VEGETATIVE COMPATIBILITY GROUPS OF VERTICILLIUM-DAHLIAE FROM ORNAMENTAL WOODY-PLANTS

Vegetative compatibility groups (VCGs) of 42 strains of Verticillium dahliae obtained primarily from ornamental woody plants were assessed through complementation tests of nitrate-nonutilizing (nit) mutants. Of these isolates, 30 were assigned to VCG1, two to VCG2, and four to VCG4. The VCGs of six strains were not determined because they either did not produce mutants or produced mutants that did not complement any of the test strains. The assignment of most of the woody-plant isolates from Illinois to VCG1 was in sharp contrast to the diverse range of host plants from which the strains were isolated. Data could indicate that an important inoculum source of the pathogen in urban areas is from established populations in nurseries that get distributed along with transplanting trees. NitM mutants appeared to be more stable than nit1 mutants in maintaining mutant phenotypes. Employing multiple mutants of many isolates, complementation tests showed that the extent of prototrophic growth at the mycelium interface was characteristic of the mutants and may not necessarily reflect that of their parent strains. Thus, caution should be exercised in quantifying relationships among natural strains within VCGs solely based on the extent of the prototrophic growth of their complementing mutants. Also, restriction banding patterns of a polymerase chain reaction-amplified mitochondrial rDNA were used to identify strains of V. dahliae that could not be identified by morphology.