March 2015
The Evolution of PVY By Dave Wilkins, Spudman Correspondent

Potato virus Y (PVY) refuses to go away.

Seed growers know the threat all too well. If virus levels exceed certain levels (2 percent in Idaho) in winter grow-out testing, the affected seed lot can’t be recertified.

Failure to recertify G2 seed (second field generation) means a downgrade to G3 and a reduced return to the seed grower.

If you don’t make recertification, then your seed doesn’t have as much value as if it had made it,” said Jonathan Whitworth, a plant pathologist with the USDA-ARS station at Aberdeen, Idaho, at the Idaho Seed Potato Growers Seminar in January.

PVY was a major topic of discussion during the seminar but it’s an issue that goes well beyond seed production. It can be an economic drag on any spud grower regardless of target market, Whitworth told producers.

The disease has changed. Some new recombinant strains cause tuber necrosis — defects that can significantly lower market value, he said.

“If external defects are more than 5 percent, the value of the lot can go from seed, fresh pack or processing price to the dehy price,” Whitworth said.

“If there’s a problem with tuber quality, it’s a quality loss, so now it becomes more of a problem for the commercial grower and for the industry in general,” he said.

A recent study in Idaho showed PVY-related tuber damage in several varieties, including Alturas, Highland Russet, Ranger Russet, Yukon Gold and Yukon Gem.

“The good news is that (Russet) Burbank and (Russet) Norkotah didn’t produce any tuber symptoms with the isolates that we tested,” Whitworth said.

Yield reductions caused by PVY can also take a bite out of profits.

Growers can expect a 1.5 hundredweight (cwt) per acre reduction in yield for every 1 percent of PVY infection in the field, according to a University of Idaho study that looked at the effects on three varieties: Russet Burbank, Shepody and Russet Norkotah.

A reduction of 1.5 cwt per acre may not seem like much, but it can quickly add up, Whitworth said.

“If you had 100 acres and it had 10 percent PVY, you’d be looking at a yield decrease of 15 sacks per acre,” he said.

That would result in a loss of nearly $12,000 for an Idaho grower, based on the 2013 (all-potato) market price of $7.75 per cwt, Whitworth said.

In other states, North Dakota, Wisconsin and Maine, where market prices were $10 to $11 per cwt, the loss would be even greater — nearly $16,000.

“I think that’s a significant amount,” Whitworth said.

When PVY first appeared in the United States several years ago, it was typically expressed by mild or no mosaic leaf symptoms, which is characteristic of PVYo, the ordinary strain of the disease.

In recent years there’s been a significant decrease in the ordinary strain. Unfortunately, there’s also been a sharp increase in new PVY strains that cause tuber necrosis.

“It’s been a shift from something that is not very damaging to something that is considerably more damaging,” said Alex Karasev, University of Idaho (UI) professor and an expert in plant viruses.

Aphids are the primary means of PVY infection. Transmission of the disease can occur in a matter of minutes or even seconds, making insecticides largely ineffective, said UI entomologist Erik Wenninger.

“In a very, very short period of time (aphids) can feed on a potato plant and transfer the virus to that plant,” he said.

UI research has shown some age-based resistance to the disease, meaning that potato plants are more vulnerable to PVY when they’re young. Controlling aphids early in the growing season makes sense, Wenninger said.

Green peach aphids are the most efficient transmitter of the disease, he said.

The bird cherry-oat aphid is also a vector of concern. UI research showed that it was far more efficient at transmitting the disease than previously thought.

“Bird cherry-oat aphid transmitted the virus at a much higher rate than has been shown in a lot of other studies, which is a little bit troubling given how abundant this aphid can be,” Wenninger said. “It might be a more important vector than we thought.”

Plant breeders are developing new varieties with resistance to the disease.

One promising cultivar is Payette Russet, one of five new russet varieties that the tri-state breeding program has proposed for release this year.

Payette Russet has good processing quality and no susceptibility to PVY, including some of the newer strains, researchers said.

“So far we’ve got good results and our goal in the breeding program is to get to this point where we don’t have to worry about PVY,” Whitworth said.

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