculture, aphid counts in weeks 8 in all but one sample were low. There was one Green Peach Aphid (GPA) trapped in southern seed growing area, but not anywhere else. Potato aphids were trapped in southern and central areas. One field showed a sudden influx of aphids, probably from nearby crops being harvested or desiccated. There were low counts of Aster leafhoppers were trapped in all seed areas.
Some of the seed fields are being desiccated, so Bisht reports there will be one more week of aphid monitoring. The results from suction and pan traps in seed fields for the 6th and 7th week of sampling can be seen in a chart (please click here):
In 2016 season, as in 2015, as part of the Canadian Hort Council, Growing Forward 2, Canadian Potato Psyllid and Zebra Chip Monitoring Network project, yellow sticky cards are being sent to Dan Johnson, Univ of Lethbridge. One potato psyllid adult was confirmed today (August 22) in a card (in field July 12-18) from Northfolk-Treherne Rural Municipality. This is the first find for 2016 in a province outside Alberta.
North Dakota has also reported occurrence of potato psyllids in their fields. "We have confirmed that psyllids are present potato fields in western ND. Psyllids are the vector of zebra chip disease and can do damage without the Lso bacterium (Gary Secor, NDSU)".
August 15, 2016 - New late blight finds were reported on potato from Carman, Winkler areas and on tomato crops east of East of Portage and east of Highway #75, according to Vikram Bisht. Frequent fungicide applications are being applied to control the disease; and in one case the tomato plants have been pulled out and destroyed.
Samples have been collected for strain identification. All of the previous samples, tested by Lethbridge Research and Development Center were determined to be US-23 strain.
"There is increasing metalaxyl insensitivity in the Pi from these samples and the use of Ridomil would probably have only marginal benefit," says Bisht.
It is important, he continues, to scout for late blight, especially in low lying, irrigation pivot center, wheel tracks of irrigation systems (guns/pivots), tree-line protected areas and under hydro-power lines (areas where applicators may have difficulty covering).
It is also critical at this time to monitor potato and tomato plants in home gardens. The DSVs (late blight risk values) accumulated over 7-days at various weather stations suggest mostly moderate risk in most of the province. There is forecast for rain and risk of thunderstorms today afternoon in many potato growing areas. Full fungicide coverage of foliage in high risk areas should be maintained.
Due to continued high moisture levels in many fields, it may be helpful to harvest the low lying areas last, so it will be easier to manage the storages.
Also, a post-harvest treatment with phosphorus acid / phosphite fungicide could be considered for such fields, adds Bisht.
Heat edema has been observed in Ontario, and should disappear when humidex values drop below 30 C. Photo courtesy of Eugenia Banks.
July 27, 2016, Ontario – Two potato problems have developed due to high temperatures, writes Eugenia Banks in her latest Ontario potato update.
The long and relentless heat wave that is affecting Ontario has provided favorable conditions for diseases and physiological problems that we do not often see. One of them is Fusarium wilt caused by Fusarium oxysporum, a relatively common soil fungus that thrives when soil temperatures are around 29 C.
A grower found patches of Fusarium wilt this year in a field of Andovers. Also wilted plants here and there were noticeable in the field. The base of the stems emerging from the seed was woody. At ground level the stems were dark brown, rotten or hollowed but did not have a strong fishy smell. The upper part of the stems was still green but wilted. Most of the plants pulled had rotten seed, but with no fishy smell.
Gary Secor from North Dakota State University helped with the identification of this problem. He isolated Fusarium oxysporum from the diseased stems. There was no sign of blackleg, dickeya or verticillium.
How to control this disease?
Fusarium oxysporum is more prevalent in very hot summers, just like our 2016 growing season. There is nothing that can be done about the weather, but:
● Avoid ammonium nitrogen as a source of N
● Keep potassium levels adequate
● Acid soils favour Fusarium wilt. Liming soils help, but may increase the risk of common scab.
This problem has been observed in two fields of the variety Canela Russet, growing in two different production areas. Small bumps form on the leaves that, with time, rupture leaving round, brown necrotic spots. The centres of the spots often drop out leaving holes in the leaves. Holes in leaves usually means insect feeding, but there were no insects in the fields. Ian MacRae, an entomologist at the University of Minnesota, did not think that the holes looked like insect feeding. Jeff Miller from Miller Research in Idaho suggested heat edema, a physiological disorder. Edema is related to water retention (see page 122 in the Potato Field Guide), but heat edema is different. The cells expand trying to diffuse the heat and eventually rupture. Because it is a physiological problem, heat edema should disappear when humidex values (temperature plus humidity) drop below 30 C. Yesterday, I checked one of the fields and noticed that, because of the hot weather, necrotic spots were developing on new leaves.
Prolonged heat waves bring all sorts of different problems to potato plants. Working with potatoes is a never-ending learning experience!
July 27, 2016, Manitoba – Several fields in western Manitoba (Carberry area) reportedly have light blight symptoms, according to Vikram Bisht's latest potato disease reports.
Some photographs of the diseased leaves show symptoms typical of late blight. Three samples received have been confirmed to have sporangia. The infections are mostly on the top foliage and found in pivot wheel tracks, along the irrigation gun run and edge of field in low areas. There are very few infected plants in the fields and they are difficult to find. The five-day spray schedule in these high risk fields may have kept the infection low.
So far, there is no report of late blight from any other part of Manitoba. The DSVs (late blight risk values) accumulated over seven days at various weather stations continue to suggest high late blight risk in most of the province.
It is extremely important to scout for late blight now, especially in low lying, irrigation pivot center, wheel tracks of irrigation systems (guns/pivots) and tree-line protected areas. It is also critical at this time to monitor potato and tomato plants in home gardens.
There is no forecast for rain events for rest of the week, but it will be warm.
June 28, 2016, Manitoba – The crops continue to look very good, says Vikram Bisht in the Manitoba potato report from June 24. Most early planted crops are row closed and many are nearly so. The rainfall distribution has been very good, and very few fields need irrigation. The accumulated Disease Severity Risk Values (DSVs) for late blight are currently low and below the critical value of 18 across the province; however, it will be helpful to have at least one protectant fungicide application before complete row closure to protect the lower and hidden canopy. It is good to see that many farms are now clearing away the cull piles near their yard and fields.
A few fields are showing herbicide drift injury symptoms. It is important that the applicators are cognizant of the crop injury potential to potatoes, and avoid spraying when the wind speed is above an acceptable level, toward the wrong direction or it is dead calm (no air movement).
First identified in Ontario potatoes in 2015, Dickeya is shaping up to be a problem. Photo courtesy of Tracy Shinners-Carnelley.
Potato growers are familiar with the problems that stem from blackleg and are adept at managing it. However, two new strains have been identified and one has been spreading in North America for the last two years. Dickeya dianthicola has been affecting potatoes in Europe since the 1970s, but is now found frequently in the United States, particularly in Maine and south along the eastern seaboard. Dickeya is most probably spread on seed and is shaping up to be a problem in North America.
“Dickeya may have two problem features,” says Amy Charkowski, a potato specialist at the University of Wisconsin. “It needs fewer cells to cause disease and it can remain in a kind of dormancy until the right conditions trigger the disease.” She adds the bacteria thrives under wet conditions and is an equal opportunity pathogen so can be found on other vegetables and some ornamental plants. Dickeya does not seem to favour grains and legumes, which makes crop rotation a good option to lessen its spread.
Blackleg seems to resemble the common cold because it changes. Growers use management practices to control the disease, but this new species is not so easily harnessed using this familiar method. Crop protection products are not effective on Dickeya dianthicola. Meanwhile, researchers have identified Dickeya solani in Europe that is very aggressive and is not easily controlled, but it has yet to cross the ocean. This proves that complacency, when it comes to blackleg control, is not an option.
“We found Dickeya in Ontario potatoes in 2015,” says Gary Secor, a plant pathologist from North Dakota State University in Fargo. “The seed came from Maine, which is an example of how easily it spreads.”
To minimize the spread of Dickeya, both Charkowski and Secor recommend not cutting seed and, instead, using the whole tuber. Since it does not survive well in soil, they also recommend diligent crop rotation. “There are no food safety issues, but there is still a lot we don’t know about Dickeya,” Secor says. “We also don’t have any idea what the economic thresholds might be.”
“We are applying for grants to allow us to focus more research on Dickeya,” Charkowski says. “We want to learn what breeders need to know to enable them to breed for resistance, and we need to determine what the thresholds are for seed.
The symptoms are similar to the strains of blackleg growers are familiar with, so tubers need to be tested to identify Dickeya. “If it is present the most noticeable symptom is plant wilting,” Charkowski explains. “There will be rotten potatoes at harvest and there could be rotting at plant emergence.” She says determining an accurate laboratory test may be part of the planned research, but a field assay would be more helpful. Knowing the strain may help determine the most effective control as well.
“A lot of co-operation is required with all agencies working together to make progress on identification and control,” Secor admits. “Certification agencies need to determine if it requires certification at the seed stage.”
According to Tracy Shinners-Carnelley of Peak of the Market in Winnipeg, the Canadian Food Inspection Agency’s seed potato inspection program has strict tolerance for blackleg infection. “This is likely a factor in how the incidence of blackleg in Canada is quite low,” she says. In the United States, she adds, blackleg is not part of the industry’s seed certification process, which puts Canadian growers at a bit of an advantage when seed changes hands because it is screened. However, accurate tests for particular strains of Dickeya may be necessary if the more virulent versions enter North America.
“Canadian growers need to be aware of the risks and be proactive in order to prevent the introduction or establishment of any new disease,” Shinners-Carnelley continues. “My main message to growers is to follow best management practices and this, combined with the use of certified seed, will help to reduce the risk and spread of Dickeya.”
June 2, 2016 – Blackleg, caused by strains of soft rot bacteria known as Dickyea, has traditionally had little impact on North American potato production, but it now appears to be on the move throughout Europe and could increasingly threaten growers in the Eastern United States.
The Plant Management Network (PMN) has released a new presentation entitled “Dickeya: A Scottish, UK and European Perspective” to provide growers and crop consultants with an overview of the history of the disease in Europe and an introduction to Dickeya solani, a new aggressive pathogen strain contributing to the increase in incidence and spread of blackleg.
The webcast, developed by Gerry Saddler, deputy head of science and advice, Scottish Agriculture with the Scottish Government, details their country’s potato production practices and explains why they have adopted a national zero-tolerance approach to the presence of Dickeya strains.
The presentation discusses in detail:
• Causes of blackleg and symptoms exhibited by different strains
• Conditions that encourage infection and common transmission methods
• Inspection and testing practices employed in Scotland
• Effective control measures to limit spread
The 40-minute presentation will remain open access through July 31 in the Focus on Potato (www.plantmanagementnetwork.org/fopplantmanagementnetwork.org/fop) webcast resource.
April 28, 2016, Charlottetown – Christine Noronha, an entomologist with Agriculture and Agri-Food Canada’s Charlottetown Research and Development Centre, has designed an environmentally green trap that could be a major breakthrough in the control of wireworms, an increasingly destructive agricultural pest on Prince Edward Island and across Canada.
In this exslusive webinar hosted by Potatoes in Canada magazine, Christine will share details about the Noronha Elaterid Light Trap (NELT). Don't miss the opportunity to ask questions and learn more from Christine Noronha.
Date: May 12, 2016
Time: 2 p.m. ADT (1 p.m. EDT)
April 26, 2016 – Eugenia Banks is offering free training sessions for potato scouts in May.
The first session will be held Monday May 30 from 9:30 a.m. to 3 p.m. at the Shelburne Agricultural Society Community Centre in Shelburne, Ont.
The second session will be held Tuesday May 31 from 9:30 a.m. to 3 p.m. at the Royal Canadian Legion, Branch 190, Norwich, Ont.
March 14, 2016, Prince Edward Island – Agriculture and Agri-Food Canada entomologist Dr. Christine Noronha has designed a simple and environmentally green trap using hardware store items that could be a major breakthrough in the control of wireworms, an increasingly destructive agricultural pest on PEI and across Canada.
The Noronha Elaterid Light Trap, or “NELT”, is made with three pieces - a small solar-powered spotlight, a plastic white cup and a piece of screening. The light is set close to the ground to attract the source of the wireworms, the female click beetles that emerge from the ground in May and June. Each of these beetles can lay between 100 and 200 eggs that produce the larvae known as wireworms. In a six-week test with 10 traps, more than 3,000 females were captured in the plastic cups, preventing the birth of up to 600,000 wireworms. The screening prevents beneficial predator insects from being caught in the trap.
Agriculture and Agri-Food Canada’s Office of Intellectual Property is trademarking the trap name and design and work is underway to find a manufacturer who might be interested in mass-producing the trap.
The NELT is the latest in a series of wireworm control measures being developed by a team that includes Agriculture and Agri-Food Canada, the PEI Potato Board, the PEI Department of Agriculture and Fisheries, the Pest Management Regulatory Agency, Cavendish Farms, the PEI Horticultural Association, growers and consulting agronomists. Wireworms live in the soil and drill their way through tuber and root crops like potatoes and carrots. The PEI Potato Board estimated wireworm damage to the province’s potato crop alone at $6 million in 2014.
To learn more about the NELT, be sure to sign up for an exclusive webinar with Christine Noronha, hosted by Potatoes in Canada magazine, on May 12.
March 11, 2016, Edmonton, Alta – Mike Harding doesn’t usually favour sensational titles for the presentations he gives to farmers.
On March 1, the Alberta Agriculture crop pathologist made an exception with his talk to potato growers on Fusarium: The Silent Storage Killer. He said fusarium in potatoes “flies under the radar” be-cause it can develop slowly in stored product and do major damage before it is noticed. READ MORE
New hope is on the horizon for potato growers engaged in the ongoing battle against Colorado potato beetle (CPB). Researchers are currently field-testing one of the most effective controls ever developed for the potato’s chief insect villain, and it is entirely chemical-free.
RNA interference (RNAi) is a biological process whereby RNA (ribonucleic acid) molecules activate a protective response against parasite nucleotide sequences by inhibiting their gene expression. In other words, it is the method by which organisms – including pests such as CPB – defend themselves against threats and regulate their own genes. But the same process that is used by a beetle to protect itself can be used to destroy it when it consumes the long double-stranded RNA (dsRNA) in genetically modified plants.
Since 2009, researchers at the Max Planck Institute in Potsdam and Jena, Germany, have been developing genetically modified potato plants to enable their chloroplasts to accumulate dsRNA targeted against essential CPB genes. After feeding on a potato’s leaves – and ingesting the dsRNA – the beetles in the study showed 100 per cent mortality within five days.
Why chloroplasts, rather than plant cell nuclei? In past breeding projects, expression of dsRNA in potato plants’ nuclei has proven inefficient because natural RNAi pathways in nuclei prevented the plant from producing enough long dsRNA. But long dsRNA are free to accumulate in chloroplasts – which have no RNAi mechanism – and the plants are fully protected against CPB.
According to Jiang Zhang, a professor with the College of Life Science at Hubei University in China, and the lead on the project, the technology shows great promise for the future of pest management, but there are no immediate plans for commercialization until all regulatory hurdles have been overcome.
“We encourage more scientists and industry involvement in this field for a better future,” Zhang says. “There is still a long way to go to make it really useful in daily life and be accepted by customers.”
RNAi for control of CPB has also gained significant momentum in private research and development. Monsanto and Syngenta have both devoted major investments toward the technology.
In early 2015, Monsanto’s BioDirect technology platform targeted at CPB advanced to Phase 2 – early product development – of the company’s research and development pipeline. The product will have to complete advanced product development and pre-launch before broad commercialization early in the next decade.
While the principle is the same, Monsanto’s product works differently than Max Planck’s modified potato plant: it is sprayed onto the plant’s foliage. Rather than expressing dsRNA in its leaves, dsRNA is applied exogenously to the plant.
“The Colorado potato beetle consumes the leaves of the potato plant where we can focus the BioDirect application, versus needing a plant to produce the dsRNA targeting the pest below-ground where sprays cannot reach,” explains Greg Heck, weed control team lead for Monsanto’s chemistry technology area.
Heck says there are thousands of dsRNA naturally present in host plants that serve a variety of functions, and beetles consume and incorporate dsRNA all the time. “When targeting them for pest control, we seek to supply one additional dsRNA that will turn down a specific gene critical to their ability to feed and grow on the plant.
“Field research conducted on our BioDirect treatment for Colorado potato beetles has already demonstrated some early positive results. This includes reduced Colorado potato beetle larva infestation and plant defoliation in multiple geographies.”
Syngenta’s most advanced RNA-based biocontrol targets CPB in potato – and is also applied via a spray. The company has tested the product in multiple geographies over several years with positive results, says Luc Maertens, Syngenta’s RNAi platform lead based in Belgium. The company hopes to commercialize the product early in the next decade pending continued development and regulatory reviews.
Maertens says the company’s biocontrol is highly selective and starts to work before CPB can cause too much damage.
“The biocontrol is not systemic [in the plant], nor does it work through contact,” he says. “It does not change or have any effect on the DNA of the pest, nor does it involve genetic modification of the plant.”
No technology can work forever, however. Insect resistance to RNAi is a potential risk – one companies and researchers alike are keen to avoid so the technology has maximum benefit and longevity.
Maertens says that as resistance emerges to existing technologies, and the pest spectrum shifts along with climate change and other factors, growers’ needs will change. “Those challenges cannot be answered by only one technology,” he says. “It is imperative to gain insights into probable resistance mechanisms to RNAi triggers in insects, to monitor possible resistance in the field, and to support the use of the technology with appropriate stewardship requirements.”
Of the two methods of RNAi application (genetic modification and spray-on), Zhang believes the former might be better for growers. “Applying dsRNA exogenously is much less cost-effective than expressing dsRNA in the plant itself,” he says. “Spraying may also cause other potential problems in the environment.”
RNAi is not meant to be a silver bullet and should be used as part of a multifaceted pest control strategy. Regardless of the method of application, RNAi may soon be working in a field near you.
Careful management of irrigated potato crops over the long-term may help maintain crop productivity and nutrient availability within acceptable levels for agricultural production. This is the conclusion Agriculture and Agri-Food Canada (AAFC) researchers reached when they did a field experiment the year after completing a long-term irrigated potato rotation study in Brandon, Man.
During that initial 14-year rotation study, Ramona Mohr, a sustainable systems agronomist at AAFC, worked with a team of researchers to identify economically and environmentally sustainable rotations for irrigated potato.
From 1998 to 2011, Mohr and her colleagues grew six different rotations: potato-canola, potato-wheat, potato-canola-wheat, potato-oat-wheat, potato-wheat-canola-wheat and potato-canola (underseeded to alfalfa)-alfalfa-alfalfa, and looked at their effects on various factors including crop yield and quality, diseases and weeds, soil quality and economics.
“Generally accepted management practices were used during the 14-year rotation study,” Mohr says. “We soil tested on an annual basis and adjusted our fertilizer management practices accordingly.”
In the year after the rotation study ended, the researchers conducted a follow-up study to assess the impact preceding rotations had on phosphorus, potassium and micronutrient concentrations in the soil, as well as on soybean yield, quality and nutrient concentration.
For the follow-up study, the group picked a glyphosate-tolerant variety of soybean as an indicator crop to try to minimize possible confounding effects of the previous rotations with respect to disease, weeds and nitrogen.
It was a unique opportunity to study the relative effects preceding rotations have on crop productivity and nutrient status of the plant-soil system, given the limited information available for Western Canada.
“This rotation study was one of only two longer-term irrigated potato rotation studies conducted in Western Canada over the past couple of decades,” Mohr says.
Taking multiple soil samples from each rotation in the spring of 2012, the researchers were able to compare them with samples taken in 1997 at the beginning of the long-term study. They discovered that the soil nutrient levels and the yield and quality of the soybean crop were all typical of the region.
“We saw some differences in nutrient levels among the different rotations, but no substantial depletion or build up of nutrients, and no large differences in soybean yield among the rotations,” Mohr says.
Preceding rotations affected soybean yield to a limited degree. Soybean yield was six per cent higher following the potato-oat-wheat rotation than the potato-canola-wheat rotation, although the reason for this difference wasn’t clear.
“We also noticed when soybean followed potato, it had a slightly higher yield than following cereals or canola, we suspect, because of greater availability of water because the potato crop was irrigated and none of the other crops were,” Mohr says.
The samples also showed that where preceding rotations included alfalfa, seed protein increased and oil concentration decreased. The researchers believe the limited yield differences may have been due, in part, to the selection of soybean as an indicator crop. This likely minimized the differences among rotations arising from disease, weeds and nitrogen. Soybean had not been used in the rotation study and so was not susceptible to those diseases that had built up in the preceding rotations. Also, it is a nitrogen-fixing crop that could supply its own nitrogen. As well, by selecting a glyphosate-tolerant soybean, weeds could be effectively controlled regardless of the previous rotation.
For nutrients, the researchers looked at phosphorus and potassium in the top 15 centimetres of the surface soil and found they fell within the same general range of what they saw in 1997.
Soil test phosphorus was slightly higher except for the rotation that included alfalfa hay. That rotation saw a reduction of both phosphorus and potassium, which the researchers determined was likely a result of the high nutrient removal rates of alfalfa hay and their fertilizer management practices during the long-term study.
For example, Mohr says they applied phosphorus in the alfalfa establishment year, but not potassium because soil levels did not call for it. “When we saw a decline in soil potassium levels, we started applying potassium at that point.”
Soil test phosphorus levels were also found to be higher in the shorter rotations. “The two-year was greater than the three-year was greater than the four-year rotation,” Mohr says. Again, the researchers attribute this to their fertilizer management practices since they applied a higher rate because they broadcast the fertilizer in potato, adding more phosphorus than the potato crop removed.
“The more frequently we grew potatoes, the higher the soil test level we tended to see,” Mohr says. “Although we saw some difference in soil test levels, again they were within the range we see in agriculture fields.”
Mohr and her colleagues also looked at micronutrients during their follow-up study. “The site had sufficient micronutrients for the crop so we didn’t need to add any fertilizer,” Mohr says, although certain of the fungicides applied to potato would have contained some copper or zinc. While the preceding rotation had minimal effects on soil copper and zinc levels, soybean established after the potato-canola-alfalfa rotations or directly following a potato crop contained comparatively higher seed copper and zinc concentrations.
This suggests, Mohr says, that including mycorrhizal crops, like potato and alfalfa, might have increased the availability of micronutrients to the following soybean crop. “The roots of potatoes and alfalfa form a relationship with mycorrhizal fungi in the soil, which may increase the availability of micronutrients. We didn’t measure mycorrhiza in this study, but we know that based on previous studies.”
The results of the follow-up study in 2012 do suggest that careful, long-term management of irrigated potato systems may help maintain crop productivity and nutrient availability within acceptable levels for agricultural production. However, Mohr stresses the impacts of disease, weeds and nitrogen fertility on crop growth may have been minimized because the researchers selected soybean as the indicator crop in this study.
“If producers were to grow other crop species, there might be a greater impact on crop productivity,” she says.
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