2000 Canola Disease Survey

in Minnesota, North Dakota and South Dakota

 

Arthur Lamey, Janet Knodel, Kent McKay, Gregory Endres, North Dakota State University,

Zachary Fore, Karen Andol, University of Minnesota,

Martin Draper, South Dakota State University

 

            A field survey in North Dakota, Minnesota and South Dakota evaluated the incidence (percent infected stems) of aster yellows, blackleg, Sclerotinia stem rot, and the pod severity (percent infected pod area) of Alternaria black spot in 2000.  The survey was conducted in 282 fields in 31 counties of the three states: 206 fields in 20 counties of North Dakota, 55 fields in six counties of Minnesota and 21 fields in five counties of South Dakota.

 

            Materials and Methods.  The survey techniques were adapted from those used by Petri et al. in Saskatchewan (3).  The fields were inspected for disease when the crop was in the swath and the stubble was freshly cut.  All fields surveyed were Argentine canola, Brassica napus.  Five stems were sequentially pulled from the soil at each of eight randomly selected locations for a total of 40 stems examined in each field.  The lower stems and roots of the plants were visually inspected for symptoms of blackleg and Sclerotinia stem rot.  These diseases were easily identified on freshly cut 6- to 18-inch stubble left in the fields.

            Blackleg was identified by the black girdling lesions at or near the soil surface.  Tiny black pycnidia characteristic of the virulent strain of the blackleg fungus often were associated with the lesions.  In cases where the cause of the disease was in doubt, the roots of plants were split with a knife and the interior tissues inspected.  If they were dark gray to black, rather than white, the root was considered to be positive for blackleg.

            Sclerotinia stem rot was identified by the bleached stems, often accompanied by a shredding of the tissues.  The presence of sclerotia inside the bleached tissues helped confirm Sclerotinia.

            At each of eight random sampling locations, the swath was inspected for symptoms of aster yellows.  Counts of aster yellows might be considered estimates rather than accurate counts because it was difficult to randomly sample five plants at each stop.  Aster yellows symptoms used to identify the disease included bladderlike upper pods and a witch’s broom or proliferation of tissues on the upper portions of the plant.

            Ten pods were inspected for Alternaria black spot at each of four random locations per field for a total of 40 pods.  Pod severity (percent of pod area affected) was assessed using the scale of 1%, 5%, 10% and 20% devised by Conn et al (1).  Pod severities and incidence in northwestern North Dakota were extremely low and are not included in this report.

            Counties were surveyed based on acreage in 1998 and 1999.  The objective was to the survey counties with the greatest acreage.  An attempt was made to visit at least 10 fields in any county surveyed so there would be an adequate sample size to compare results among counties.

 

             Results.  Aster yellows incidence was 4.5% in North Dakota, 3.9% in Minnesota and 3.8% in South Dakota. The highest incidences were 9.4% in Roseau County, MN; 9.4% in Cavalier County, ND; 8.3% in Polk County, MN and 6.7% in Campbell County, SD.