ABSTRACT
D-factors are cytoplasmically transmitted genetic determinants that have the poten tial to cause degenerative disease in Ceratocystis ulmi.' Their biological characteristics and implications for Dutch elm disease have been reviewed. The molecular nature and mode of action of d-factors are of considerable interest since, once identified, it may be possible to manipulate them to provide effective biological control agents for Dutch elm disease. The d2-factor shows a close, but not absolute, correlation with double-stranded ribonucleic acid (dsRNA) segments 4, 7, and 10. It remains possible that the d2-factor is a cytoplasmic element other than dsRNA, e.g., a DNA plasmid, which was fortuitously associated with dsRNA in the original isolate, and which is eliminated in a manner similar to dsRNA in conidia and ascospores. The d1-factor, in isolate H321 of the NAN subgroup of the aggressive strain of C. ulmi, was the first ct-factor to be discovered.'
