Aster yellows is a phytoplasma infection that causes misshapen pods and flower buds. Most fields in most years have less than 1% infection, which is not an economic level of loss.
However, some infection will go unnoticed because not all infected plants show the obvious signs of infection. The plant looks normal but some pods are empty or contain misshapen seeds. Photos below show various aster yellows symptoms.
Aster yellows phytoplasma is carried by aster leafhoppers, which come up each year on south winds from the U.S. Canola varieties are not known to have any difference in infection rates. If aster yellows is found in one variety, it will be found in all varieties.