Canola School: New test to detect aster yellows in leafhoppers

Aster yellows infection of canola. 2012. Photo credit: Tiffany Martinka, CCC

In the last couple of years, aster leafhoppers have arrived around the third week of May, perhaps on the same wind that diamondback moths fly in on from the U.S. The aster leafhopper is a small, bullet-shaped insect that doesn’t directly damage the plant; instead it is a vector for the disease aster yellows.

In 2012 in particular, a lot of canola fields were infected with aster yellows on the Prairies, which appears as deformed or bladder-like pods in canola, and can affect yield in severe cases.

Tyler Wist, field crop entomologist and research scientist with Agriculture and Agri-Food Canada (AAFC) at Saskatoon, Sask., explains how the small insect can be such a cause for concern in this Canola School episode. (story continues below video):

Aster leafhoppers carry the aster yellows phytoplasma in their salivary glands and can infect a canola plant when they start feeding. Aster yellows is rated on a severity scale from zero to five; anything above a two on the severity scale and seed production goes way down, says Wist.

“Down in the states, aster yellows is a really big problem in carrot production, lettuce production, as well, and celery production,” says Wist, with other crops potentially affected, too. In the U.S. they’ve developed an aster yellows risk threshold calculated from the number of leafhoppers caught in a sweep net multiplied by the percentage of leafhoppers infected with the disease. In order to know how severe aster yellows may be, the leafhopper must be tested for the disease.

To gain an idea of this same risk threshold in Canada, Wist needed to know when the leafhopper would arrive and needed a test to confirm that the leafhopper has aster yellows. The first piece of information he could get is from the Prairie Pest Monitoring Network.

To get the second piece of information, whether or not the leafhopper carried the aster yellows phytoplasma, Wist’s research lab first used a process called Nested PCR—a method that amplifies a small amount of DNA from an organism in order to study it— but Wist and his PhD student, Karolina Pusz-Boschenka found the test wasn’t sensitive enough.

“Dr. Tim Dumonceaux was working on a different technique, the acronym is LAMP,” says Wist, and using a different molecular target for amplifying the DNA, they were able to get a more sensitive test for aster yellows. Before, the test would take at least a week, but now it can be done in about an hour.

The aster yellows risk index in field edges or directly in the field hasn’t been determined yet for canola, but colleagues of Wist’s found that it only takes three aster leafhoppers to transmit the disease in 10 hours of feeding. The yield of the affected plant will be completely wiped out, and that aster leafhoppers can live in a field for up to a month.

Aster leafhoppers do have a parasitoid that protrudes out of the side of the leafhopper at a certain growth stage, but it doesn’t do a great job at biologically controlling the leafhopper. Beneficial insects such as lacewings, ladybugs, and ground beetles might eat leafhoppers, but the leafhoppers are hard to catch by these insects.

“You always have to make that call. Do I want to protect the beneficials in my field, and is it worth it spraying leafhoppers in my field? Right now, we don’t have a great answer,” says Wist.

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