Lack of Association of Mitochondrial Plasmids and Pathogenicity in Nectria haematococca (Fusarium solani f. sp. cucurbitae). Deborah A. Samac. Department of Plant Pathology and Plant Disease Resistance Research Unit, respectively, USDA, Agricultural Research Service, University of Wisconsin, 1630 Linden Drive, Madison, 53706 U.S.A. Sally A. Leong. Department of Plant Pathology and Plant Disease Resistance Research Unit, respectively, USDA, Agricultural Research Service, University of Wisconsin, 1630 Linden Drive, Madison, 53706 U.S.A. MPMI 2:128-131. Accepted 15 February 1989. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1989.

Two linear plasmids, pFSC1 and pFSC2, are present in mitochondria of a highly pathogenic strain (FS37) of mating population I of Nectria haematococca (Fusarium solani f. sp. cucurbitae). Strains derived by curing FS37 of plasmid DNAs were less pathogenic than FS37, suggesting that plasmid DNA played a role in pathogenicity. To test whether the reduced pathogenicity was due to loss of the plasmids, cured strains were recurrently backcrossed to the plasmid-containing parental strain to introduce cured mitochondria into a near wild-type nuclear background. Pathogenicity tests with backcrossed progeny revealed nuclear inheritance, but not extrachromosomal inheritance, of pathogenicity determining factors. Also, transfer of plasmid-containing mitochondria into a less pathogenic strain did not result in increased pathogenicity. Thus, the original cured strains apparently had sustained nuclear DNA mutations, induced by the curing process, that reduced pathogenicity. Mitochondrial plasmids did not appear to be directly involved in pathogenicity.