Aphids, virus harangue spud industry, scientists
PRESQUE ISLE, Maine — On the flip side of farmers worrying about pests are entomologists, who earn a living understanding odd creatures like aphids, a huge family of self-cloning, virus-spreading insects.
To Jim Dwyer, a University of Maine Cooperative Extension specialist in Presque Isle, “aphids are really fascinating.” The tiny, lice-sized bugs spend their short lives mostly as populations of females, cloning themselves during the summer, feeding on plants, and having males born in the fall.
With pesticides, fungicides, crop rotations and other treatments, aphids and the diseases they transmit have been manageable, ebbing and flowing problems for Maine’s potato industry. But in some respects, the insects and at least one virus — potato virus Y — are ahead of the game, as Dwyer and other researchers are finding.
Over the last decade in Europe, the traditional strain of potato virus Y (PVY-O) “has been completely displaced” with mutated variants, Dwyer said.
“It’s all been taken over by the necrotic strains, PVY-NTN and PVY-NO,” which are “more vigorous and more fit.” They don’t show symptoms in the potato plant leaves — which is how field scientists have traditionally identified infections — and they can still cause damage to the tubers.
While in the 1990s most of the potato Y viruses found in Maine were the traditional, more manageable O-strain, the mutated varieties from Europe have etched out small colonies in North America, Dwyer said. They’re especially a hazard for Maine’s 100-plus certified seed potato growers, who supply potato farms across the East Coast and Canadian Maritimes.
“If you can’t see it, how do you manage it?” Dwyer said, outlining the dilemma in a recent presentation at the University of Maine Presque Isle, where some students and faculty are contributing to research.
Workers assessing quality for certified seed potatoes have traditionally walked through the fields, looking for mottling, yellowing and leaf crinkling. In Maine, certified seed potatoes must come from fields containing less than five percent of virus-infected plants. Foundation-class seed, the source of certified seed classes, can only have less than half of one percent of plants with virus.
Viruses have gone through cycles in Maine’s potato fields in the past. In 1991, the state agriculture department raised the tolerance level for potato virus Y from 5 percent to 7 percent of seeds, as food processors and their growers struggled to rid the virus of their spuds. Potatoes with PVY aren’t harmful to eat, but they have blemishes that the market has generally rejected.
Today, with the new strains “there is a movement in the state to go toward laboratory tests” for seed potato certification to have a more accurate view of the viruses present, Dwyer said.
In the meantime, he and other researchers also are learning as much as they can about the aphids flying through potato country, which feed on plants and transmit potato virus Y, including some unknown number of the “necrotic strains, among other infections.
“What are the chances of an aphid hitting a virus-laden plant, and moving onto another plant and inoculating the next plant?” Dwyer said, looking at the situation from a farmer’s perspective. “The odds become very high, and if you’re a seed grower, you can have serious issues.”
Unlike other viruses, the potato virus Y can transfer to a potato plant within a few seconds of contact with an aphid, so insecticides that take longer to work may not be as effective. Potato plants can be visited as many as 2,000 times in a day by aphids during migrating swarms of the bugs, Dwyer said.
The PVY virus also has been found in dandelion, hairy nightshade, lamb’s quarters, American germander, and maple trees — possible transmission links in the agricultural ecosystem that researchers are trying to understand by trapping aphids in field catchers and analyzing the viruses they carry.
Maine and New Brunswick farmers have been faring well using stylet oil, Dwyer said. The refined mineral oil, approved for organic growing, is sprayed on plant leaves and interferes with the ability of aphids to spread the virus, although that can get expensive. (Ladybugs also are prolific consumers of aphids, among other insects that may be beneficial for farmers.)
In 2015, Maine potato growers were spared a huge wrath of aphids, thanks in part to the relatively cool June, Dwyer said. Potato samples during the last two seasons also suggested that the mutated strains are for the time being less common than the type-O strain.
“Things aren’t continuing to get worse, which is a positive from our standpoint,” Dwyer said.
