[UPDATED: Nov. 28, 2022] Aster yellows is a sneaky disease, arriving in canola via aster leafhoppers infected with the parasitic organism aster yellows phytoplasma (AYP). The leafhoppers spread the disease to plants as they feed, but the symptoms don’t show up until the crop is too far gone to treat.
In bad years — which seem to happen cyclically every five to eight years — aster yellows can cause significant yield losses in canola as well as in cereals and flax.
Tyler Wist, a research scientist in field crop entomology for Agriculture and Agri-Food Canada (AAFC) in Saskatoon, calls the aster leafhopper a “gregarious” pest, meaning it’ll feed on almost any plants out there and can infect them with phytoplasma. Canola is highly susceptible to infection with AYP.
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The last “bad year” was 2012. But even in typical years, the disease can be a problem.
Symptoms in canola are, frankly, weird — infected plants are often taller than healthy ones, and they stay green when the rest of the field is senescing. Flowers turn green and seeds sprout inside pods.
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On the plus side, the symptoms can make infected plants easy to spot, even without leaving the car.
“Sometimes when we’re out looking for aster yellows, we’ll slowly drive by and we can see it in the field,” says Wist.
He is the lead and co-lead, respectively, on two projects aimed at identifying infected fields in time to manage the disease with fungicide. One Western Grains Research Foundation (WGRF) project, which Wist refers to colloquially as “Aster Yellows Winds,” tracks insect arrival from the U.S. each spring and summer.
The other project, funded by WGRF, Saskatchewan’s Ministry of Agriculture, and SaskCanola Wist calls “Aster Yellows Weeds” — this one focuses on identifying aster yellows reservoirs in weed populations in the province.
Invaders from the south
The Winds project started with the Saskatchewan ministry monitoring diamondback moth arrival with pheromone traps.
“Diamondback moths and aster leafhoppers might be arriving on the same winds from the southern U.S.,” says Wist. If the moths are present, the leafhoppers often are too. Aster leafhoppers aren’t attracted to pheromone traps, but they are to sticky yellow cards, which are collected weekly during the spring.
If the cards indicate that leafhoppers are present in a given week, Wist and his collaborators can go back and use U.S. National Oceanic and Atmospheric Administration software called HYSPLIT, which models the trajectory of air particles to find their likely origin.
“What we’ve been finding over the four years of the project is that they’re just coming straight north — they’re likely being tossed into Saskatchewan from the Nebraska area,” he says.
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Part two of the Winds project is figuring out which aster leafhoppers that arrive in Saskatchewan are infected with AYP. If they’re not, they won’t do any damage, Wist says. How to tell if they’re infected? One way is to use nested PCR (polymerase chain reaction, the same technology as COVID testing), but it can take two weeks to get results from the lab.
The lamp test
There is a better way. Wist’s colleague at AAFC, research scientist and molecular microbiology specialist Tim Dumonceaux, has developed a loop-mediated isothermal amplification (LAMP) test that can be used in the field to diagnose AYP in leafhoppers
“Tim developed a method where you could run the LAMP and you’d get a colour reaction in the tube — it would go from pink to orange. We realized it was an order of magnitude more sensitive than the nested PCR. And you don’t have to run a gel,” Wist explains.
The test works by placing a leafhopper on a “plant-saver card” which extracts its DNA. Then, the extracted DNA is placed inside an ironically named “Genie” machine to run the LAMP reaction. The test needs a power source, but can be plugged into a car dashboard; it also needs a source of steady heat, which can be supplied by a thermos of hot water. “After half an hour, if your tube has changed colour, that leafhopper has aster yellows,” says Wist.
“Tim has run this test on blueberries in the back of a truck, and we’ve run it in an Ag Canada SUV. But you don’t have to do it in the field — you can run it back to the lab and it’s less of a pain for us.”
The test has been licensed to Harvest Genomics, as well as a lab in Quebec, so it could potentially be delivered to producers in a kit format in the future.
Native plant hosts?
Wist says the Weeds study provides some important insight on aster yellows reservoirs that may be harbouring the disease in Saskatchewan. “The next generation (of aster leafhoppers) in Saskatchewan has a higher percentage of infection, which doesn’t make sense unless they’re picking it up from plants in the province,” he says.
“Most of the alfalfa we tested had aster yellows in it, and also some common biennial weeds — they’re the last green thing in the fall. We think the leafhoppers congregate in areas that have rosettes of weeds, and the next year they grow up into a plant and lo and behold, they’re infected.”
Wist says the leafhopper migration comes in before the crops are established, lands in those plants, picks up the aster yellows phytoplasma, and moves it into the crops.
“As if you needed another reason to dislike dandelions,” he says.
While they wait for in-field test kits to become a reality, producers who suspect they have an aster yellows problem can sweep-net grassy verges and field margins and contact Wist’s lab.
“What we’ve been doing is limited by the number of leafhoppers we can get,” he says. “It’s tough for me to cover off the entire province with a sweep net.”
*Update: the article was updated to add SaskCanola as a funder of aster yellows’ research.
