Red River IPM Update - Sept 12, 1999

Impact of Aster Yellows on Canola May be Greater Than You Think

In response to questions from crop consultants and growers about aster yellows, we’ve been examining canola from the Canola Production Center in Roseau over the last couple of weeks. The disease is caused by a phytoplasm (like a bacteria with no cell wall) and is vectored by the aster leafhopper. Many plots at the Center show symptoms of Aster Yellows, including a discoloration of leaves and blossoms, bladdering of pods, and unfilled pods. A table of symptoms observed at the Production Center is presented below. There was a disruption in the planting of plots at the Center this year, with rain separating the planting events into earlier (July 03) and later (July 19) planted plots. It was the earlier planted plots (those planted on July 03) which seemed to be hit hardest by Aster Yellows. Whether this represents a difference in susceptibility due to plant age or merely reflects an immigration event of aster leafhoppers is unknown

A number of environmental conditions have combined to make this disease more widespread than it has been in previous years. Aster leafhopper populations in northern Minnesota and North Dakota were very high this year. Populations of this insect are re-established every spring as overwintering individuals are brought in on weather systems from the south. This summer, we experienced an unusually high number of these weather events, resulting in a high number of successful colonization events by these insects. Conditions, once they arrived, were favorable for population increase, resulting in a boom in leafhopper numbers (true also for potato leafhopper). In addition, the amount of precipitation experienced in a lot of the canola production areas in northern Minnesota resulted in stressed plants. Between an unusually high number of disease vectors and the previously stressed plants, we suspect canola may have been more susceptible than normal to the disease. Unfortunately, little can be done about aster yellows; aster leafhoppers are very mobile and populations can be re-established easily by individuals arriving on the next weather front from the south. Hopefully, next year, we will not suffer the same environmental conditions and the disease outlook will be better.

Last week, Dave LeGare and Karen Andol, both of the U. Minnesota Canola Project, examined pods from infected plants. They found that even in plants not showing heavy aster yellows symptoms, seeds within pods (sometimes green pods) were germinating, many with the cotyledon leaves already developing. This indicates yield impacts might be far greater than growers anticipate from a visual inspection of the plants. What appear as healthy pods may actually still be partially filled with premature, germinating seeds. A significant proportion of seeds from infected plants were germinating in the pod, regardless of pod maturity. This means that a significant proportion of seed from infected plants may not have the weight to settle in the bin while combining. Even if these seeds do enter the bin, they will contribute little or no oil during processing.

Ian MacRae, Dept of Entomology
Dave LeGare, Dept. of Agronomy & Plant Genetics
Karen Andol, Dept. of Agronomy & Plant Genetics
Curt Nyegaard, Roseau County Extension