Features Agronomy Diseases
Ahead of the curve on zebra chip

One of the features used to distinguish the potato psyllid from other psyllid species is the broad white stripe on the adult potato psyllid’s abdomen. Photo by Dan Johnson/University of Lethbridge.

Zebra chip is a devastating potato disease costing millions of dollars to the potato industry in affected areas of the U.S. and other countries. So far, the disease hasn’t been found in Canada, but it is moving toward our border. Now a major multi-agency project is taking a proactive approach in case zebra chip arrives here.

The project involves a nation-wide monitoring network for zebra chip and the potato psyllid – the tiny insect that transmits the disease – and related research and technology transfer activities. The overall goal is to help the Canadian potato industry be better prepared to deal with the disease if it comes to Canada.

The impetus for the project came from the Potato Growers of Alberta (PGA). “About four years ago, I was at a Potato Association of America conference and the hot topic was the potato psyllid and zebra chip. I talked to some of the presenters and scientists about what was happening with this disease and the potential for it to migrate north. So I became interested in the disease,” PGA executive director Terence Hochstein explains.  

Then in 2011, zebra chip was found in Washington, Oregon and Idaho. Hochstein says, “In the Pacific Northwest, they didn’t realize they’d had it until the end of the season and the damage was already done. We discussed what it did to their crop that year, the economic cost to growers and the effects on the whole industry, and we decided to start looking at the problem rather than waiting until we have it here.”

Dr. Dan Johnson, an entomologist specializing in large-scale insect forecasting, ecology and biogeographic analysis at the University of Lethbridge, is the project’s principal investigator and national co-ordinator. He notes, “Potato psyllid and zebra chip are poised to enter Canada, but they are not here yet. It may be just a matter of time before the disease moves into Canada. We think it will probably arrive first in British Columbia, Alberta and Manitoba, and then perhaps Eastern Canada.”

About the disease and its vector
Zebra chip is caused by the pathogen Candidatus Liberibacter solanacearum. Infected potato plants show above-ground symptoms such as stunting, zigzag stems, and misshapen and discoloured leaves. Tuber symptoms include enlarged lenticels, collapsed stolons and brown “zebra” streaks and flecks that are especially noticeable in fried potatoes. The disease kills potato plants, severely reduces yields and makes the tubers unsalable because of the brown discoloration.

“This disease is so new that there are a lot of unanswered questions,” notes Dr. Larry Kawchuk, a plant pathologist and molecular biologist at Agriculture and Agri-Food Canada (AAFC). Kawchuk is one of the main collaborators on the project.

Zebra chip was first discovered in Mexico in 1994 and in Texas in 2000. Since then it has spread through much of the western half of the U.S., into several Central American countries and New Zealand.

Only potato psyllids (Bactericera cockerelli) transmit the zebra chip pathogen. They become infected by feeding on plants that have the disease, and then they spread the pathogen to other susceptible plants by feeding on them. Infected adults can sometimes transmit the pathogen to their offspring.

Potato psyllids go through three life stages: egg, nymphs and adult. The adults are about 2 mm in length, with long, clear wings. They prefer to lay their yellowish, football-shaped eggs on solanaceous plants, such as potatoes, tomatoes and peppers and on weeds such as hairy nightshade. The nymphs are flat with a spiny fringe.

In the U.S., only a small percentage of potato psyllids carry the zebra chip pathogen. Infected psyllids are referred to as “hot.” Some U.S. research indicates hot psyllids can transmit the pathogen to a plant in less than an hour or two of feeding. The adults hop around, so hot psyllids can infect multiple plants.

Potato psyllids are able to survive temperatures below freezing and seem to be overwintering in the Pacific Northwest. Over the years, they have occasionally come into Canada. For instance, in 2012 two purported potato psyllid immatures were found near Carberry, Man. Both were free of the pathogen.

At present, the main way to control the spread of zebra chip is by controlling the potato psyllids. Some potato cultivars are more tolerant of the disease than others, and U.S. potato breeders are working to develop varieties with improved zebra chip resistance.

Monitoring underway
The Canadian monitoring network is a collaborative effort. The three research leads are: Johnson; Kawchuk; and Scott Meers, insect management specialist with Alberta Agriculture and Rural Development, with responsibility for insect pest monitoring in the province.

Along with these three researchers and the PGA, the network includes people from potato grower associations in other provinces, the Canadian Potato Council, the Canadian Horticultural Council, agronomists, federal and provincial specialists, university researchers, potato processors and others in the potato value chain.

The project, which started in 2013 with field sampling in advance of the main project, is funded through the Canadian Horticultural Council in the Canadian Agri-Science Cluster for Horticulture supported in part by AAFC Growing Forward 2 funding. The project’s industry funding component is provided by contributions from the PGA, the Stuart Cairns Memorial Chipping Research Fund and the University of Lethbridge.

Yellow sticky cards are used to monitor adult psyllids. These cards have very light layers of sticky surface and do not harm birds or mammals. Hochstein outlines how the psyllid monitoring works in Alberta: “We work with the local agronomists to ensure good coverage across the growing area. While they are scouting the fields, the agronomists put the cards in, usually just inside the perimeter of a field,” he says. “The cards are left in place for seven days of clear weather. Then the agronomists collect them and drop them off at our office.”

As well, Johnson places cards at additional locations, such as roadside vegetation in Alberta and B.C., to improve the chances of catching invading potato psyllids. He has also established sentinel plots for more detailed sampling, including sweep net sampling for adults, and leaf sampling for eggs, nymphs and adults.

Co-operation from growers to establish sampling sites at field edges will be an important part of the continuing program.

Project staff examines every one of the collected cards under a microscope to identify and count all the insects, including things like aphids, beetles, flies, wasps and other psyllid species. Over 1000 cards were examined in 2013. Cards examined in 2014 include: over 500 from Alberta, 59 from Manitoba, 500 from New Brunswick, 11 from P.E.I. and 45 from Quebec.

“So far we have found many types of psyllids, but no potato psyllids,” Johnson says.

The project’s disease monitoring involves testing potato psyllids caught by the monitoring network and any plants suspected of having zebra chip. Kawchuk has been working on refining laboratory methods for testing for the disease based on techniques developed by some of his U.S. colleagues. His lab extracts DNA from each sample and analyzes it for the zebra chip pathogen, using a very precise technique.

“We are encouraging people to send in samples of plants with foliage symptoms that might possibly indicate zebra chip,” Kawchuk says. “And if processors see any examples of tubers with the distinctive zebra chip speckling, we’re asking them to bring in the tubers.”

During 2013 and 2014, Kawchuk’s lab examined over 150 foliage samples. He says, “All of our testing was negative for zebra chip.”

So far, so good
“We’ve done enough sampling to know zebra chip is not here yet,” Johnson says. “We have a little breathing room to test and refine our sampling and analysis methods, and get ready if and when it does hit. So we are ahead of the curve on this research.”
The monitoring network will expand this year, with more fields and more provinces participating.

Johnson is also interested in sampling additional sites beyond those monitored in the provincial monitoring efforts, and he’ll provide free yellow cards to people who want to put the cards out for a week and then send them to him for checking. Greenhouses, potted plant suppliers, gardens and tomato crops are important additional sampling sites.

As well, he plans to continue fieldwork to try to determine the current distribution and prevalence of solanaceous weed species. He hopes to start monitoring some of these weeds for the potato psyllid, and to determine whether the psyllid, and/or the pathogen, is able to overwinter in any of these weeds.

If and when potato psyllids arrive in Canada, the researchers will work on refining monitoring techniques to suit Canadian conditions. They will also assess how closely the total potato psyllid numbers and the number of hot psyllids are related to the incidence of zebra chip. And they will evaluate to what extent they should be monitoring fields for symptomatic plant tissues. 

In addition, Johnson will be investigating ways to predict potato psyllid infestations, for example, by using factors like air mass movements, temperatures, sources of populations and so on.

And if the psyllid arrives, Meers and Johnson will start research on control methods. Possible directions for their research include integrated control practices, minor use registrations for insecticides and testing psyllids to check for insecticide resistance, since some U.S. potato psyllids have developed resistance to certain insecticides. Johnson notes, “Some research in the U.S. indicates that it may even be possible to develop a biological control agent for suppression of the potato psyllid, perhaps using the new insect biocontrol agents we have recently developed in Canada.”

If the zebra chip pathogen is found in Canada, Kawchuk will study it to determine whether the Canadian biotype is different from those found in other regions.

Information aids for growers and agronomists are also being developed. For example, Johnson says the team is working on an iPhone app for psyllid identification. “We have about 30 species of psyllids, so I’ve started preparing a guide for psyllid recognition and sampling, with photographs. You need to use a hand lens, but you can recognize the potato psyllid quite quickly once you know the different psyllid species.”

At present, the key for Canadian potato growers is to watch for the insect and disease symptoms.

“It would be great if zebra chip doesn’t arrive here,” Johnson says. “Certain attributes of our landscape, vegetation and climate might help to slow potato psyllid establishment. For example, perennial solanaceous weeds can allow the potato psyllid to overwinter and we have almost no perennial solanaceous weeds,” he says. “So we might end up in the situation where occasionally the psyllid will fly in and cause trouble but not overwinter. The only way to find out will be to stay on top of sampling and develop computer models of where and how they could survive here.” 

Hochstein adds, “It’s a matter of watching and monitoring. I hope we never see zebra chip. But if we do, we are going to try to be prepared.”

March 23, 2015  By Carolyn King