PVY research protects seed, prepares growers
For potato growers across the U.S., potato virus Y (PVY) is a serious problem that causes foliar and necrotic tuber symptoms that reduce size and overall yield in a wide range of potato varieties.
A University of Idaho study estimates losses from PVY cost the potato industry in that state alone $34 million a year. The same study estimates a 10-50% reduction in potential yields.
Few options exist for controlling PVY, except for growing and maintaining disease-free seed. While strains change and vectors persist, USDA Research Plant Pathologist Jonathan Whitworth and University of Minnesota Professor and Extension Entomologist Ian MacRae work to gather data that helps potato growers make informed management decisions.
Current PVY research disseminated in demonstration plots
In recent years, tracking results in the western U.S. states show that PVYO, the once predominant strain, has been displaced by PVYNWi. In eastern Canada, the same predominant strain has been displaced by PVYNTN. Depending on the variety grown, PVY strain, and the environment, foliar symptoms differ in intensity. Whitworth, working in collaboration with the now-retired Stewart Gray, evaluated the impact of different strains on some 62% of potato varieties grown in the U.S. Their aim was to determine which were most susceptible and how symptoms manifested under the different strains.
Whitworth’s work helps bring resistant genes into new varieties, and involves finding ways to keep breeding material free from viruses and other diseases. It also involves taking advanced breeding lines and evaluating how they respond to PVY. Host resistance offers a solution for growers, helping them to reduce their use of insecticides and crop oils, said Whitworth.
Using multiple isolates from five strains of PVY, the researchers infected and evaluated varieties representing 62% of the seed acreage submitted for certification in 2019. Five market classes were represented and a total of 67 potato varieties were used. Based on an analysis of visual foliar ratings, the researchers found the most severe symptoms were associated with the PVYO strain. The mildest symptoms were associated with PVYN:O.
While mild PVY symptoms are ideal in commercial production, in seed production mild symptoms allow a virus reservoir to build up in seed crops. In the seed industry, the common practice for reducing the virus is to rogue out plants and remove seed lots based on visually detected symptoms. In recent years, certification agencies have based virus percentages on ELISA testing leaf samples rather than visual inspections. This has helped improve the overall situation.
Several varieties, including Eva, Ciklamen and Payette Russet, showed extreme resistance to PVY due to the presence of a resistance gene. Whitworth and his team did not detect virus in any plants inoculated with any of the virus isolates.
Varieties such as Russet Norkotah and Silverton Russet, have mild or transient PVY symptoms and have contributed to increased levels of PVY in seed crops.
Whitworth said studying and cataloging the symptoms of different strains on different varieties provides important decision-making knowledge to growers. Knowing the symptoms in the different varieties helps them to determine what course of action to take. Knowing what action to take can be especially challenging as new strains develop and change.
In 2016, growers in Washington were invited to view PVY demonstrations plots displaying 42 different varieties. In 2018, 20 varieties were displayed in plots in Washington, Wisconsin and Maine. That year, there were five common varieties used across all demonstration plots. The remaining 15 were specific to that region.
“In these trials what we did is we had four rows of each variety,” said Whitworth. “One row was healthy, one row was infected with PVYO, one with PVYN:O, and one with PVYNTN. It provided growers with the opportunity to come look and see what plants actually look like when infected with the different strains of PVY and compare them directly with a healthy plant.”
Similar trials are planned for 2022 in three locations as part of a Specialty Crop Research Initiative grant. Doing so provides a great opportunity for researchers to disseminate the latest PVY research. It also allows them to show growers directly how they can control and limit PVY in their location.
Monitoring vector flight provides early warning system
Over the past 10 years, Ian MacRae and his colleagues at the University of Minnesota have been monitoring vector flight. Depending on the year, they have anywhere between 17 and 21 traps set up throughout central Minnesota all the way into North Dakota, covering about 400 miles in total.
“It covers most of the seed production that’s going on between North Dakota and Minnesota,” said MacRae.
The project is the result of grower demand; grower cooperators who host traps supplied by the university mail in samples weekly. Samples are sorted and identified in MacRae’s lab.
MacRae and his lab have collected 10 years worth of data, looking at seasonal flights, and as a result, have a good idea when they’ll start seeing vector pressure.
“All of the individual regions, their dynamics are a little different, and so this gives them a tool to try and start thinking about how they’re going to do their PVY management,” said MacRae.
Not all aphids are created equal. And while a number of species may be present in the trap, they aren’t all as efficient as a vector of PVY. Myzus persicae, or the green peach aphid, is the most efficient. Soybean aphid is only about a tenth as efficient as green peach aphid. Risk assessments can now be done based on species present as well as number.
“We have what we call a PVY vector risk index, and it’s species dependent,” said MacRae. “If we have 10 soybean aphids we think that’s equal to one green peach aphid.”
“You can cumulatively add these over the weeks and know how much risk a grower has to potential PVY transmission by aphids,” he added.
The results are used as a warning system for growers. The long-term data reveals when there should be a major push.
“Regional flights and the amount of risk that an area is going to have is not equal across the entire continent,” said MacRae.
In some regions, growers may be able to use early vine kill to avoid end-of-season pressure. If the vines are still green then they’re attractive to aphids, he said.
Using digital media, the results are disseminated via a blog site (aphidalert.blogspot.com), newsletters and Twitter.
“It’s been a great project and growers seem to appreciate it,” MacRae said in conclusion. “It’s giving us some very good data that’s helping us think about how and when we want to do our management tactics and what we want to do at different times of the year.”
Top photo: Silverton Russet showing mild PVY symptoms in a PVY plot. First row on the left is PVYO infected. Third row is PVYNTN. Photo: WSU