Aster yellows were a problem in the majority of canola fields across the prairies this year, and have been positively identified in cereal crops as well. The yield loss attributed to this disease is up for some debate, but we do know it had more impact this year than previous years. We normally have levels of one to five per cent infection in fields on a yearly basis, but this year we saw up to 60 per cent infection in canola. Is this a disease we should be watching closely and what management options do producers have? In order to start planning for next year, we need to understand more about this disease and what aspects we can control.
Understanding the spread of the disease is the first step. Aster yellows does not spread by wind or rain, and field infection does not start with stubble from last years crop. Aster yellows is transmitted by insects called leafhoppers. The leafhoppers are sucking insects that suck sap from plants as they feed. When they feed from an infected plant they acquire the organism. After a week or two the leafhopper becomes a carrier and can infect other plants. Once a leafhopper becomes a carrier, it is a carrier for life, and each plant they feed on becomes infected. With a life span of 100 days or longer, there is a lot of feeding that can occur, and a lot of plants that can become infected. Once a plant is infected, it also remains so for life.
We know how the disease spreads in a field or local area, but where does the initial infection come from? There are three potential sources of infection. The first involves leafhoppers migrating from the southeastern and central United States through wind currents. These migrant leafhoppers may have been infected with the virus before they cross the border. We know in 2012 in North Dakota there was a high level of infected leafhoppers in early June. This means that some of the leafhoppers migrating in were carrying the disease and could have been the early sources of infection.
A second source of infection could be from leafhoppers overwintering as adults. The early appearance of leafhoppers in the spring supports overwintering adults but this has yet to be confirmed in Western Canada.
The third source of infection involves leafhopper eggs overwintering in Western Canada and emerging in spring. The newly hatched leafhoppers are not infected as the disease does not get transmitted from the adults to the eggs. Upon hatching in the spring the young leafhoppers can become infected by feeding on infected plants. These infected plants are called host plants and include perennial, biennial or winter annual crops and weeds.
Hosts of Aster yellows include over 300 species of plants in 48 plant families including broadleaf crops, vegetables, ornamentals, weeds and, to a lesser extent, cereal crops. Perennial, biennial or winter annual weeds can be hosts which means control of weeds in the fall or early spring may be one management tool we can use to prepare for next year.
Monitoring the leafhopper populations in the spring and identifying the level of infection in the population will also be important to allow producers to make management decisions. There are no products registered to control the disease itself, and no varieties with resistance. Once the symptoms appear there is nothing that can be done to reverse the infection or kill the organism. Ways to minimize the potential for aster yellow infection for future years include seeding early, controlling perennial, biennial and winter annual weeds in and around susceptible crops, avoid planting near perennial crops that are known to be infected, and watch for the presence of leaf hoppers. Application of an insecticide registered for use on leafhoppers may be beneficial if populations are high enough early in the season.
For more information on this, or other crops related topics, contact Sherrilyn Phelps, Regional Crops Specialist, at 306-446-7475 or the Agriculture Knowledge Centre in Moose Jaw at 1-866-457-2377.
