Features Agronomy Diseases
Hitting a moving target

Dr. Xianzhou Nie, virologist at AAFC’s Potato Research Centre in Fredericton, is studying strains of PVY in order to  determine resistant varieties. Photo courtesy of Agriculture and Agri-Food Canada.

Any Canadian potato farmer knows well the half dozen major viral diseases that can damage their potato crops. The PVY group is one of the most destructive.

PVY occurs worldwide and mutations occur regularly, so new strains are being created on an ongoing basis. The virus is spread in the field by aphids from infected plants to uninfected plants. Those with the virus rapidly lose vitality and produce fewer and smaller potatoes. A severe strain of PVY called PVYNTN can also cause potato tuber necrotic ringspot disease on some cultivars such as Yukon Gold, AC Chaleur and Cherokee, making them unmarketable. There is no cure, so planting clean seed is paramount.

However, within a few years, potato growers in Canada and beyond will be able to access varieties with extreme resistance to PVY. This is thanks to the hard work of potato breeder Agnes Murphy and her colleagues, such as virologist Dr. Xianzhou Nie, at Agriculture and Agri-Food Canada’s Potato Research Centre (PRC) in Fredericton, N.B. They are studying strains of the virus, as well as potato plant genes that confer resistance to these strains, with world-renowned methods they created themselves.

“There are many strains of the virus that now exist, with varying characteristics and properties,” says Nie. “We have to stay on top of what strains exist because different potato varieties respond differently to each of them.” His updates on the latest strains are provided to the Canadian Food Inspection Agency and to farmers, so they can do a better job at spotting symptoms of the virus in the field.

Genetic analysis
“PVY is a very complex virus,” notes Nie. “It can be categorized into potato- and non-potato groups, and within those that infect potatoes, at least five strains have been recognized to date.” The most common ones are PVYO (ordinary strain), PVYN (tobacco necrosis strain), PVYN:O (N:O strain) and PVYNTN (potato tuber necrosis strain). “PVYO and PVYN are the conventional strains, and from which PVYN:O and PVYNTN have emerged due to natural genome recombination processes,” Nie explains. “PVYO causes symptoms ranging from mosaic to leaf drop and plant death in potato, depending on cultivars, and mosaic on tobacco. PVYN causes severe venial/petiole necrosis on tobacco plants, but mild symptoms on most potato cultivars.” Both PVYN:O and PVYNTN cause severe symptoms on tobacco plants, and mild to severe mosaic on most potato cultivars. PVYNTN also causes potato tuber necrotic ringspot disease in several varieties.  

Being able to differentiate between these existing PVY strains means you must have a reliable diagnostic method. Nie and colleague Dr. Rudra Singh were the first researchers in the world to develop this over 2002 and 2003, and their technique is now used commonly around the globe. “It is a polymerase chain reaction-based approach methodology that amplifies a part of the virus genome under the direction of two primers that are specific to the virus strain you are targeting,” Nie explains. “The amplified DNA fragment is then detected after electrophoresis in an agarose gel, and visualized using an image analysis system.”

Previous methods for differentiating PVY strains were not as effective, relying on enzyme-linked immunosorbent assay (ELISA). “Our polymerase chain reaction-based method targets the virus genome sequence directly and therefore can detect and differentiate different strains of PVY more effectively,” Nie says. Their method is not only used widely to survey and characterize PVY in potatoes, but also to detect and identify new strains.

Transmission of the disease by aphids – the only insect known to be capable of transmission of PVY – is also a concern, and Nie works with entomologists at the PRC to understand it more thoroughly. PVY is transmitted when the aphids feed, carried from plant to plant through sticking to aphid mouthparts. “Green peach aphid is the most effective vector of PVY,” Nie notes. “Nevertheless, many other species of aphid – both potato-colonizing and non-potato-colonizing species – can transmit PVY during the aphid’s dispersal phase when they are in search of new hosts.” Farmers may be able to prevent some of the insects from carrying the virus to uninfected plants by spraying the crop with mineral oil, but planting clean seed is the best management practice and disease-prevention strategy.

However, growers will have another strategy soon enough. Researchers at the Potato Research Centre are hard at work developing new potato varieties that are resistant to the virus. “There are two types of resistance to PVY,” Nie says. “These are called hypersensitive resistance (HR) and extreme resistance (ER), and different sets of potato genes produce them.” HR is strain-specific (typically to PVYO), whereas ER is broad-spectrum, effective to all strains of PVY. In 2013, the PRC released two new potato selections (AR2013-08 and AR2013-14) with extreme resistance to PVY, and they are now being tested by industry potato evaluators. The test is done in the greenhouse by mechanically inoculating the selections with PVY, followed by graft-inoculation with the virus to test for extreme resistance, Nie explains.

There are now also several promising lines at PRC with extreme resistance to PVY that are currently being evaluated. “To put all desirable traits – a cultivar with extreme PVY resistance and acceptable agronomic and economic characteristics – into a single variety selection is a tremendously challenging job,” he says. “However, to be optimistic, it will likely be available in the next several years.” 

January 21, 2015  By Treena Hein