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Vegetative Compatibility and Pathogenicity of Verticillium albo-atrum. J. C. Correll, Department of Plant Pathology, University of California, Berkeley 94720; T. R. Gordon, and A. H. McCain. Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 78:1017-1021. Accepted for publication 26 January 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1017.

Strains of Verticillium albo-atrum, isolated from alfalfa, cucumber, Ceanothus, and Pelargonium, were compared for virulence on 10 different host species in greenhouse inoculation tests. The strains tested were pathogenic on cantaloupe, cotton, eggplant, potato, and Pelargonium but were not pathogenic on pepper or cucumber. Only strains recovered from alfalfa were virulent on alfalfa. All of these strains, including several recovered from potato and hop, were tested for vegetative (heterokaryon) compatibility with one another by pairing nitrate nonutilizing (nit) mutants. Fifteen strains recovered from and virulent on alfalfa, from throughout North America and from France, Yugoslavia, and the USSR, were vegetatively compatible with one another and, therefore, in the same vegetative compatibility group (VCG01). These data indicate that alfalfa strains of V. albo-atrum may represent a genetically homogeneous clonal population with a common origin that has subsequently become distributed worldwide. Thirteen other strains of V. albo-atrum, recovered from several diverse hosts and geographical locations, were vegetatively compatible with one another but vegetatively incompatible with all of the alfalfa strains of V. albo-atrum; these strains were placed in a second VCG (VCG02). Two hop strains from the United Kingdom were in VCG02, which indicates that VCG02 may also have a worldwide distribution. Four strains recovered from hop were heterokaryon self-incompatible; that is, phenotypically distinct nitrate nonutilizing mutants derived from the same parent were unable to form a complementing heterokaryon. These four hop strains were vegetatively incompatible with two other hop strains as well as with all the other strains examined. Although strains in VCG02 were recovered from several different hosts, the three strains in this VCG that were tested could not be differentiated on the basis of virulence on 10 host species.

Additional keywords: heterokaryosis, host specificity, nitrogen metabolism, nit mutants.