Potatoes in Canada

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Managing potato virus Y

Potato virus Y, which used to be called common mosaic virus, has plagued potato growers for many years with its potential to cause severe yield reductions. But in the last two decades the problems caused by PVY have grown due to a combination of factors – including emerging strains that damage tuber quality and make the potatoes unmarketable. These days, managing PVY is more important than ever.

June 10, 2013  By Carolyn King


Spraying a mineral oil (tank mixed with an insecticide and/or a fungicide) to reduce transmission of PVY by aphids.

Higher virus levels
“Recently most of the potato-producing areas in North America have been experiencing high PVY levels for several reasons. One of the most important factors is the emergence of new strains of PVY, which are necrotic strains,” explains Dr. Mathuresh Singh, a potato virologist with the Agricultural Certification Services (ACS) in Fredericton, New Brunswick. (ACS is a diagnostic laboratory owned and operated by New Brunswick potato growers; it tests potatoes for various pathogens, including PVY.)

“Necrotic” strains are strains that cause dead spots somewhere in the plant. For example, PVYntn causes potato tuber necrotic ringspot disease in susceptible cultivars – the tubers get brownish necrotic rings. This serious problem is compounded by the fact that PVYntn causes only very mild symptoms in the plant’s foliage so that infected plants may go undetected. In contrast, PVYo, which is the “ordinary” strain of PVY, can cause very obvious foliar symptoms in many cultivars, such as mottling, crinkling and dropping of leaves, and stunted plants. This foliar damage can reduce yields, but the obvious symptoms also make it easy for growers to “rogue” (identify and dispose of) infected plants and for seed potato inspectors to visually identify infected plants.

A second factor is the introduction and widespread use of several potato varieties that exhibit mild or no foliar symptoms when they carry PVYo. Singh explains, “You could call them ‘latent varieties.’ Without clear symptoms, it’s very hard to identify infected plants and take them out of the system.”

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Another issue that can make it difficult to identify infected plants is if the infection occurs late in the season. “It takes about two to three weeks or longer after infection for the disease to cause foliar symptoms. If the virus is transmitted at about the end of July to the middle of August, then by the time the symptoms would be appearing, the plants are old or the foliage has been top-killed. So you can’t see the foliar symptoms, even though the tubers may be infected,” says Singh.

A fourth factor is an increasing problem with aphids, which are an important vector for PVY – they acquire the virus from infected plants and spread it to other plants. “There’s been more aphid activity in recent years, possibly because of climate change, or new aphid species are coming into the system, or some different aphid species are transmitting the virus,” says Singh.

He adds, “We’re finding that it’s not just potato-colonizing aphids that can carry PVY; most aphid species, whether they are coming from trees or strawberries or other plants, are able to transmit PVY.”
So it has become easier for PVY-infected plants to remain as undetected sources of the virus and easier for the virus to spread. And the potential impacts on potato production have become even more serious.

PVY strains in Canada
“The most recent national survey of PVY in Canada was conducted from 2004 to 2006 in collaboration with the US. It showed that Canada had all the North American PVY strains known at that time. PVYo was still the most predominant strain, comprising about 60 to 80 percent of the population, followed by PVYn:o and PVYntn,” says Singh. PVYn:o is another necrotic strain capable of causing tuber symptoms in susceptible potato varieties.

More recent studies are indicating further changes in PVY, including changes to the proportions of the different strains. Singh notes, “In recent years in the US, they have been finding in some states that most of the PVYo population has been replaced with either PVYn:o or PVYntn. In New Brunswick, in most cases the population is still about 70 or 80 percent PVYo, but the proportion of the other strains is increasing. The trend in North America is similar to what they found in Europe, where PVYntn displaced PVYo as the predominant strain during the last two decades.”

In a very recently completed study of PVY strains detected between 2010 and 2012 on Prince Edward Island, Canadian researchers found PVYo accounted for only 30 to 40 percent of the total PVY samples, while PVYntn accounted for 30 percent and PVYn:o accounted for about 20 percent. Singh says, “This is what is going to happen in most of the potato-growing areas where PVY is a problem and if not managed properly.”

Post-harvest testing for PVY
A key step in controlling PVY is to accurately detect infected seed potatoes because they can be an important source of the virus in potato fields. “In Canada, our national seed potato certification system is conducted by the Canadian Food Inspection Agency (CFIA). All seed potatoes planted across Canada are visually inspected twice during the growing season for visual symptoms of PVY in the foliage. This system used to work, but it’s not very effective at detecting late infections or infections in latent varieties. To do that, you have to do post-harvest testing of the tubers. At present two provinces – New Brunswick and P.E.I. – are doing mandatory post-harvest testing,” explains Singh. (Other provinces such as Quebec, Alberta, Manitoba and Saskatchewan conduct post-harvest testing but it’s not mandatory.)

In spring 2011, New Brunswick initiated mandatory post-harvest testing of seed potatoes to be planted in the province. “Growers have to test their seed for PVY, and the virus level must not exceed a certain number. That number varies from year to year because we look at all the test results for the year to decide what percent of infected seed is appropriate, for example, three percent, five percent, six percent. Any seed with a virus level higher than that can’t be planted in New Brunswick, even if it has passed the CFIA’s visual certification system,” says Dr. Khalil Al-Mughrabi, potato pathologist with the New Brunswick Department of Agriculture, Aquaculture and Fisheries.

He adds, “In the beginning, the growers weren’t sure if they wanted mandatory post-harvest testing because it costs money to test and it will cost money if the virus level is high in their seed because they can’t plant it in New Brunswick.
“But the growers concluded that this requirement is to their benefit. It might be hard in the beginning, but at the end of the day, the quality of their seed would be higher and they would be able to compete in the marketplace.”

Advice for growers
“Potato virus Y is not forgiving. It can reduce yield between 10 and 80 percent, so if the virus level is high the grower loses big time. It’s really important to take steps to manage the virus,” emphasizes Al-Mughrabi.
He groups PVY management guidelines into three broad principles: eliminate sources of the virus; prevent aphids from spreading the virus; and use resistant potato varieties.

Eliminate sources of PVY: “The easiest way to spread PVY is with infected seed, so growers should use certified, disease-free seed. Always make sure the seed has been tested for PVY and get the levels of the virus when you get the lab test results,” says Al-Mughrabi.

Other important practices to eliminate PVY sources include: disinfecting cutting and planting equipment before coming in contact with seed and before going from seedlot to seedlot; controlling volunteer potato plants; eliminating potato cull piles; roguing infected potato plants; and controlling weeds that could host the disease (such as hairy nightshade) and weeds that are aphid hosts (such as wild mustard, wild rose and wild radish).

Prevent aphids from spreading PVY: When an aphid probes an infected leaf with its piercing mouthpart, called a stylet, the stylet picks up the virus. Then the aphid transfers the virus to other plants as it probes their foliage with its infected stylet. The aphid can acquire and transmit PVY within seconds.

“The best option is to spray oils registered for this use on potato foliage,” says Al-Mughrabi. “When an aphid probes through that film of oil into a leaf, the oil cleans the stylet, and the virus dies on the film of oil.” He advises growers to follow the oil’s label recommendations to ensure effective applications.

Singh has been visiting New Brunswick potato growers to look at what they are doing to control PVY and find out how well it’s working for them. “We recommend to our growers here to apply mineral oil as soon as the potato plants start coming up,” he notes. “We are finding that one of the most effective methods of managing this virus is to apply about 2 to 2.5 litres of mineral oil per acre about every five to six days [until vine-kill] and also add a dose of insecticide to the oil spray about every two to three weeks to keep the aphid population down.”

Al-Mughrabi explains that it’s better to use an insecticide in combination with the oil because insecticides alone aren’t very effective at controlling PVY. “An insecticide doesn’t kill instantly, so the aphid will stay alive for a while and probably try to feed until it dies, [and when it feeds, it transmits the virus].”

He also recommends that growers make use of their local aphid alert program, if one is available in their area. ”In New Brunswick, we put aphid traps in the fields of participating growers, and every few days scouts collect the aphids from the traps. We identify the species and their numbers in the various parts of the province. The information goes on our website and our hotline, so growers can find out what’s in their area and whether the threshold for aphid control has been reached.”

That information helps growers make decisions regarding aphid control, for example whether early vine-kill would be a good aphid control option for their situation.

Crop borders are another practice used by some growers. Al-Mughrabi says, “A crop border consists of a non-PVY host crop, typically a cereal crop, planted around a small, early-generation seedlot to provide a buffer against aphids flying into the field. Aphids usually land on the interface between fallow ground and a green crop, so in this case they would land on the cereal crop. They probe the cereal plants – PVY doesn’t harm the cereals in any way – and that probing helps clean off their stylets before they enter the potato crop.”

Use PVY-resistant varieties: This sounds simple, but choosing resistant strains has become more complicated with the evolution of PVY. Recent research by Singh and his colleagues at Agriculture and Agri-Food Canada’s Potato Research Centre in Fredericton showed the impact of PVY varies significantly depending on both the potato cultivar and the virus strain and even the variants within strains.

A PVY website (www.potatovirus.com) sponsored by the United States Department of Agriculture lists various potato varieties with their susceptibility to foliar symptoms and to tuber symptoms. For example, Yukon Gem shows little or no foliar symptoms of PVY infection, but it is very susceptible to potato tuber necrotic ringspot disease. Yukon Gold and Highland Russet are also considered very susceptible to tuber necrosis. In contrast Russet Norkotah, Classic Russet and Premier Russet are considered very resistant to tuber necrosis.

“A lot more research still has to be done because of the new PVY strains. These strains could potentially drive some varieties off the market, like Yukon Gold, a very common variety which is very sensitive to tuber damage by PVYntn,” says Singh. In that 2010-12 PEI study, over 50% of the tubers from PVYntninfected Yukon Gold plants showed symptoms of potato tuber necrotic ringspot disease, with many of them having severe symptoms.

Although Canadian researchers have made significant progress in investigating PVY, Singh says more work is needed to improve methods to manage PVY. He cites examples like the need for more information about the symptomology of the various PVY strains in different potato varieties, and about mineral oil applications, including the effects of these oils on potato yields and the most effective application rates and timing.

Singh and his colleagues are hoping to obtain funding for a national study to enable a coordinated effort to learn more about key PVY issues. He emphasizes, “Certainly problems with PVY and its emerging strains will increase if the virus not managed properly.”


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