Pandemic sets back disease discovery

Aster yellows disease forecasting for western Canadian field crops is difficult in a normal spring, but this year it presents a whole new level of issues because of the COVID-19 pandemic.

Agriculture Canada laboratories in Saskatoon are usually well into testing leafhoppers for aster yellows by June, but this year they are yet to test an insect because of work restrictions brought on by the pandemic.

Tyler Wist, Agriculture Canada researcher, said the Agriculture Canada test is the only game in town when it comes to testing for the disease in Saskatchewan.

Wist was first able to access the department’s research farm near Saskatoon this spring on May 19, when the Government of Saskatchewan initiated its second phase of re-opening the province from pandemic lockdown restrictions.

“As soon as I put a sweep net down, there were leafhoppers there in the alfalfa,” Wist said.

“So we can’t really confirm if they were here earlier, but the diamondback moth pheromone traps were recording diamondback moths about a week before that.”

Wist is working on a project that is looking to see if there is a correlation between the diamondback moth arrival with the leafhopper arrival, and wind patterns that carry the pests up from the United States.

The project uses pheromone traps in sites across Saskatchewan growing regions to monitor for diamondback moths, and yellow sticky cards at the same sites to capture leafhoppers.

This spring the research team got a hit at Rouleau, Sask., where both leafhoppers and diamondback moths were found at the same site, around the same time he started finding leafhoppers in sweeps near Saskatoon.

Wist said it’s possible the diamondback moths came in a week earlier than the leafhoppers or they may have come through multiple introductions, however it seemed the leafhoppers had one big introduction to the province this spring.

The timing of the leafhopper arrival to the Prairies is important because an early arrival of the pest has been associated with severe outbreaks of aster yellows, including the 2012 infestation that likely cost Canadian canola growers hundreds of millions of dollars.

This spring has also had strong winds from the southwest.

Environment Canada provides Agriculture Canada with weather data called reverse and forward trajectory data. Modelling from that says if a packet of air left an area in the U.S., it will arrive at a specific Canadian location at a predicted time.

“With the diamondback moth traps and the leafhoppers, we’re trying to ground truth those winds and make sure that when the Prairie Pest monitoring says we’ve got winds that could bring diamondback moths, they actually are bringing diamondback moths,” Wist said.

This year, the leafhoppers were on the Prairies in force by May 19, and possibly a week earlier, but it’s unknown if they are carrying the phytoplasma that causes aster yellows.

“We haven’t been able to do any testing; our labs are still completely shut down,” Wist said.

“We’re putting those leaf hoppers on ice and we’ll get a picture of how infected they are later on when labs are back running again.”

Last February, Wist’s PhD student, Karolina Pusz-Bocheńska, spearheaded a paper published in Plant Health Progress that details protocols for a mobile field-ready test for aster yellows in leafhoppers.

“We’ve taken this tool and compared it against all the other molecular tests to show that it’s more sensitive. We allude to it being able to be done in the field, but we have done it in the field. It’s just for what we’re doing, its more cumbersome to do it in the field then to drive the leaf hoppers an hour back to the lab and have the lab people do it,” Wist said.

The new test extracts and analyzes DNA from the insects and can be conducted in the field in one hour, which will allow farmers to quickly decide if they should spray their crops for leafhoppers.

This test also can also indicate whether plant tissues are infected with aster yellows.

Wist said molecular scientists Tim Dumonceaux helped drive the development of the aster yellows tailgate test.

Harvest genomics is looking into taking these protocols and commercializing a product farmers can use to identify aster yellows, that would make testing for the disease much more accessible than it is today.

This year, however, growers will likely be in the dark when it comes to whether the leafhoppers on their field are infected with aster yellows.

“We are in that window of when another aster yellows cycle is going to happen, but we don’t really know what is driving those cycles,” Wist said.

In 2012, there was drought in the Midwestern U.S., and Wist said he read reports of leafhoppers in this area picking up and leaving because there was nothing for them to eat, and then southwest winds brought them up here.

“We’ve got some data from the cereal crops (from 2012) where we had infection levels of the leafhoppers of about 10 percent, which is way higher than the 0.5 to one percent that we usually find in any given year. But we don’t know, did they bring it up with them? Did they pick it up when they got here, which is possible,” Wist said.

Wist is also looking to see if aster leafhoppers pick up aster yellows locally in host crops such as alfalfa before spreading them into crops, instead of bringing the disease with them from the U.S.

“It could be what happened in 2012. The leafhopper showed up in mass and there wasn’t any crop to feed on so they fed on something else and picked up aster yellows and then moved it into the crop,” Wist said.

He said canola growers won’t know until the end of July if their crops are affected by aster yellows because it takes six to eight weeks to start seeing symptoms in the canola, including misshapen pods and flower buds.

There hasn’t been a lot of research on how economically damaging aster yellows can be to a crop, however researchers Chrystel Olivier and Bob Elliott have shown how damaging it can be to an individual canola plant.

“Three leafhoppers may cause enough damage after feeding on a canola plant for 10 hours that we get symptoms on a canola plant that there is zero yield from that canola plant,” Wist said.

Even if growers know they have leafhoppers infected with aster yellows on their crop, economic thresholds for spraying haven’t been developed.

“We don’t know what 100 leafhoppers in 100 sweeps at 10 percent levels of infection actually means for a canola field,” Wist said.

He said he and his colleagues have data from 2012, but it’s hard to make the leap to make a recommendation when it comes to spraying, partially because leafhoppers don’t seem to like canola very much.

“We often see aster yellows in a canola crop along the edges, so I think the leafhoppers are just sort of moving around and just popping into the edges of the field.

“Back in 2012, I think they just sort of blanketed the landscape and were just sort of feeding on everything,” Wist said.

He said funding from ADF and Western Grains Research Foundation has been essential.

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