Etiology of Canola Blackleg in Kentucky and Seasonal Discharge Patterns ofLeptosphaeria maculansAscospores from Infested Canola Stubble

A limited survey was conducted to determine the pathogenicity groupings (PG) of Leptosphaeria maculans associated with canola (Brassica napus var. oleifera) blackleg in south central Kentucky. In addition, multiyear ascospore discharge patterns were determined for infested canola stubble from seven fields. Strains PG3 and PG4 of L. maculans were most commonly associated with blackleg in Kentucky (18.4 and 79% of the isolates tested, respectively). One weakly virulent PG1 isolate (2.6%) and no PG2 isolates were found. L. maculans-infested canola stubble discharged ascospores within I week of harvest in 1990 and 1991. Early discharge patterns were not determined in 1989. Stubble collected each year released significant quantities of ascospores in the fall and winter months following crop harvest. However, few ascospores were released from infested stubble after the first year following crop production. The implications of these data in blackleg management are discussed.