We will attempt to determine if there is any relationship between crop rotations, weather patterns and disease. In east Polk County Minnesota and in Stutsman County, North Dakota, very little blackleg was found last week and Sclerotinia incidence was moderately high in only one field in each county. Aster yellows was present in almost every field in both counties. It is difficult to determine the incidence of aster yellows in swathed fields, but it may have been as high as several percent in some fields. It has not been observed in previous surveys, so apparently was not common.
Affected plants have a cluster of green leaf-like flowers near the tip of the raceme.
In some cases, the cluster has shortened internodes, resulting in a witches' broom at the tip of the raceme. Some of the upper pods form into short, flat, broadened bladder-like structures. Although prevalent and striking, I doubt if the incidence is that high in most fields observed to date. Aster yellows is a mycoplasma-like organism that is transmitted by the aster leafhopper. The leafhopper blows in each year from areas to the south of us.