Hung jury on aster yellows
It’s widespread, hard to detect and impossible to cure. No wonder its making headlines.
Aster yellows is a chronic, systemic plant disease commonly found in canola. It is caused by a bacterium-like organism called a phytoplasma, a pathogen that requires living plant and insect hosts to survive, spread and reproduce.
Dr. Chrystel Olivier, entomology specialist from Agriculture and Agri-Food Canada’s (AAFC) Saskatoon Research Centre, explained that the disease is transmitted by infected leafhoppers and survives in living plant tissue.
“The main source is the aster leafhopper which is blown into the Prairies by winds from the United States every spring,” said Olivier. “The extent of the disease depends on the migration of leafhoppers from the U.S. to Canada and their infectivity, the proximity of infected plants to healthy plants and the weather.”
While aster yellows is normally associated with canola, “DNA tests on
normal-looking plants reveal the presence of phytoplasma DNA in up to 30 per cent of cereals,” she said.
“Additionally, on the Prairies this year, there were more aster leafhoppers than we normally see. High rainfall renders plants more succulent (juicy) and therefore more attractive to leafhoppers.”
But Dr. Kelly Turkington, research scientist from AAFC’s Lacombe Research Centre, cautioned against overreacting to the aster yellows’ threat.
“Of the samples submitted by producers for testing due to aster yellows symptoms—such as chlorosis and head malformation—only 20 to 25 per cent tested positive for the disease,” said Turkington. “People were looking at any unusual symptoms in the field and attributing them to aster yellows.”
If you plan to tweak your crop and pest management due to concern about aster yellows, make certain you know what you’re dealing with or it may create problems in subsequent growing seasons, explained Turkington.
Since there is no cure for aster yellows, prevention is crucial. Turkington advised producers to monitor their crops closely and avoid having barley near canola or other susceptible crops.
“You can spray with insecticide to kill leafhoppers, but they are migratory, so you would have to spray several times. By the time you see symptoms it is probably too late,” he said.
Turkington said it is important to rule out other possibilities, rather than assume you have an aster yellows problem. This can include soil and tissue tests, as well as a review of your chemical program, root systems and a topographical field assessment.
After that, if you’re still adamant about using insecticides, “Put some check strips in and see if there’s a difference in the areas you sprayed. Some producers say, ‘if I spray the whole field I can sleep at night,’ but at what price?” said Turkington.