Canola growers should beware of aster yellows

Source: Saskatchewan Agriculture and Food

A disease that causes strange malformations is showing up in greater concentrations in this year's canola crop.

The disease is called "aster yellows," and it is caused by something called a phytoplasma - a micro-organism somewhere between a bacteria and a virus.

Penny Pearse, the Provincial Plant Disease Specialist with Saskatchewan Agriculture and Food, says, for the past six years, the incidence of aster yellows has been at trace levels of less than one per cent, but the number will be much higher this year, with some crops experiencing up to 10 per cent infection.

"We have received more reports of aster yellows in canola from both growers and agronomists, and it appears to be quite wide-spread across the province," she noted. "What makes this disease look so unique is that it causes malformations in the plant, so the plants are often taller, discoloured, and have malformed pods and flowers. The infection may look worse than it actually is, since the symptoms are so dramatic, so we recommend that growers do a count of infected and healthy plants to determine the actual incidence value."

Pearse says the phytoplasma causes the disease, but it needs a helping hand.

"A phytoplasma will not survive on its own, so it gets transferred from plant to plant by an insect vector. In this case, the most common vector is the aster leafhopper," she stated.

"When an insect feeds on an infected plant, it will pick up this pathogen and transfer it to healthy plants. So, in a year when we have more leafhoppers, we tend to see more aster yellows."

The damage done by aster yellows is complete and irreversible, with the yield loss dependent on the number of plants affected.

"Most of the diseases we have in Saskatchewan are caused by fungi, which can be controlled through the use of a fungicide. Whereas something like aster yellows, once it is in the plant, there is nothing you can do... the damage is done," Pearse said.

"We don't know a lot about this disease," she said. "Leafhoppers have been found to over-winter in Saskatchewan and can keep the phytoplasma alive from one season to the next. In addition, some of the perennial crops that we grow here - meaning crops with root systems that over-winter - offer a way for the pathogen to over-winter. Crops like echinacea and caraway are also at risk," Pearse stated.

A survey of canola fields was conducted this summer to look at aster yellows and other canola diseases. Aster yellows was present in all surveyed fields, ranging from trace levels to as high as 15 per cent infection. The overall average infection level in 2007 is approximately two per cent, which is similar to 2000's levels. It is likely that this summer's heat amplified the aster yellows symptoms. In addition, the phytoplasma multiplies more quickly in the plant under hot conditions.

For more information, check out the aster yellows fact sheet on the Saskatchewan Agriculture and Food website at http://www.agr.gov.sk.ca/. It can be found in the "Production" section under "Disease."

For more information, contact:
Penny Pearse, Provincial Plant Disease Specialist
Saskatchewan Agriculture and Food
Phone: (306) 787-4671