The incidence of aster yellows disease appears to be on the increase this year. And although it continues to be a fairly insignificant problem in most canola fields, it is hurting herb and spice crops.
“At this point we don’t know if it is a unique situation for this year or if it is because it overwintered from last year in some of our perennial crops,” said Penny Pearse, Sask-atchewan plant disease specialist.
The disease is called aster yellows because it was first discovered on asters and because it turns the leaves of some plants a brilliant yellow.
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It is caused by a viral-like pathogen that is transmitted by leafhoppers. Although the disease can overwinter in biennial or perennial crops and weeds such as dandelion, plantain and quackgrass, most infections are caused by leafhoppers migrating from the United States.
“We are doing a canola disease survey again this year, but at of yet we don’t have any numbers,” Pearse said.
Aster yellows in canola can cause direct yield loss as flowers and pods are malformed with poor or no seed fill, she said. Symptoms become more obvious toward harvest as infected plants remain more upright, are purplish-green and malformed.
“It can look worse than what it is. They stick up above the crop canopy, they are malformed and purple so they are obvious. But if you actually start counting what percentage is infected, it is usually one to two percent, if that much.”
However, in 1999, researchers were surprised to count the infection rate as high as six percent in one field.
“So that farmer was looking at a six percent yield lost and that is significant.”
There has been no research program regarding the disease and there are no registered insecticides or fungicides.
Pearse said the bug itself does not harm the plant. Rather, plants are damaged by the disease that some leafhoppers carry. There is no test to determine whether the leafhoppers in a field are carriers.
To be effective against the migratory insect, the crop would have to be sprayed several times and that would be costly.
Agriculture Canada and the Saskatchewan Herb and Spice Association are in the second year of a program to determine the effect of the disease on echinacea, caraway, coriander, valerian and feverfew.
They are monitoring leafhopper numbers using suction traps at the plots to pull in air and insects to monitor arrival of the leafhoppers, and sticky cards to check the species and density within the plots.
They are testing several organic insecticides including pyrethrines, diatomacious earth, garlic oil and a product called retenone.
“When the numbers of leafhoppers started to increase in early June, that is when they started their control. But the rains this year made it difficult. A lot of these products need to come in contact with the leafhoppers.”
As of July, the organic insecticides were having little success, Pearse said.
Management of aster yellows in herbs and spices is limited to: removal and destruction of infected plants; application of insecticides to surrounding weeds to control leafhoppers; and, when dividing crowns and roots, selection of healthy plants for vegetative propagation.
The options for control of aster yellows in canola are also few. Perennial weeds within and around crops should be controlled, and planting canola near crops of perennial forages or echinacea known to have aster yellows should be avoided.
