Black Leg of Canola (Brassica napus var. oleifera) in Kentucky. A. Mengistu, Department of Plant Pathology, University of Wisconsin, Madison 53706. P. H. Williams, D. E. Hershman, and D. W. Sippell. Department of Plant Pathology, University of Wisconsin, Madison 53706; University of Kentucky, Lexington 40546; and Allelix, Mississauga, Ontario. Plant Dis. 74:938. Accepted for publication 1 August 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0938C.
Black leg is a major disease of rapeseed (Brassica napus L. var. oleifera)-canola in Europe, Canada, and Australia-and is caused by Leptosphaeria maculans (Desmaz.) Ces & de Not. (anamorph, Phoma lingam (Tode:Fr.) Desmaz.). An epidemic of this disease was detected in an area southeast of Russellville, Kentucky. Approximately 100 ha representing eight fields planted to various rapeseed cultivars were severely affected with basal cankers, lodging, and up to 80% yield loss. Three isolates of L. maculans recovered from stubble collected from these fields were evaluated for aggressiveness on cotyledons of the differential B. napus cultivars Westar, Quinta, and Glacier and on six rapid-cycling Brassica spp. and Raphanus sativus L. from the Crucifer Genetics Cooperative collection. The Kentucky isolates were compared with three standard isolates of L. maculans from Australia and Europe showing differential pathogenicity on Westar, Quinta, and Glacier. All three Kentucky isolates were highly virulent on all B. napus differentials and were classified as pathogenicity group four (PG4), the severely virulent strain. B. rapa, B. olearacea, B. napus, and R. sativus were susceptible to these isolates, whereas B. nigra, B. juncea, and B. carinata were resistant. This is the first report of a major epidemic of black leg on canola and of the occurrence of a severe (PG4) strain of L. maculans in the United States. This is a serious disease of crucifers worldwide and is important because canola is being developed as a new crop in the United States.