The tiny insect that can transmit the disease that’s decimated many potato fields from Mexico to the northwestern U.S. but has so far eluded Canada, was caught in researchers’ traps in southern Alberta this past summer. How concerned should growers be on this side of the border?
When Joseph Munyaneza first started working with zebra chip as a U.S. federal researcher, the disease — a bacterial infection spread by potato-tomato psyllids — was confined to Texas.
Munyaneza is a research entomologist with the United States Department of Agriculture’s Agricultural Research Service (USDA-ARS) and is based in Washington, where he also serves as an adjunct professor at Washington State University.
“As a federal government employee, I conducted research in Texas. But most potatoes [in the U.S.] are grown in the Pacific Northwest. So nobody paid much attention,” he says. “Everybody was complacent. But in 2011, [the disease] travelled to the Northwest, and then people started waking up.”
The Canadian Zebra Chip and Potato Psyllid monitoring network was established in 2013 to help alert growers on this side of the border to dangers of the disease. This past November, Munyaneza was invited to talk about the U.S. experience in battling zebra chip at the Potato Growers of Alberta’s annual potato conference held in Red Deer, Alta.
First reported in Mexican potato fields in 1994, zebra chip disease then spread to Texas and has since been reported in numerous U.S. states, including Nebraska, Colorado, Kansas, Wyoming, New Mexico, Arizona, Nevada and California. In recent years, the disease has moved into prime potato-growing regions in Idaho, Washington and Oregon. It’s also turned up in a few other countries, including New Zealand.
In the summer of 2015, researchers in Canada found potato-tomato psyllids in the Lethbridge, Alta. area. The psyllids were not carrying the bacterium that causes zebra chip — Candidatus Liberibacter solanacearum (Lso) — but the finding is still considered a major red flag. “If the psyllid is in Canada, and no one has reported zebra chip yet, it doesn’t mean you won’t get the disease,” says Munyaneza.
Zebra chip can attack any potato — processing, seed, or fresh — and causes unsightly black stripes in the tubers of infected plants, rendering them unmarketable.
It can be a devastating disease economically, with the capacity to wipe out production and cost growers millions of dollars annually. In Mexico, says Munyaneza, the disease has decimated so many potato fields that many growers have switched to other crops. “And when it first showed up here in the Northwest in 2011, there were some growers who lost millions of dollars in two to three weeks,” he says.
“Now, people are spraying on average 10 [insecticide] applications a season, and these applications can cost up to $300 per acre, so even if you don’t lose your crop you’re spending too much money trying to protect it.”
Even though zebra chip has not been found in Canada, Canadian producers cannot afford to be complacent, says Munyaneza. As he says, potato growers need to “be careful, be on the lookout.”
The potato-tomato psyllid is a tiny insect, about the size of a green peach aphid. Though it’s an invasive species, psyllids have been in North America for more than a hundred years. But the psyllid was not connected to zebra chip until 2007, says Munyaneza.
“We knew the insect was a pest and we believed it was injecting a toxin into the plant. Then we found out the bacterium was being transmitted by the insect,” he says.
Psyllids can acquire Lso in two ways — by feeding on infected plants, or by inheriting the bacterium (mother to offspring).
“Once the insect gets the bacterium they are infected for life,” says Munyaneza. A single psyllid can have hundreds — or even a thousand — offspring, which means once a population is infected with Lso it’s nearly impossible to contain.
Initially, researchers believed psyllids were migrating northward. But then they discovered that psyllids are “basically resident” throughout the U.S. Pacific Northwest, says Munyaneza. Gradually, Lso-carrying psyllids have spread the bacterium to existing psyllid populations in the region.
People are spraying on average 10 applications a season, and these applications can cost up to $300 per acre, so even if you don’t lose your crop, you’re spending too much money trying to protect it.
– Joseph Munyaneza
Dan Johnson, a professor of environmental science at University of Lethbridge in Alberta, suspects psyllids have also been resident in Canada for a long time. In the past, psyllids have been found from British Columbia to Manitoba.
“My belief is that the psyllids we’re finding did not suddenly arrive,” he says. “They did not have wear on their wings. They had nice new wings … that indicate they hatched in those fields.”
When he went through the literature, Johnson found evidence that psyllids migrated north from Texas in the 1940s and 1950s. He believes they have been resident in Canada since then at very low levels.
According to Johnson, the fact that potato-tomato psyllids are established in Canada is not necessarily cause for alarm — yet.
“I don’t think it’s getting worse,” he says. “I think what it indicates is that we have a very low population, much lower than Oregon or Idaho, and there’s no need to start spraying. If we started spraying we’d make it worse — not much of it would kill the target, but it would kill the enemies of the target.”
In the summer of 2015, Alberta growers generally didn’t spray for Colorado potato beetle, says Johnson, which meant there were enough of these natural predators of psyllids around to help control psyllid populations.
Monitoring is the most important method for minimizing the risk posed by zebra chip, says Johnson. Scouting fields for psyllids is step one in a proactive management plan.
Johnson is the lead on the Canadian Potato Council’s zebra chip monitoring project. Along with research partners Larry Kawchuk, an Agriculture and Agri-Food Canada potato pathologist, and Scott Meers, Alberta Agriculture and Rural Development’s provincial entomologist, he has set up a national monitoring system that involves surveys and sampling in fields across the country. Johnson’s team is also analyzing where psyllids lay and hatch eggs, as well as potential control strategies.
The national monitoring project collects data in B.C., Alberta, Manitoba, Quebec and New Brunswick, with more limited data collection from Saskatchewan and Ontario, says Johnson. “We keep track of about 10 psyllid species, and recognition of colours, stages, and sexes is an important part of maintaining some certainty in the results,” he says.
A graduate student at the University of Lethbridge, Summer (Qing) Xia, has begun building statistical forecasting models that will look at weather patterns and distributions in the U.S. to get a sense of where the pest will live in Canada.
Potato Growers of Alberta has partnered with the national monitoring project, and has also partnered with a group called Promax Agronomy Services and the Alberta Crop Industry Development Fund Ltd. on another project, says Jay Anderson, PGA’s project manager.
PGA helps fund both projects, and Anderson himself participates in in-kind work, putting out traps and monitoring.
In the summer, volunteers from Alberta’s potato industry collect Promax’s sticky traps weekly. The traps are then analyzed in a lab and psyllids are counted. That information is sent out in a weekly email to Alberta growers. Sampling runs from late May to September.
Scouting for psyllids is not always easy. One of the biggest problems with zebra chip is that the psyllid is difficult to identify, says Johnson. “There are many psyllids but there is only one that transmits Lso to potato crops in North America, so helping people to learn to recognize it has been tough — it’s very small, and even entomologists have trouble with it,” he says.
Johnson is putting together a photo guide to psyllids to make it easier for growers and researchers alike to identify potato-tomato psyllids as well as their natural enemies.
This summer, the team plans to expand its monitoring network of sticky traps across the country. “One thing we need badly is more methodical gridded sampling, a pattern of sampling that hits as many locations as possible,” he says. “So we’re going to do more sampling and get more cooperation from growers.”
It’s important to keep monitoring and to stay on top of this. We started proactively and we need to continue to be proactive.
– Jay Anderson
Johnson is pushing for as much sampling as possible. Currently, he’s building statistical models that will help the team determine how many sticky traps should be set up. “There are good economic reasons to take bigger samples than we’ve done in the past,” he says.
Munyaneza maintains many insecticides are effective against psyllids, but spraying should occur as a last resort. In Canada, beneficial insect predators of psyllids still appear to be effectively controlling psyllid populations, and as psyllids are notoriously prone to developing resistance to chemical controls, insecticide use should be delayed as long as possible, he says. So far, psyllid populations are low enough in Canada to necessitate careful monitoring, but further controls are unnecessary, Munyaneza adds.
Both the national and provincial monitoring teams are working hard to gather data and get an accurate sense of psyllid populations and the risk of zebra chip in Canada. But the experts say growers should also get involved: if they spot psyllids in their fields, they should put out their own sticky traps and start counting.
“Especially in Alberta, it’s important to keep monitoring and to stay on top of this. We started proactively and we need to continue to be proactive,” says Anderson.